Radio stations for a wide variety of purposes operate on VHF: radar, communications, television, radio broadcasting, etc. Amateur radio transceivers have recently begun to operate on the same waves.
The receiving and transmitting antennas used on VHF differ significantly from those for long, medium and even short waves.
VHF antennas have relatively small sizes with very good quality indicators. Within the VHF range, antennas of different subbands also differ sharply from each other both in principle of operation and in design. For example, centimeter-wave antennas are very different from meter-band antennas. It is difficult to find even any external similarity between them.
We will talk about antennas in which radio amateurs and television viewers are currently showing the greatest interest: about antennas meter range(10-1 m) and the long-wave part of the decimeter range (1 m - 50 cm). These antennas are used in everyday practice for receiving television and as receiving and transmitting antennas for communication amateur radio VHF stations.
The selection and design of receiving and transmitting antennas is a very serious stage in the practice of a radio amateur. Therefore, we want to talk about some of the most important properties of VHF antennas, which will help you choose antennas wisely and justifiably for various VHF installations.
Directional properties of VHF antennas. The directional properties of antennas refer to their ability to emit electromagnetic energy in relatively narrow beams in certain desired directions. The fact is that there are no antennas that radiate electromagnetic energy evenly in all directions.
Let us first consider the simplest and at the same time the most common VHF antenna - a symmetrical half-wave vibrator (Fig. 1). This vibrator consists of two metal rods located on the same axis. The total length of the vibrator is approximately half the wavelength. Let's place the vibrator. horizontally, i.e. parallel to the ground, and mentally draw a plane perpendicular to the axis of the vibrator (vertical plane). In this plane, the radiated power is distributed evenly in all directions. Therefore, they say that a horizontal vibrator is non-directional in the vertical plane. In the horizontal plane, the radiation is directional, and highest power is emitted perpendicular to the vibrator, and in the direction of its axis there is no radiation at all
Accordingly, a vertically located vibrator radiates uniformly in all directions in the horizontal plane and unevenly in the vertical.
Rice. 1. Directional patterns of a half-wave vibrator.
For clarity, the directional properties of antennas are depicted graphically in the form of radiation patterns in the horizontal and vertical planes (Fig. 1). It must be emphasized that the radiation pattern does not make it possible to determine what power the antenna emits in a certain given direction, since the amount of this power depends not only on the shape of the pattern, but also on the total power of the transmitter. The antenna radiation pattern characterizes. only the distribution of transmitter power in space, regardless of the total value of this power, is determined only by the design of the antenna.
In Fig. Figure 2 shows, as an example, some possible radiation patterns of VHF antennas in the horizontal plane.
An antenna with a type a diagram radiates in a horizontal plane evenly in all directions. The antenna of an amateur radio transmitter should have such a diagram, if the direction to the correspondent is unknown in advance, as well as a television transmitting antenna.
Diagrams of types b and c have two symmetrical lobes. Antennas with such patterns radiate equally in two opposite directions. It is often useful to concentrate radiation in only one direction. Then you need to use unidirectional antennas with radiation patterns of type d.
As can be seen from the figure, these diagrams usually have, in addition to the main lobe, small “back” or “side” lobes, which indicates some transmitter power consumption for radiation in undesired directions. Note that an antenna with a type d radiation pattern emits electromagnetic waves in a narrower beam and is, therefore, more directional. The width of the main lobe of the radiation pattern is measured in degrees and calculated at half power level or 0.7 voltage (angle a in diagram d).
Rice. 2. Various forms of VHF antenna patterns.
The question arises: how to choose a VHF transmitting antenna in terms of the shape of the radiation pattern? To answer this question, you need to know within what angle the direction from the transmitting antenna to a possible correspondent can change.
It is necessary that this angle fits within the opening angle of the main lobe of the radiation pattern at half power level.
Note that the narrower the main lobe of the radiation pattern and the smaller the back and side lobes, the greater the power of the emitted waves (with the total transmitter power remaining constant) emitted in the main direction and the greater the communication range in this direction.
The main types of antennas and their corresponding radiation patterns will be shown below. So far we have looked at transmitting antennas. What about the directional properties of receiving antennas?
Let some antenna be used as a transmitter to emit signals into space and have a radiation pattern shown in Fig. 2, d. The maximum power of the emitted waves corresponds to the direction shown by the solid arrow. If the same antenna is used for reception, then the power of the signals arriving at the receiver input will be maximum when the signal comes from the same direction (dotted arrow).
Thus, it turns out that the radiation pattern of any antenna remains unchanged when it is operating both for transmission and reception. When choosing the type of receiving antenna from the point of view of the radiation pattern, the same considerations regarding the required angle of the pattern in the horizontal plane must be taken into account.
It should also be added that the narrower the main lobe of the radiation pattern and the smaller the tank lobes, the weaker the effect of various reception interferences (medical, industrial, etc.).
VHF antenna gain. Receiving and transmitting VHF antennas are characterized not only by the radiation pattern, but also by the magnitude of the gain.
Let there be two transmitters of the same power. The antenna of the first transmitter is a half-wave vibrator (Fig. 1), the antenna of the second transmitter is unidirectional with the diagram shown in Fig. 2, d. The antenna of the second transmitter creates a stronger electromagnetic field in the main direction. This is obviously explained by the fact that, firstly, the antenna of the second transmitter radiates only in one direction and, secondly, it concentrates the radiation in a narrower beam. If the antenna of the second transmitter creates an electromagnetic field at a certain distance, for example, twice the strength (intensity), then this antenna is said to have a field gain of 2 relative to the half-wave vibrator.
The gain of any antenna is determined by comparing it with a half-wave vibrator, the gain of which is conventionally assumed to be equal to unity.
The concept of gain can also be extended to receiving antennas. In this case, the field gain shows how many times the voltage at the receiver input increases when using this antenna compared to the case of using a half-wave vibrator.
It should be noted that an increase in gain is not necessarily associated with a decrease in the width of the radiation pattern in the horizontal plane. It is possible to increase the gain of the receiving and transmitting antennas of VHF stations by narrowing the radiation pattern in the vertical plane and thereby not limiting the angle within which communication is possible.
Feeders for VHF antennas. The receiving and transmitting antennas are connected respectively to the receiver and transmitter feeder.
Selecting the type of feeder and the method of connecting it to the antenna - important point in the process of designing a VHF antenna for both a transceiver radio station and a television receiver.
Symmetrical cables, shielded (RD-13) or unshielded (KATV), and asymmetrical shielded (cables RK-1, RK-3, RK-49, etc.) can be used as feeders. In Fig. 3 shows cable designs various types.
For both television antennas and antennas for transmitting and receiving VHF radio stations, it is best to use an asymmetrical shielded cable. This cable is relatively inexpensive; it can be attached with simple brackets directly to any wall: wooden, brick, etc. In addition, when such a cable is used, loss of transmitter power and distortion of the antenna pattern due to radiation from the feeder itself are practically eliminated.
Rice. 3. Cables used on VHF. a—asymmetrical shielded cable; b—symmetrical shielded cable; c—symmetrical unshielded cable.
There may be cases where the transmitter has balanced output, and the transition to coaxial cable for some reason impossible. In such cases, a shielded symmetrical cable should be used, and if the latter is not available, an unshielded cable should be used. It should be borne in mind that unshielded cable is attached to the walls using special insulators.
Connection of feeders to antennas of various types should be made only as shown in the figures below. These feeder connection schemes provide both balancing (when moving from an unbalanced cable to a balanced antenna) and matching. Incorrect connection of the feeder to the antenna leads to a decrease in the radiated power, as well as to frequency distortion of the transmitted and received signals. When receiving television, specific distortions may appear in the form of repeated image contours.
Antenna types for amateur radio stations and television reception. In principle, the same types of antennas can be used for amateur VHF radio stations and television reception. Therefore, it is advisable to talk about these antennas at the same time, making appropriate reservations if necessary.
The simplest, most common antenna for an amateur VHF radio station and for television reception is a half-wave vibrator (Fig. 4).
The half-wave vibrator can be used on any of 12 television channels in the frequency range 48.5-230 MHz, as well as in the amateur radio VHF bands: 28-29.7, 144-146 and 420-425 MHz.
There are two main types of half-wave vibrators: a linear half-wave vibrator (Fig. 4,a) and a half-wave loop vibrator (Fig. 4,6). In terms of their electrical characteristics, both vibrators are approximately equivalent; they have the same radiation patterns and the same gains. The bandwidth of the loop vibrator is somewhat wider, but this is not significant, since the bandwidth of a properly designed linear vibrator is quite sufficient to pass the frequencies of any television channel, and even more so the channel of an amateur radio station.
Both types of vibrators are usually made of tubes (steel, brass, copper, duralumin). They can also be made from metal strips or corners. Their main design dimensions are shown in Fig. 4. By wavelength I, in the case of a vibrator for television reception, we should understand the wavelength corresponding to the average frequency of the television channel; in the case of a vibrator for an amateur VHF radio station, K must be understood as the wavelength corresponding to the carrier frequency.
Possible ways to connect feeders to a linear half-wave vibrator are shown in Fig. 4, c, d, e and f. Schemes in Fig. 4,c and d are used when asymmetrical shielded cables with a characteristic impedance of 75 ohms (RK-1, RK-3, etc.) are used as feeders.
In the diagram in Fig. 4, the cable connection is made through a U-shaped elbow from the same cable; in the diagram in Fig. The 4.g cable is connected through a balancing short-circuited bridge made of tubes. Both schemes are approximately equivalent, although the scheme shown in Fig. 4g still provides transmission of a wider frequency band. Scheme in Fig. 4, d is used in the case of using a symmetrical shielded cable RD-13 with a characteristic impedance of 75 ohms as a feeder, diagram in Fig. 4,e - in the case of using a symmetrical unshielded CATV ribbon cable with a characteristic impedance of 300 ohms.
Rice. 4. Diagrams for connecting cables to vibrators.
c—linear half-wave vibrator; b—half-wave loop vibrator; c—connecting the cable through the elbow; d—cable connection via a quarter-wave bridge; d - connection of a symmetrical shielded cable; e - connection of a symmetrical unshielded cable; g—cable connection via U-elbow; h - connection of a symmetrical unshielded cable; n - connection of a symmetrical shielded cable; k - connecting the cable to an asymmetrical quarter-wave vibrator.
Possible ways to connect feeders to a half-wave loop vibrator (Pistolkors vibrator) are shown in Fig. 4, g, h and i. Scheme in Fig. 4,g is used when using asymmetrical shielded cables with a characteristic impedance of 75 ohms (RK-1, RK-3, etc.), the diagram in Fig. 4,h - when using a symmetrical unshielded cable with a characteristic impedance of 300 ohms (KATV), the diagram in Fig. 4,i - when using a symmetrical shielded cable with a characteristic impedance of 75 ohms (RD-13).
In Fig. Figure 4k shows an antenna called a quarter-wave vertical vibrator and is usually used in cases where the antenna can be placed above a large metal sheet (for example, for automobile stations).
Note that to ensure matching of the cable with the antenna in the diagrams in Fig. 4,e and cables are connected through quarter-wave matching transformers made from cable sections.
All considered schemes for connecting feeders to half-wave vibrators can be used with equal success for both transmitting and receiving antennas.
Which vibrator is better to use: linear or loop vibrator? We have already noted that from the point of view of electrical characteristics, both vibrators are approximately equivalent. The question posed should be resolved based only on constructive considerations and available materials. A loop vibrator requires, for example, twice as much tube consumption to produce. At the same time, the vibrator cable is easy to install on any mast - metal or wood, since it can be attached at the midpoint (point 0 in Fig. 4.6) directly to the mast using welding or a metal clamp without any insulators. Attaching a linear vibrator to a mast requires insulators: ceramic, plastic, polystyrene or plexiglass.
As antennas with a relatively high gain and better directional properties than a half-wave vibrator, “wave channel” antennas consisting of several vibrators are used for television reception and for VHF amateur stations.
The simplest antenna of this type is a two-element antenna, consisting of two vibrators (Fig. 5, a), located in the same plane and mounted on a boom, which is made of a metal pipe, angle or wooden beam.
Rice. 5. Directional VHF antennas of the “wave channel” type. a—two-element antenna (voltage gain 1.35); b—three-element antenna (voltage gain 1.85); c—five-element antenna (voltage gain 2.4).
As one of the vibrators, which is called active (a feeder is connected to this vibrator), a linear half-wave vibrator or a half-wave stub vibrator, described above and shown in Fig. 4. The second of the two-element antenna vibrators - passive (the feeder is not connected to it) - is a solid metal tube, fixed “to the boom directly, without any insulators. The passive vibrator, like the active one, is mounted symmetrically relative to the boom. The length of the passive vibrator and its distance to the active one are chosen in such a way as to direct the power emitted by the active vibrator only in one direction. From this point of view, the passive vibrator of a two-element antenna is called a reflector. Thus , the two-element antenna is unidirectional, as can be seen from the given radiation pattern.
Rice. 6. Contour slot antenna with reflector (voltage gain 1.9).
The three-element antenna (Fig. 5, b) contains, in addition to the active vibrator and reflector, another passive vibrator, called the director. The length of the director and its distance to the active vibrator are chosen in such a way as to further enhance the radiation in the main direction. In accordance with this, a three-element antenna has a higher gain and a narrower radiation pattern than a two-element antenna.
A five-element antenna (Fig. 5c) already contains three directors, in addition to a reflector and an active vibrator, and has an even higher gain and an even narrower radiation pattern.
Connecting feeders to active vibrators of multi-element antennas shown in Fig. 5 is produced as shown in Fig. 4, g, h and i.
It is possible, of course, to make an antenna with an even larger number of directors, but this does not make much sense, since when the number of directors increases beyond three, the gain increases very slowly, while the weight and complexity of the design increase significantly. If for something (for example, for long-distance television reception) it is necessary to have a very high gain, then so-called common-mode antennas are made, consisting of multi-element antennas of the “wave channel” type, located in several floors or rows.
In Fig. Figure 6 shows a VHF antenna, called a contour slot antenna. It consists of a rectangular frame, which is the active element of the antenna, and a reflector. The reflector is made of five tubes forming a flat lattice.
The gain of the antenna is approximately equal to that of a three-element channel-wave antenna, but the bandwidth of this antenna is wider.
Rice. 7. Antenna with a corner reflector (voltage gain 3.6).
The cable is connected to points a and b as shown in Fig. 4, g, h and m;
The maximum radiation is directed perpendicular to the plane of the frame. The contour-slot antenna with a reflector is most widespread in the amateur radio range 144-146 MHz.
Rice. 8. Horn antenna (voltage gain at l = 0.5K is 1.3, and at l = lambda is 2.b).
It should be noted that if the antenna is positioned so that the plane of the frame is perpendicular to the ground, then the structure of the radiated field is similar to the structure of the field of a horizontal vibrator (only horizontally polarized waves are emitted or received).
In the range of 420-425 MHz, an antenna with a corner reflector (Fig. 7) and one of the varieties of horn antennas (Fig. 8) are also very convenient.
The CATV cable should be connected to the antenna with a corner reflector at points a and b. The two side faces of the horn antenna are covered with a metal mesh. The cable is connected to points a and b.
Antenna designs are described, and circuit diagrams antenna amplifiers for a homemade VHF radio station (diagram and description) for the frequency ranges 144 MHz, 430 MHz and 1296 MHz.
About the characteristics of VHF antennas
The efficiency of an antenna is clearly related to its geometric dimensions; for this reason, the antenna is the only device included in the radio station that has not been affected by the process of miniaturization of radio equipment.
Manufacturing and installing an antenna is a rather complex and labor-intensive task, especially since it involves addressing issues of strength and rigidity of mechanical structures. Nevertheless, increasing the efficiency of the antenna is the only unlimited way to increase the energy potential of a radio station.
Any antenna can be represented as an equivalent platform standing in the path of radio waves. The larger its area, the greater the antenna gain, formula:
where G is the antenna gain relative to the isotropic radiator; S - equivalent area, m2; lambda - wavelength, m.
From an energy point of view, it does not matter what shape the equivalent site will have: whether it will be round, square or in the shape of an elongated rectangle. In any case, with an equal area there will be an equal gain. Another thing is the radiation pattern; the shape of the equivalent platform has the most direct influence on it. Thus, the width of the main lobe of the radiation pattern can be related to the linear dimensions of the site by the following approximate expression (formula):
A0(delta_0) - width of the main lobe at -3 dB level; hail; lambda - wavelength, m; l is the linear size of the equivalent area in the plane of measurement of the radiation pattern, m.
This formula, rewritten in a different form, allows us to estimate the size of the equivalent area from the known radiation pattern: l = 50 * lambda / delta_0.
Let, for example, tests of a 432 MHz antenna show that the radiation pattern width is 25° in the horizontal plane and 20° in the vertical plane. It is easy to determine that the equivalent area would be 1.4 m horizontally and 1.75 m vertically.
Such estimates are very convenient if it is intended to increase the gain by connecting several antennas into an antenna array. So, for the example considered, the distance between adjacent floors of the array should be 1.75 m, and between adjacent rows - 1.4 m. At smaller distances, the equivalent areas will overlap each other and the total gain will be less than the sum of the gains of all antennas.
At large distances, gaps will appear between individual areas. As a result, the overall gain will not increase, but the dimensions of the antenna will increase unjustifiably. At the same time, dips appear in the main lobe of the radiation pattern, breaking it into several components.
And although the presence of such dips can sometimes be beneficial (for example, if it is necessary to tune out interference, the azimuth of which differs little from the azimuth of the correspondent), in most cases such a radiation pattern makes it difficult to work on the air.
Returning once again to the question of antenna gain, it should be noted that in the general case, the gain is the product of the directional coefficient and the efficiency of the antenna (formula):
where K is k.n.d. antennas; n - efficiency antennas. This means that it is not enough to make an antenna of a large area; you must also be able to deliver all the energy falling on a given area to the consumer of this energy, i.e., to the input of the receiver, with minimal losses. (Here and henceforth we will use the “reciprocity principle” that is valid for antennas, which indicates the equivalence of the antenna parameters in the reception and transmission modes. For example, the radiation pattern or efficiency does not depend on whether the antenna is used for reception or transmission. This allows you to choose the most convenient mode of operation of the antenna each time.)
The emission of electromagnetic energy is associated with the flow of high-frequency current, so losses in the antenna itself are determined by ohmic losses in metal elements. Losses in cable lines have a great influence on the efficiency of the antenna-feeder path, which must be taken into account when assessing the energy potential of a radio station. It is useful to remember that the antenna-feeder path is used for both reception and transmission and, therefore, losses in the feeder will be included twice in the final result.
The table provides brief information about some high-frequency cables that are used in amateur radio practice. The table shows that with increasing frequency, losses in the feeder increase rapidly.
For example, a 20-meter section of cable type RK-75-4-11 (old name RK-1) attenuates the signal passing through it at a frequency of 144 MHz by 2.1 times (3.2 dB), at a frequency of 432 MHz - 3.4 times (5.4 dB), and at a frequency of 1296 MHz - 13 times (11.2 dB). It can be seen that in high-frequency ranges the losses increase to unacceptable values.
In addition, data is presented here for the case when there are no reflections at the ends of the line, i.e., for the case of operation at a matched load. If the load resistance differs from the characteristic impedance of the cable, then part of the energy is reflected from the end of the cable and moves in the opposite direction.
This reflected portion of the energy can only return to the load after it has traveled a double path from the load to the generator and back from the generator to the load. If the losses in the feeder are small, then such multiple reflections are quite acceptable.
This “tuned feeder” mode, in particular, is used in some types of multi-band HF antennas. On VHF, where losses in the feeder increase sharply, we can assume that part of the energy reflected from the load is almost completely lost. The situation, however, is not as bad as it might seem at first glance. In order to estimate mismatch losses, we write down the r.s.v. as a function of reflection coefficient (formula):
here G is the reflection coefficient;
from here it is easy to obtain an expression for calculating the amount of losses (formula):
Rice. 31. Technical and wave parameters of coaxial cables.
This expression is shown graphically in Fig. 32. It can be seen that even with r.s.v. = 3, losses reach only 25%. If the losses in the feeder itself are not very large, then due to the partial return of the reflected energy, the reflection losses will be even less.
So, for the case of losses in the feeder of 2 dB, reflection losses at V.S.V. = 3 decreases from 25 to 20%. It is clear that there is no point in striving for r.s.v. = 1.1 or even 1.01, cap this is given in the description of some amateur radio antennas. So, with r.s.v. = 1.5, reflection losses, even in the worst case, will be only 4%. It also follows that, without any significant losses, you can power an antenna with an input impedance of 50 Ohms using a coaxial cable with a characteristic impedance of 75 Ohms, since in this case the d.r.s. will be equal to 1.5.
Rice. 32. Dependence of reflection losses on c.s. V.
Let us now consider the features inherent in the antenna-feeder system in reception mode. In this mode, the noise properties of the antenna begin to play a significant role. For this reason, the concept of noise temperature is often introduced for a receiving antenna. If, for example, the noise temperature of the antenna is 200 K. then this means that the antenna generates the same noise as it generated
would be an active resistance heated to a temperature of 200K. Antenna noise consists of external and internal noise. External noise is a source of interference that fundamentally limits the ability to receive weak signals.
With an antenna aimed at the horizon, these are primarily thermal noise from the earth's surface, various types of industrial interference, as well as noise of cosmic origin. Internal noise is determined by the presence of losses in the antenna and feeder. Like any active resistance, loss resistance generates thermal noise.
For this reason, the sensitivity of the receiver deteriorates not only due to the fact that the received useful signal is attenuated in the feeder, but also due to the fact that the feeder generates additional noise. Both of these factors are taken into account in the simple formula “for an attenuator heated to a temperature environment. The noise figure of the receiver, taking into account losses in the feeder, is equal to (formula):
where Ftotal is the resulting noise factor; L - attenuation in the feeder or in any other passive quadrupole; Fpr is the receiver's own noise figure.
Thus, knowing the noise figure of the receiver and calculating the attenuation in the feeder using the table, you can easily determine the resulting noise figure of the receiver from the antenna terminals. You can also solve the inverse problem, that is, by measuring the noise figure with and without a feeder, determine the losses in the cable. This is a more reliable way, since for various reasons the actual losses in the cable may differ significantly from the tabulated ones.
It can be seen that losses in the feeder have a significant impact on the potential capabilities of the radio station. As a result, the effort spent on manufacturing a large and complex antenna can be negated. And if in the transmission mode it is still possible to somehow compensate for losses in the feeder by increasing the power, then in the reception mode the losses are irreversible. Allow this problem Antenna preamplifiers located in close proximity to the antenna help.
The question of the need to use such an amplifier must be decided in each specific case, comparing the external noise of the antenna and the internal noise of the receiver. In order to ensure normal operation of the receiver input circuit, instead of the antenna it is necessary to connect a resistor whose resistance is equal to the characteristic impedance of the feeder.
If, even in the most favorable night hours, the antenna noise is noticeably (2 times or more) higher than the resistor noise, an antenna amplifier should not be used. Moreover, an extra gain stage will make the receiver more vulnerable to interference from nearby radio stations.
In order to connect a preamplifier in receive mode, you need to have two high-frequency relays or one relay and a separate feeder connecting the preamplifier output to the receiver input.
VHF antenna preamplifier circuits
Antenna preamplifier circuits can be borrowed from transverter circuits of the corresponding ranges. For example in Fig. 33, a shows the circuit of the antenna amplifier for the 144 MHz range, and in Fig. 33.6 - for the 432 MHz range.
The method for setting up preamplifiers does not differ from the method for setting up the corresponding stages of transverters.
If the antenna relays do not provide sufficient isolation, the problem arises of protecting the preamplifier from the transmitter signal. As one of the protection measures, diodes D1 are included in the basic circuit of transistors. When setting up, be sure to check whether connecting a protective diode does not degrade the noise figure of the preamplifier.
Rice. 33. Antenna amplifier circuits.
Protection problems completely disappear if you use a powerful multi-emitter transistor KT610 or KT911 as a preamplifier. The circuit of such a preamplifier, designed for the 144 MHz range, is shown in Fig. 34. Coil L1 contains two turns of silver-plated wire with a diameter of 1.0 mm.
The diameter of the mandrel is 10 mm. Setting up the amplifier must begin by setting the transistor mode to DC. By selecting resistor R1, it is necessary to ensure that the collector current of the transistor is 15-25 mA.
Fig. 31. Antenna amplifier 144 MHz range, made on a multi-emitter transistor.
The preamplifier has the following characteristics: gain about 20 dB, noise figure 1.5-1.8. To prevent failure of subsequent amplification stages, it is advisable to remove the supply voltage from transistor T1 in transmission mode, and even better, connect the preamplifier power wire to ground.
VHF antenna designs
Let's now look at some practical designs antennas For many years, the most popular among radio amateurs have been antennas of the “wave channel” type, which are also known as; “director antennas” and “Uda-Yaga antennas”. These antennas, belonging to the class of antennas with axial radiation, have the best gain to weight ratio and are also very simple in design.
The main drawback that has limited the use of such antennas in industrial communications is their narrowband. However, for radio amateurs this disadvantage does not play a big role, since the width of the ranges allocated for amateur radio communications is also small.
Recently, numerous attempts have been made to improve the channel wave antenna in order to increase its gain. A section of a log-periodic antenna (a “Swan” antenna) was used as an active element, or more complex passive elements were used, consisting, for example, of four half-wave vibrators (numerous types of antennas produced by Western countries for receiving television on decimeter waves).
However, all these tricks do not provide a significant gain, since ultimately the gain of any antenna with axial radiation is determined by its length. The use of more complex vibrators is equivalent to the use of several conventional “wave channel” antennas located at a very small distance from each other. As already indicated, this is equivalent to almost complete mutual overlap of equivalent areas, and therefore the resulting gain is also small.
Rice. 35. Quagi eight-element antenna for the 144 MHz band, dimensions for the 432 MHz band are given in brackets.
Of the improved wave channel antennas, perhaps the most interesting are the Quagi antennas. The name is made up of two English words "Quad" and "Yagi" and indicates that the antenna is a hybrid of a "quad" and "Yagi" type antenna.
Actually, only the active element and the reflector frame are taken from the “square”, and all the directors are the same as in the “wave channel” antenna. The antenna is powered by a cable with a characteristic impedance of 50 Ohms. The cable is connected directly to the gap in the active frame without any matching device.
The reflective frame has a perimeter of 2200 mm (711 mm), and the active frame has a perimeter of 2083 mm (676 mm). Here and below, the dimensions for the 432 MHz range are indicated in parentheses.
Both frames are made of copper wire with a diameter of 2.5-3 mm and secured to the supporting cross-arm using strips of organic glass. The supporting traverse has a length of 420 cm (140 cm) and is made of a wooden, preferably pine, block with a cross-section of 2.5X8 cm (1.2x5 cm). To facilitate the design, the height of the bar can be reduced towards the ends of the antenna. The directors are made from aluminum or copper wire with a diameter of 3 mm.
The output impedance of the antenna is 50 Ohms, but without large losses it can be powered by a cable with a characteristic impedance of 75 Ohms. When using multiple antennas, the distance between adjacent floors and rows should be 3.35 m (1.09 m).
The more efficient Quagi antenna, designed for the 432 MHz band, has a similar design. The supporting traverse is made from a wooden block with a length of 370 cm and a cross-section of 2.5x5 cm. The height of the block smoothly decreases towards the ends to 1.5 cm.
The length of the reflective frame is 711 mm, and the active frame is 676 mm. Both frames are made of copper wire with a diameter
2.5 mm. The directors are made of wire with a diameter of 3 mm. Other dimensions are shown in Fig. 36.
The antenna is powered by a coaxial cable with a characteristic impedance of 50 Ohms without a balun. In principle, this antenna can be used for the 1296 MHz range, while the wire diameter and all other dimensions should be reduced by 3 times.
Rice. 36. Quagi fifteen-element antenna for the 432 MHz band.
Of the antennas specifically designed for the 1296 MHz range, the antenna proposed by the English ultrashort wave G3JVL is of interest. The antenna is a “wave channel” with ring vibrato
ramie, a kind of multi-element loop antenna. The antenna contains 28 elements, including an additional reflector made of aluminum mesh and 27 ring vibrators. The main reflector and all directors are made of aluminum strips 4.8 mm wide and 0.7 mm thick.
At the ends of the strips there are holes drilled for an M3 screw. The distance between the centers of the hole is 246 mm for the reflector, 210 mm for the first 11 directors and 203 mm for the remaining directors. Then the strips are rolled into a ring and screwed to a supporting duralumin tube with a diameter of 12-15 mm. The distances between elements are shown in Fig.
37. The dimensions of the additional reflector are shown in Fig. 38, a.
Rice. 37. Twenty-eight element antenna for the 1296 MHz range, distances to the elements are measured from the additional reflector.
Rice. 38. Antenna for the 1296 MHz range.
The design of the active element is shown in Fig. 38.6. Unlike other elements, the active frame is made from a copper strip. Frame perimeter 235 mm.
The frame is attached to the supporting tube using a MB threaded bolt. A thin cable with fluoroplastic insulation is passed through a hole drilled along the axis of the bolt. A hole for the cable is also drilled in the middle of the strip from which the active frame is made. The frame is attached to the bolt head using soldering. The cable braid is also soldered to the bolt head.
A thin cable with increased attenuation should be as short as possible. It ends with a high-frequency connector to which the main feeder is connected. An option is possible in which a thicker cable is passed through the mounting bolt, and through a hole drilled in the support tube behind the active frame.
In this case, it is also necessary to ensure contact of the cable braid with the base of the frame.
The given antenna descriptions intentionally do not include gain data. The fact is that precise measurement Gaining an antenna is quite a difficult task, requiring special conditions. As a result, various data often appear in amateur radio literature.
Thus, the figure given by the author of the antenna described above for the 1296 MHz range - 20 dB - seems somewhat inflated. The data given for the Quagi antenna looks more realistic - 12 dB for an 8-element antenna and 15 dB for a 15-element antenna.
Zhutyaev S.G. Amateur VHF radio station, 1981.
Time and time again, ultrashortwave operators ask their senior colleagues: “Which antenna should I choose?” It is impossible to answer this question accurately, since it all depends on the purpose for which the antenna is being built. If communications are expected in all directions, for example within a city, then antennas with a circular diagram are very convenient, which often allow operation at distances between stations of 50-100 km. For long-distance communications, directional antennas are more suitable. In areas “densely populated” with ultrashort wavelengths or in cases where there is interference from some directions, it is undoubtedly better to use highly directional antennas.
These few examples are enough to understand that there is no antenna that is equally suitable for all cases. The radio amateur must choose an antenna that meets his basic requirements. Better yet, build two or three antennas and use them as needed.
It is unwise for a beginner ultrashortwave operator to choose any bulky and complex structure as his first antenna, during the construction of which he can make many mistakes due to inexperience. You should start with the construction of simple antennas and, as experience and knowledge grow, move on to more complex systems.
When choosing the type of antenna, you need to take into account what basic materials are available to the designer. If you cannot purchase pipes or rods for the antenna elements, then you can choose, for example, a “double square”, the construction of which only requires wire, wooden slats and a small amount of insulating material. It is also important how the supply line will be made - from coaxial or ribbon cable, or simply in the form of a two-wire line.
We must not lose sight of whether any measurements are needed when building the antenna. For a beginner, who also does not have measuring equipment, it is better to choose an antenna that will probably work well without tuning.
Let's look at a number of antenna types. Among them there are simple designs, accessible for every beginner to repeat, and complex, including antenna systems, which may be of interest to more experienced DX “hunters”. Since most of our ultra-shortwave radios operate in the 144 MHz range, antenna sizes are given specifically for this range.
The reader will note that no technical design details are provided for either antenna. But this should not interfere with construction, since operating techniques and many details are described in any amateur radio manual.
CIRCULAR RADIATION ANTENNAS
Cross-shaped dipole. The antenna consists of two half-wave vibrators 1, located at an angle of 90° to each other (Fig. 1). The radiation pattern of this antenna is far from a perfect circle, but in practice it produces quite good circular radiation. Since the characteristic impedance of one dipole is approximately 70 Ohms, when two dipoles are connected in parallel, the characteristic impedance is about 35 Ohms. We do not have such a coaxial cable at our disposal, so it is best to power the antenna through a quarter-wave transformer 3 made from a 50-ohm cable. A 75-ohm cable 4 runs from the transformer to the equipment. The balancing U-elbow 2 is made from the same cable.
Vertical antenna (Ground Plane). Emitter 1 (Fig. 2) and radial conductors 2 provide a circular diagram in a horizontal plane. The angle between the radial conductors and the emitter determines the characteristic impedance of the antenna.
rice. 2
At an angle of 90°, the wave impedance is approximately 30 Ohms, at an angle of 180° - 70 Ohms. Typically, an angle of 145° is chosen, which allows the antenna to be fed with a 50-ohm cable. The cable is connected to connector 3, mounted on a metal plate to which radial conductors are electrically connected. The emitter, to which the central conductor of the cable is connected, is installed on insulator 4.
DIRECTIONAL ANTENNAS
"Double Square" This popular directional HF antenna is also used on VHF (Fig. 3, a). Its gain (compared to a half-wave vibrator) reaches 5.7 dB, the forward/backward radiation ratio is 25 dB.
rice. 3
The distance between active vibrator 1 and reflector 2 is chosen to be 0.15 lambda, which allows the antenna to be powered with a 75-ohm coaxial cable 3. Experience has shown that the antenna fed in this way works quite satisfactorily. You can tune the antenna using a short-circuited cable connected to the gap in the reflector frame.
To balance the antenna, you can use a quarter-wave glass (Fig. 3, b), connecting it to the ends of the active vibrator 1. The glass consists of a metal cylinder 4 with two covers - metal 5 and dielectric 6. Cable 3 runs inside the glass, the cable braid is connected to the cover 5. The diameter of the glass should be 3-4 times larger than the diameter of the cable.
To make antenna elements, you can use copper or aluminum tube, tape or wire of various diameters. The “double square” takes up very little space and is structurally simple. This antenna has relatively good characteristics. The possibility of placing antennas of different ranges on the same cross-shaped rails is noteworthy.
Triangle Antenna (Delta Loop) belongs to the same family as the “square”, since the perimeter of the active vibrator is approximately equal to the wavelength. A special feature of this antenna is that all elements of its design are metal. The author of the antenna advised feeding it with a 50-ohm coaxial cable, but a 75-ohm cable is also successfully used for this purpose. The simplest triangular antenna is shown in Fig. 4. Active vibrator 1 is adjusted using a gamma matching device to which cable 3 is connected. Depending on the availability of measuring instruments, the adjustment is carried out according to the minimum SWR or the maximum signal strength. To simplify things, reflector 2 can be made unadjustable.
rice. 4
UA1WW experimented a lot with the triangular antenna. He advises using 5- and 9-element options. The latter, due to its small horizontal radiation angle, is especially suitable for long-distance communications. A drawing of a 5-element antenna is shown in Fig. 5. Here 1 is an active vibrator, 2 is a reflector, 3-5 are directors. Since this is a completely new antenna for our ultrashort wavelengths, we present some design data.
rice. 5
A 4-sided duralumin pipe with a square side of 18-20 mm is most suitable for a load-bearing traverse; it is much more convenient to mount elements on it than on a round pipe (see Fig. 6).
rice. 6
The antenna elements are made of a copper or aluminum tube or rod with a diameter of 6 mm, the horizontal side is made of wire with a diameter of 3 mm. The dimensions of the elements (in accordance with Fig. 6) are as follows:
Triangular antenna- an object of interest for ultrashort wavelengths around the world. Taking into account the positive experience with it, we can assume that it will soon become one of the most popular antennas. Therefore, we draw the attention of those who want to experiment to one special type of it - a double triangular antenna (Fig. 7). The triangle dimensions of this antenna are slightly larger than those of a single antenna; the perimeter of the reflector is 2266, the active vibrator - 2116 and the director - 1993 mm. The distance between the reflector and the vibrator is 0.2 lambda, between the vibrator and the director is 0.15 lambda.
rice. 7
According to some data, the following gains were obtained for a double antenna (compared to a half-wave vibrator): one element (active vibrator) - 3-4 dB: two elements (vibrator and reflector) - 8-9 dB: three elements (reflector, vibrator in director), - 10-11 dB. This seems like a promising type of antenna and worth pursuing.
10-element antenna (Yagi). Undoubtedly, this is the most popular VHF antenna (Fig. 8). It gives a gain of 13 dB. The author carried out meteor communications with England and Belgium using such an antenna, and many long-distance communications due to tropospheric passage and “aurora”.
rice. 8
The passive elements of the antenna are made of bimetallic wire with a diameter of 4 mm, and the active loop vibrator is made of a 15 mm copper tube and the same wire. The characteristic impedance at the feed point is 300 ohms, so the 75 ohm cable is connected through a U-elbow, the length of which is 68 cm.
The length of the supporting beam is slightly more than 3.5 m, the diameter is 20 mm. The length of the reflector is 7-1060, the vibrator is 2-990, the directors are 3-10 - 933, 930, 927, 924, 921, 918, 915 and 912 mm, respectively.
Multi-band antenna. There are circumstances when it is not possible to install more than one antenna. But in addition to an antenna, a radio station often also needs a television antenna! Then the way out is a multi-band UKB antenna. One variant of such an antenna is shown in Fig. 9, a (top view) and 9, b (axonometric projection). It can be successfully used in the ranges from 50 to 220 MHz. The antenna gain at a frequency of 50 MHz is 7 dB, 144 MHz is 12 dB, and at 220 MHz is even 13.5 dB. This antenna is a two-story one. At a frequency of 50 MHz, two corner vibrators 1 operate on each floor, located at a distance of lambda/4. At a frequency of 144 MHz their length is approximately 3/4 lambda and therefore the result is a V-shaped antenna. At 220 MHz the vibrators are 5/4 lambda long.
rice. 9
The vibrators are connected to each other by 2 two-wire lines, and both floors by 3 lines, the length of which, depending on the range, is from 1/4 to 5/4 lambda. The distance between floors, if desired, can be changed within the limits allowed by the length of lines 3. The input impedance of the antenna at feed point 4 at frequencies of 50 and 144 MHz is about 300 Ohms, at a frequency of 220 MHz it drops to about 200 Ohms.
Antenna elements can be made from a tube or rod: vibrators - with a diameter of 10 mm; line 2 - with a diameter of 12 mm (10 mm is possible, then the distance between the centers of the line wires should be chosen equal to 64 mm): line 3 - with a diameter of 6 mm.
RADIO No. 8, 1973 p.20-23.
First, why is the word “tuning” the antenna in quotation marks? Let’s say you made an antenna “according to concepts” with all the bells and whistles, boxes, plumbing, etc. that you think should be included in it, and if the SWR is “not the right one”, there is nothing easier for you than to move something, cut it, adjust it and etc. in the antenna itself. Also in terms of concepts, because somewhere someone wrote about what, where and how to move so that the SWR is “the one”. However, in a VK antenna, for example with a split vibrator, there are only three general parameter: diameter, length and position of elements. The accuracy of their observance is quite sufficient for its compliance with the model and coordinated operation with the cable. This means that if the SWR is “wrong,” it’s not a matter of moving, cutting, or adjusting the elements, guided by the SWR, i.e. by agreement means introducing into the antenna another “non-such” of the opposite sign - just to “eat up all the power”, but we don’t know where and how it emits and we sleep peacefully. Something like “tuning” the TV according to power consumption without looking at the screen.
So, the antenna is made, but the SWR is not the same. Let's put the hacksaw aside and figure out why.
♣- If the antenna is made according to pictures and dimensions in reputable publications, it is advisable to get an objective idea of it before sawing it. Type promises "R in 60 ohms, gain 11.5 dB, highly efficient, no special settings required" may turn out to be a deception or delusion of the author and agree on it (Rothhammel vol. 2 p. 70 Quagi antenna) or achieving the promised parameters is impossible in principle (Rake Avrika. Magazine "OST". May 1997, pp. 58, 59). Therefore, it is better to look at it in the calculation program before making it. This will take less time than fruitless attempts to revive scrap metal.
It is likely that the reason is outside the antenna, because in addition to it, the SWR meter itself, the connecting cable and the surroundings of the antenna are also involved in the measurement.
♣- The antenna environment can have a strong influence on the SWR, up to 2 or more. These are objects located in the near zone (up to 1λ of the antenna size) that are opaque to the operating frequency (with strong reflection or absorption), including a cable unsuccessfully removed from the vibrator, and reflectors in the alignment of the main lobe at a distance λ closer than the antenna gain in db. This influence can be avoided by taking measurements in an open area and pointing the antenna towards the sky. If possible, make the traverse longer than the reflector during manufacturing and testing. At the first stage of checking the antenna, you can hold it by the shank and rotate it and change the direction, making sure that the SWR changes by no more than 0.1. This suggests that the influence of the antenna’s surroundings is within acceptable limits, otherwise it is necessary to change the position to one freer from such influence, where you can continue working with the antenna, pointing it upward and securing it to the shank.
♣- A 10% SWR meter error will give you an SWR of 1.1. MFJ 269 has this error. Therefore, SWR 1.1 and lower must be accepted as an inevitable tolerance for measurement error, although it is not difficult to match, or rather mismatch, the antenna for this error up to SWR 1.00.
♣In addition to the error, there is another factor that affects the readings, these are the harmonics in the SWR meter signal. For MFJ they are -26 dB or 5% of the fundamental frequency level, for T 100 -30 dB or 3%. AA 600, moreover, has a rectangular output signal (square wave), in which the level of the 2nd and 3rd harmonics is about 30% of the fundamental frequency. On a reference resistive load, they will all show an SWR of 1.00. But for a real antenna it is unlikely that at double and triple frequencies its SWR is also 1.0; rather, the harmonics will be completely reflected and with the actual SWR of the antenna 1.0 at the main frequency, MFJ and T100 can and should show no better than 1.05, and AA 600 is no better than 1.5. Otherwise, one must either consider their readings unreliable, or the attenuation of the cable on harmonics is high, so that both their direct and reflected power is lost in it.
♣- Cable wave impedance tolerance according to GOST +-4%. Actually measured deviations of wave impedances of cables RG8x, RG58 and other products of the free market are up to 25%. These deviations will give you an SWR of up to 1.08 for antennas with Soviet cables and up to 1.5 with bourgeois brands. This can be avoided by using a tested section of cable to test the antenna with a length that is the minimum possible, but a multiple of half the wavelength, taking into account K shortening, which in this case will work as a half-wave repeater in a fairly wide frequency band.
An idea of what SWR of a normal antenna the SWR meter will show with an unfavorable combination of these factors can be obtained by multiplying the extreme values of each of them: 1.1x1.5x2.0 = SWR 3.3. (With the same degree of probability, you can get an SWR of 1.0 from an antenna with a SWR of 3.3 with a “favorable” combination of these factors for it.)
There are still three or four steps left to the antenna. In almost every design except the VK antenna itself, i.e. elements of a certain shape, length and position, there are devices that are difficult or impossible to include in the model and therefore taking into account their influence and methods of solution remain the most difficult and controversial issues:
Device for matching R antenna with cable (transformer)
Current cut-off device on the outer side of the braid. (1/4 λ glass, ferrite, etc.)
A device for connecting a cable to an antenna, if you can call it that, a pigtail and a central core between the cable and the antenna.
Device for fixing elements in position and position in accordance with the model (traverse and fasteners)
I won't repeat myself. More than half of the material on the pages of the site is devoted to how to minimize their influence and avoid errors in the manufacture of antennas.
There is only one reason left for the discrepancy between the SWR of the antenna and its model.
This is a model, or rather there are errors in it. Spicy (< 45°) углы между проводами, близкорасположенные провода и сегменты в них длинее, чем расстояние между проводами, толстые провода и сегменты в них короче, чем их диаметр, - самые распространенные ошибки.
It is advisable to make sure that there are no errors in the model before making the antenna. If you delve into the subtleties of segmentation, etc. It's still difficult, send the file by email me or my friend who has mastered the program.
If everything mentioned above is followed, then I can assure you that after careful manufacturing of even the most narrowband and error-sensitive antenna, you won’t need to cut, adjust or move anything in it. All that remains is to install it on a mast or in a stack and route the cable so that they have minimal impact on the antenna.
You can tune the antenna only under the control of all the parameters for which it is responsible. This is primarily the radiation pattern (and its derivative - gain). This type of setup is a difficult task even for professionals on a professional range. Hence the simple conclusion and solution is to abandon in a real antenna what is not modeled in the model, but affects its parameters or reduce it to a minimum. Then you can be sure that even the “right” SWR is not a randomly zeroed sum of “non-conformities” of different signs, but a sign of compliance of other antenna parameters, besides SWR, with the parameters that were achieved in the model.
You can and should tune an antenna that has to be installed in conditions that are difficult or impossible to take into account or simulate, an antenna of the HF range, next to which there is ground, wires, etc. Moreover, already at 21 MHz, and even more so at 28 MHz, their influence easy to reduce or eliminate altogether. And on any VHF band such an influence is unacceptable, because it distorts the diagram and it is already pointless to configure something in the antenna itself “at SWR 1.0”. You have to look for and find the error, there is no other option.
In conclusionFor many, the argument: “I’m using this antenna...” is still the most powerful. It’s not in my rules, but I’ll use it too: Of the site’s antennas at 1296 MHz, 8 antennas (3 stacks of 2 and 2 single) were made and sent without monitoring not only the parameters, but without checking the SWR at all or just working. Two stacks then passed the SWR control of UA6EM. Single 1.1 m worked in the 2008 field day for the UA6FW/6 team with 10 watts at 1296 MHz. In “Cup 1296” this antenna lost only 2 connections to the leader. The credit goes mainly to the tactics and skill of the operators, but still. At 144 they had my stack of 2 x 4.5 m. And it was also given away and installed without control before the competition. Immediately after PD 2008, its participants wrote on the VHF DX forum:
RA3AQ "The long-distance stations of the 6th region UA6EM (907 km), UA6FW/6 (2+70cm qrb 972km) passed the test perfectly." UY9IA "The farthest connection (1296) 07/06/2008 03:51 UA6FW/6 606.7km" In general, on the forum in the reports of participants in the line "the farthest connection with..." the call sign UA6FW/6 appears 10 times, RA3AQ - 6 times, RA6AX-4 times, others 1-2 times.
Once again: these antennas were given away and taken to the PD without monitoring the SWR and checking their operation. Although there is something to check at 144 MHz, I was too lazy. All antennas on traverses are made of pine.
Amateur radio for RA3LE has been and remains the main component of that part of life that is allocated to a man in the family for his favorite hobbies or activities. And it began in 1956, with the first complex receiver. It started once and for all. Already in 1958, the first radio station was built for the 38-40 MHz range, a year later the call sign RAZLAG was received, and soon the first diploma for 4th place in republican competitions.
After graduating from the Kharkov Polytechnic in 1965 with a degree in radio engineering, I, having missed the change in the range to 28-30 MHz, without hesitation, switched to VHF. With the new call sign UA3LBO, for the All-Union competitions in 1966, he produced good equipment using 6S17KV, GS4V/GS6V lamps and antennas of his own design. The result was 3rd place at 144 MHz in the individual competition, and in 1968 - 2nd place in the team competition, which allowed him to receive the title of Master of Sports. Then there was a break while serving as an officer in Lyakhovichi.
The seventies of the last century were a wonderful time for the construction of high-quality equipment and antennas, the beginning active work through Tropo and Meteora (MS), first victories in Field Days and other VHF radio competitions. Every three years I built a new transceiver and antenna system. By 1981, “at the top” there were 8x13 elements at 144 MHz and 16x25 elements at 432 MHz with a BFT66 LNA; “bottom” - power amplifiers on GI7B and GS31B, respectively. At 1296 MHz - 4 × 37 elements, LNA and power amplifier on GI41B. All these designs were of our own design, but, of course, the experience of foreign radio amateurs was taken into account during the design.
All these years, my main and constant “on-air” companions were Georgy, UC2AAB (now EU1AB), and Victor, RA3YCR. On VHF at that time, Tula residents were active in the “east”, and Dnepropetrovsk residents were active in the “south”. Honor and praise to them. Muscovites, as now, were rarely heard. By this time, I held VHF radio range records in Europe and the USSR, for a long time - first places in the “table of ranks” on all bands in the USSR and on 432 MHz in the European TOP list. I was the first in Russia to start working on 432 MHz with signals reflected from the Moon (EME), and QSOs via Aurora in this range became as familiar to me as on 144 MHz.
Since 1985, I began to simplify antenna systems, reducing the number of antennas, but improving their quality, because... Experience in creating such systems gradually accumulated. During this time, seven antenna systems were replaced. When calculating and designing antennas, I adhere to the rule - to design highly efficient antennas that have maximum gain at the best gain/bandwidth (G/T) ratio. The bandwidth reserve must provide compensation for the influence of weather conditions at the place of residence. My antennas have never let me down. Perhaps one of the few, I work on transceivers of my own design, made by myself.
In certain periods there were declines in activity on my part, caused by circumstances, some “satiety” and a small number of new correspondents in new squares. In addition, since 1983, I stopped working on MS and EME “skeds” - I simply became uninterested. Many, on the contrary, began to work exclusively on “skeds”. Who likes what. After all, many radio amateurs like to work with weak equipment within the area or even EME (at someone else’s expense). Complete dependence of broadcast work on the Internet and telephone is also not for me.
Since 2004, I again began to actively work in Russian competitions. Experience, high-quality equipment and antennas have allowed me to win or take prizes more than once. The two Russian Cups are very valuable to me. The most interesting connections for me were and remain connections through Tropo and Aurora. It is a pity that in recent years the Aurora has become a rarity in mid-latitudes.
Everyone goes their own way, depending on knowledge, opportunities and conditions. But still, real satisfaction from doing our favorite thing can only be obtained by having good equipment and antennas, which we must constantly strive for.
We bring to your attention antennas for the bands 144, 432 and 1296 MHz- they are simple, have high parameters and good repeatability. However, it makes no sense to describe the design of antennas in detail, because only one out of ten radio amateurs will have exactly the same materials and tools for their manufacture. It is enough to describe the requirements for the manufacture of antennas, and the radio amateur himself will select everything necessary to meet these requirements, otherwise endless questions will begin: “What if....?”
The main parameters of the described antennas are given in Table 1, and all the necessary physical dimensions of antennas for the ranges of 144, 432 and 1296 MHz are given respectively in Table. 2-4.
The MM AN A program is a convenient tool for the antenna designer, but theoretical preparation is required. When calculating models, they must be checked and adjusted - to achieve the best G/T value - in other programs, for example, in YA354. Numerous experiments and measurements on professional equipment allow us to conclude that with the selected element diameters, the calculated frequencies in MMANA correspond to the following actual frequencies: 144.6 MHz - 144.3 MHz, 435.0 MHz - 432.0 MHz, 1307.0 - 1296 .0 MHz.
All elements of the 144 MHz antenna are made of tubes with a diameter of 6 mm. Active vibrator - loop. Its length is 940 mm, width is 73 mm, and the total perimeter is 2026 mm.
Antennas in the 432 MHz and 1296 MHz bands use simple “split” active vibrators with a diameter of 6 and 2.5 mm, respectively. The remaining elements of the 432 MHz antenna are made of tubes (rods) with a diameter of 5 mm, and the antenna elements for 1296 MHz are 2.5 mm. The deviation in the diameters and lengths of elements for antennas in the 144 MHz range should not exceed ±0.5 mm, 432 MHz - ±0.2 mm, 1296 MHz - ±0.1 mm.
The 1296 MHz antenna uses a reflector, two elements of which are spaced vertically up and down by 29.5 mm relative to the plane of the active vibrator and directors.
The elements are attached to the metal crossbeam at a distance of at least 0.6 from the diameter of the crossbeam. Homemade or purchased plumbing “clips” are suitable for fastening.
The metal parts for fastening elements to them (clamps, brackets, “screws”) should not be massive, i.e. significantly increasing the diameter of the elements themselves. On the “clips” mark the center and make a groove for placing the element. When using dielectric (wooden) traverses, any method of fastening elements is acceptable (including through the traverse). After assembling the antenna, the wooden crossbars must be painted with white paint PF115.
The recommended diameter (section) of the traverse for antennas in the 144 MHz range is 25-30 mm, 432 MHz - 18-20*mm, 1296 MHz - 10-15 mm. The best material is D16Tit.p. When using wooden traverses of this size, there must be a place for fastening the elements.
In antennas at 432 MHz and 1296 MHz, active vibrators must be located exactly in the plane of the other elements, otherwise a vertical radiation angle will appear. In a 144 MHz antenna, the active vibrator must be symmetrical to the plane of the vibrators. It is advisable to make vibrators from copper - this will allow you to solder a coaxial cable to them along the shortest path, without additional petals, screws, nuts, etc. If a radio amateur knows how to solder aluminum, then in the antennas of the 144 and 432 MHz bands the active vibrators can be made of aluminum. The soldering area should be painted with PF115 paint. The dimensions of active vibrators indicated in the tables are their finished dimensions!
In antennas of the 144 and 432 MHz bands, copper, D16, AD, aluminum, bimetal can be used to make directors, and in antennas at 1296 MHz - PEV wire or aluminum (soft!) wire from household electrical wiring. Avoid cross-scratching elements.
In antennas of the 144 MHz and 432 MHz bands, the method of mounting active vibrators does not differ from that of directors. Between the halves of the active vibrators of the 144 MHz and 432 MHz antennas, the gap is about 10 mm when connecting a cable with a diameter of no more than 11 mm along the outer insulation. To improve the rigidity of the active vibrator, a rod made of caprolon or from a fishing rod can be installed at the site of its cut. In the 1296 MHz antenna, the gap between the halves of the active vibrator should be no more than 6 mm.
In the author’s version, the active vibrator of the 1296 MHz antenna is mounted like this: the halves are inserted from the sides into a polyethylene foam rectangle. The transition length of the central core of the cable is 1 mm; the second half of the vibrator is soldered end-to-end to the braid of the cable, cut at an angle of 45°.
I recommend using adapter cables in any VHF antennas. They will allow you to accurately measure/adjust the input resistance and are at the same time a glass (stocking) type balun. The length of the adapter cable from the end of the braid of the active vibrator to the body of the connector sealed at the other end of the cable is 1/2 wave. Almost from the end of the braid of the active vibrator, a screen from the same cable, a quarter of a wave long, is placed on the external polyethylene insulation of the cable, taking into account the shortening of the cable, i.e. The length of the stretched additional braid for the 144 MHz range is 344 mm, 432 MHz - 114 mm, 1296 MHz - 38 mm. The end of the braid of the active vibrator is isolated from everything, and its other end should be connected (soldered) to the main braid of the adapter cable. The resulting structure should be placed in a heat-shrinkable tube or carefully wrapped with electrical tape.
Antennas of two polarizations can be placed on one traverse by moving the elements of each antenna 50-70 mm from each other. The antennas are switched using a relay installed directly on the antenna.
If the antennas are for the bands 144, 432 and 1296 MHz. will be installed on one mast, and the height of the mast is no more than 6-8 m from the conductive surface, then the top should be a 144 MHz antenna, 1.5 m lower - a 432 MHz antenna, 1 m lower - 1296 MHz.
When checking and adjusting the input impedance, it is enough to install the antenna vertically on a table at a height of 1-1.5 m from the ground.
In conclusion, I recommend studying other sources on this topic before making antennas. They contain suitable tips and recommendations that you can use if they do not contradict the information provided in this article.
You can download the file of the described antennas for the MMANA program
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