A. Kalashnik
Radio hobby 1/2001
VHF antennas
In recent years, the interest of radio amateurs in the 2-meter range has been constantly growing due to the increase in the number of FM repeaters and, accordingly, the improvement of conditions for the development mobile communications, a network of various BBS and portals, incl. with Internet access, as well as satellite repeaters. The increase in activity is also facilitated by the permission from March 1, 1998 to work on VHF for novice radio amateurs.
When working on the 2-meter range, antennas are used both with vertical polarization (mainly for mobile communications and when working through repeaters) and with horizontal polarization. In this case, it is desirable to have an antenna with a circular radiation pattern in both the horizontal and vertical planes. The latter is very important when working through satellite (AES) repeaters. For these purposes, as a rule, several antennas are used, which reduces efficiency in operation, provided that long-range unstable passage on a 2-meter range is required.
The author managed to solve this problem by implementing an antenna with an almost spherical radiation pattern. In this case, the antenna can emit and, accordingly, receive electromagnetic waves with both vertical and horizontal polarization.
The design is based on the popular J-antenna (Fig. 1). It is a vertical dipole fed from the lower end using a short-circuited quarter-wave line. As you know, this antenna works only with vertical polarization and has a circular pattern in the horizontal plane with a deep minimum in the vertical direction.
The author proposed to change the shape of the vertical radiator of this antenna, bending the dipole in half at 90 °. In this case, the horizontal part of the dipole in the first version consisted of two opposite elements of length L / l each (Fig. 2) and was first described in the collection "Infoix" No. 4/1990, p. 42,43.
In the last modification, the author proposed to make the horizontal part of the radiator from 4 mutually perpendicular segments of length l / 4, having electrical contact with the vertical part of the radiator (Fig. 3). The antenna design is easy to manufacture and easily repeatable even for novice radio amateurs. The vertical parts of the antenna are made of a pipe with a diameter of 32 mm. Material - bronze, brass, copper, as well as aluminum alloys, provided that reliable electrical contact of all parts of the antenna and the matching device is ensured (soldering or welding). The horizontal cruciform part is made of a rod or tube with a diameter of 6 mm (the material is similar to the used tube with a diameter of 32 mm).
This design retains the advantage of a J-antenna in that the lower end of the short-circuited quarter-wave line can be grounded, for example electrically connected to a grounded mast, in which case the entire antenna can serve as a good lightning conductor. The setting consists in selecting the place for connecting the power cable to the matching line (Fig. 4) at a minimum SWR. The author used the RC-75. however, a 50 ohm feeder can also be used. With the dimensions indicated in Fig. 3, 4 and a 75-ohm feeder, VSWR = 1.0 near 145.5 MHz.
The antenna is mounted on a metal earthed mast, 7 m above the ground, but a mast of any material and design can be used. Foreign conductive objects should be removed from horizontal elements by more than 2 meters. With a corresponding change in geometric dimensions, such an antenna can be built for other VHF bands.
This antenna has been with the author since 1983. It showed good results for all types of transmission, as well as for communications through amateur satellites in their visibility zone and without a signal failure "overhead". During "Field Day 2000" an experiment was carried out on the basis of UT0H, during which the signals of my beacon, on which the described antenna was used, were received by antennas with both vertical and horizontal polarization with approximately the same loudness.
From the editor. Figures 1 and 4 show two options for connecting the cable to the matching line. In the first case (Fig. 1), the central core is soldered to the line conductor connected to the emitter, and in the author's version (Fig. 4) - vice versa. Both options are equivalent, although in the publication "the cable connection method shown in Fig.1 is more common.
Literature
- Benkovsky 3., Lipinsky E. Amateur antennas of short and ultra-short waves: Per. from Polish / Ed. O. P. Frolova. - M .: Radio and communication, 1983.. 480 p., Ill. - (Mass radio library; Issue 1052)
The classic version. Both antenna sections are installed vertically. Used mainly at HF.
Such a vertical antenna is well known to VHF lovers as a J antenna. The figure shows general scheme antennas are a classic option. It consists of two sections: emitting L / 2 and matching L / 4. The high impedance of a half-wave emitter can be matched (reduced) to a low impedance using a quarter-wave end-of-line. This method has been known for a long time and is widely used in practice.
On a quarter-wave line, you can find two XX points with an impedance of 500m (or 750m) to connect the corresponding coaxial cable (feeder). When connecting the cable, it is advisable to provide a balancing choke or transformer. The quarter-wave line itself is easiest to make from 450-ohm ribbon cable.
DK7ZB has manufactured and tested several similar antennas for the amateur bands 2, 6, 12 and 30 meters. I brought the data for the remaining ranges into a table, which is very convenient to use when experimenting. Below he gives mathematical calculations for independent calculations, based on the specific calculation conditions:
Emitter: L / 2 = 0.471 X (m) - copper insulated wire 2mm;
Quarter wave line: L / 4 = 0.223X (M) - 450-ohm ribbon cable (Wireman);
Points XX are located approximately 5 ... 10% from the closed end of the quarter-wave line.
When installing the antenna, in addition to the main vertical position, the following options may be considered:
Estimated dimensions:
Diap. L / 2 L / 4 XX MHz SWR Bandwidth (kHz) (m) (m) (m) (cm) (SWR
Calculated values marked with (*) require rechecking.
The quarter-wavelength (X / 4) line length is given for WireMan 450-ohm RF ribbon cable.
The values given in the table are valid for free installation of the antenna in space. If the radiating wire is fixed on any supporting insulated elements, then its length should be reduced by approximately 2 ... 3%, because the operating frequency in this case decreases
A short time ago, mostly home-made equipment was used to work on the 144-145 MHz range. VHF transverters were popular among radio amateurs, many of which were comparable in size to the transceiver used with it. Radio amateurs converted decommissioned industrial VHF-type "Palma" radio stations to the amateur VHF band of 145 MHz, receiving a radio station operating on several channels. Then "Viola" became available to radio amateurs, and later "Mayaki" operating on forty channels. These radio stations then looked fantastic in their capabilities!
At present, it is relatively inexpensive to purchase multichannel portable VHF transceivers of world famous companies - "YAESU", "KENWOOD", "ALINCO", which in their parameters and ease of operation significantly surpass both homemade equipment in the 145 MHz range, and converted industrial equipment - "Palms "," Lighthouses "," Viola ".
But to work through a repeater from home, office, while driving when working from a car, an antenna is needed that is more effective than the "rubber band" used in conjunction with a portable radio station. When using a stationary "branded" VHF station, it is often advisable to use a homemade VHF antenna with it, since a decent "branded" outdoor antenna of the 145 MHz range is not cheap.
This material is devoted to the manufacture of simple homemade antennas suitable for use with stationary and portable VHF radio stations.
Features of 145 MHz antennas
Due to the fact that for the manufacture of antennas in the 145 MHz range, a thick wire is usually used - with a diameter of 1 to 10 mm (sometimes thicker vibrators are used, especially in commercial antennas), then antennas in the 145 MHz range are broadband. This often makes it possible, when making the antenna exactly according to the specified dimensions, to do without its additional tuning to the 145 MHz range.
To tune antennas in the 145 MHz range, you must have a SWR meter. It could be like homemade device and industrial production. On the 145 MHz band, radio amateurs practically do not use bridge antenna resistance meters, due to the apparent complexity of their correct manufacture. Although with careful manufacture of the bridge meter and, therefore, its correct operation in this range, it is possible to accurately determine the input impedance of the VHF antennas. But even using only the SWR - a through-type meter, it is quite possible to tune homemade VHF antennas. The power of 0.5 W, which is provided by imported portable radio stations in the "LOW" mode and domestic portable VHF radio stations such as "Dnepr", "Viola", "VEBR", is quite enough for the operation of many types of SWR meters. The "LOW" mode allows tuning antennas without fear of failure of the output stage of the radio station at any antenna input impedance.
Before starting tuning the VHF antenna, it is advisable to make sure that the SWR meter readings are correct. It is a good idea to have two SWR meters rated for 50 and 75 ohm transmission paths. When tuning VHF antennas, it is desirable to have a control antenna, which can be either a "rubber band" from a portable radio station or a homemade quarter-wave pin. When tuning the antenna, the level of the field strength created by the tuned antenna is measured relative to the reference one. This makes it possible to judge the comparative efficiency of the tunable antenna. Of course, if a standard calibrated field strength meter is used in the measurements, an accurate estimate of the antenna performance can be obtained. When using a calibrated field meter, it is easy to remove the antenna directional pattern. But even using home-made field strength meters for measurements and having obtained only a qualitative picture of the distribution of the electromagnetic field strength, it is possible to draw a conclusion about the efficiency of the tuned antenna and approximately estimate its directional pattern. Consider the practical designs of VHF antennas.
Simple antennas
The simplest outdoor VHF antenna (Fig. 1) can be made using an antenna that works in conjunction with a portable radio station. On the window frame from the outside (Fig. 2) or from the inside, a metal corner is attached to an extension wooden bar, in the center of which there is a socket for connecting this antenna. It is necessary to strive to ensure that the coaxial cable leading to the antenna was the minimum required length. Along the edges of the corner, 4 counterweights 50 cm long are attached. It is necessary to ensure good electrical contact of the counterweights, the antenna connector with the metal corner. The shortened twisted antenna of the radio station has an input impedance in the range of 30-40 ohms, so that a coaxial cable with a characteristic impedance of 50 ohms can be used to power it. With the help of the angle of inclination of the counterweights, it is possible to change the input impedance of the antenna within certain limits, and, therefore, to match the antenna with the coaxial cable. Instead of a proprietary "rubber band", you can temporarily use an antenna made of a copper wire with a diameter of 1-2 mm and a length of 48 cm, which is inserted into the antenna socket with its sharply sharpened end.
Figure 1. Simple outdoor VHF antenna
Figure 2. Construction of a simple outdoor VHF antenna
The VHF antenna, made of a coaxial cable with the outer sheath removed, works reliably. The cable is terminated in the HF-connector similar to the connector of the "proprietary" antenna (Fig. 3). The length of the coaxial cable used for the manufacture of the antenna is 48 cm. Such an antenna can be used in conjunction with a portable radio station instead of a broken or lost standard antenna.
Figure 3. Simple homemade VHF antenna
For the quick manufacture of an external VHF antenna, you can use a connecting coaxial cable 2-3 meters long, which is terminated with connectors corresponding to the antenna jack of the radio station and antenna. The antenna can be connected to such a piece of cable using a high-frequency tee (Fig. 4). In this case, a “rubber band” antenna is connected from one end of the tee, and 50 cm counterweights are screwed on from the other end of the tee, or another type of radio technical “ground” for the VHF antenna is connected through the connector.
Figure 4. Simple remote VHF antenna
Homemade antennas portable radio station
If you lose or break the standard antenna of a portable radio station, you can make a homemade twisted VHF antenna. For this, a base is used - polyethylene insulation of a coaxial cable with a diameter of 7-12 mm and a length of 10-15 cm, on which initially 50 cm of a copper wire with a diameter of 1-1.5 mm is wound. It is very convenient to use a frequency response meter to tune a twisted antenna, but you can also use an ordinary SWR meter. Initially, the resonant frequency of the assembled antenna is determined, then, by biting off part of the turns, shifting, moving apart the turns of the antenna, the twisted antenna is tuned to resonance at 145 MHz.
This procedure is not very complicated, and by tuning 2-3 twisted antennas, the radio amateur can tune new twisted antennas in literally 5-10 minutes, of course, if the above devices are available. After tuning the antenna, it is necessary to fix the turns either with electrical tape, or with a cambric soaked in acetone, or with a heat-shrinkable tube. After fixing the turns, it is necessary to check the frequency of the antenna again and, if necessary, adjust it using the upper turns.
It should be noted that in "branded" shortened twisted antennas, heat-shrinkable tubes are used to fix the antenna conductor.
Half-wave field antenna
For quarter-wave antennas to work effectively, multiple quarter-wave counterweights must be used. This complicates the design for a field quarter-wave antenna, which must be spaced out in relation to the VHF transceiver. In this case, you can use a VHF antenna with an electric length of L / 2, which does not require counterweights for its operation, and provides a directional pattern pressed to the ground and ease of installation. For an antenna with an electrical length of L / 2, there is a problem of matching its high input impedance with the low characteristic impedance of the coaxial cable. An antenna with a length L / 2 and a diameter of 1 mm will have an input impedance on the 145 MHz band of about 1000 ohms. Matching using a quarter-wave resonator, which is optimal in this case, is not always convenient in practice, since it requires the selection of points for connecting the coaxial cable to the resonator for its effective operation and precise tuning of the antenna pin to resonance. The dimensions of the resonator for the 145 MHz range are also relatively large. The destabilizing factors on the antenna when it is matched using a resonator will be especially pronounced.
However, at low powers supplied to the antenna, quite satisfactory matching can be achieved using a P - loop, similarly to how it is described in the literature. A diagram of a half-wave antenna and its matching device is shown in Fig. 5. The length of the antenna rod is chosen slightly shorter or longer than the length L / 2. This is necessary so that even with a small difference in the electrical length of the antenna from L / 2, the active resistance of the antenna impedance decreases noticeably, and its reactive part at the initial stage increases insignificantly. As a result, matching with the help of the P-loop of such a shortened antenna is possible with greater efficiency than matching an antenna with a length of exactly L / 2. It is preferable to use an antenna with a length slightly longer than L / 2.
Figure 5. Coordination of the VHF antenna using the P - loop
Air trimming capacitors of the KPVM-1 type were used in the matching device. Coil L1 contains 5 turns of silver-plated wire with a diameter of 1 mm, wound on a mandrel with a diameter of 6 mm and a pitch of 2 mm.
Antenna tuning is not difficult. By including the SWR meter into the cable path of the antenna and simultaneously measuring the level of the field strength created by the antenna, by changing the capacitance of variable capacitors C1 and C2, compressing-stretching the turns of the L1 coil, they achieve the minimum readings of the SWR meter and, accordingly, the maximum readings of the field strength meter. If these two maxima do not coincide, it is necessary to slightly change the length of the antenna, and repeat its adjustment again.
The matching device was placed in a case soldered from foil-clad fiberglass with dimensions of 50 * 30 * 20 mm. When working from a stationary workstation of a radio amateur, the antenna can be placed in the window opening. When working in the field, the antenna can be suspended from the upper end from a tree using a fishing line, as shown in fig. 6. A 50-ohm coaxial cable can be used to power the antenna. The use of a 75-ohm coaxial cable will slightly increase the efficiency of the antenna matching device, but at the same time, it will require tuning the output stage of the radio station to operate on a 75-ohm load.
Figure 6. Installing Antenna for Field Operation
Foil Window Antennas
On the basis of the adhesive foil used in security alarm systems, very simple designs of VHF window antennas can be built. This foil can already be purchased with an adhesive base. Then, having freed one side of the foil from the protective layer, it is enough just to press it against the glass and the foil is instantly reliably glued. Foil without an adhesive base can be glued to glass using varnish or glue like "Moment". But for this you need to have some skill. The foil can even be secured to the window with adhesive tape.
With proper training, it is quite possible to make a good soldered connection of the center conductor and the braid of the coaxial cable with aluminum foil. Based on personal experience, each type of such foil requires its own flux for soldering. Some types of foil solder well even using rosin alone, some can be soldered with soldering grease, other types of foil require the use of active fluxes. The flux should be tested on the specific type of foil used to make the antenna well in advance of installation.
Good results are obtained by using a foil-clad fiberglass substrate for soldering and fixing the foil, as shown in Fig. 7. A piece of foil-clad fiberglass is glued to the glass with the help of Moment glue, the antenna foil is soldered to the edges of the foil, the cores of the coaxial cable are soldered to the copper foil of the fiberglass at a short distance from the foil. After soldering, the connection must be protected with a moisture-resistant varnish or glue. Otherwise, this connection may corrode.
Figure 7. Connecting Antenna Foil to Coaxial Cable
Let us examine the practical designs of foil-based window antennas.
Vertical window dipole antenna
A diagram of a vertical dipole window VHF antenna based on a foil is shown in Fig. eight.
Figure 8. Windowed vertical dipole VHF antenna
The quarter-wave post and counterweight are positioned at 135 degrees to bring the antenna system's input impedance closer to 50 ohms. This makes it possible to use a coaxial cable with a characteristic impedance of 50 Ohm to power the antenna and use the antenna in conjunction with portable radio stations, the output stage of which has such an input impedance. The coaxial cable should run perpendicular to the antenna over the glass for as long as possible.
Foil frame antenna
A frame window VHF antenna, shown in Fig. 2, will work more efficiently than a dipole vertical antenna. 9. When the antenna is fed from the lateral angle, the maximum of the radiated polarization is in the vertical plane, when the antenna is fed in the lower angle, the maximum of the radiated polarization is in the horizontal plane. But at any position of the power points, the antenna emits a radio wave, with combined polarization, both with vertical and horizontal. This circumstance is very favorable for communication with portable and mobile radio stations, the position of the antennas of which will change during movement.
Figure 9. Framework window VHF antenna
The input impedance of the window loop antenna is 110 ohms. To match this impedance to a coaxial cable with a characteristic impedance of 50 ohms, a quarter-wave section of coaxial cable with a characteristic impedance of 75 ohms is used. The cable should run perpendicular to the antenna axis for as long as possible. The loop antenna has a gain of about 2 dB higher than a dipole window antenna.
When making foil window antennas with a width of 6-20 mm, they do not require tuning and operate in a frequency range that is much wider than the amateur band of 145 MHz. If the obtained resonant frequency of the antennas is lower than the required one, then the dipole can be tuned by symmetrically cutting off the foil from its ends. The loop antenna can be tuned using a jumper made from the same foil used to make the antenna. The foil closes the antenna web in the corner, opposite the power points. Once configured, contact between the jumper and the antenna can be made either by soldering or using adhesive tape. Such adhesive tape should press the jumper firmly against the antenna web in order to ensure reliable electrical contact with it.
Antennas made of foil can be supplied with significant power levels - up to 100 watts or more.
Outdoor vertical antenna
When placing the antenna outside the room, the question always arises about protecting the opening of the coaxial cable from atmospheric influences, about using a high-quality antenna support insulator, moisture-resistant wire for antennas, etc. These problems can be solved by installing a protected outdoor VHF antenna. The design of such an antenna is shown in Fig. 10.
Figure 10. Protected outdoor VHF antenna
A hole is made in the center of a 1 meter long plastic water pipe, into which the coaxial cable can fit tightly. Then the cable is threaded there, protrudes out of the pipe, is exposed at a distance of 48 cm, the cable screen is twisted and soldered at a length of 48 cm. The cable with the antenna is put back into the pipe. Standard plugs are put on the top and bottom of the pipe. It is not difficult to waterproof the hole where the coaxial cable enters. This can be done with automotive silicone or a fast curing automotive epoxy. As a result, we get a beautiful, moisture-insulated protected antenna that can work for many years under the influence of atmospheric influences.
To fix the vibrator and the antenna counterweight inside, you can use 1-2 cardboard or plastic washers tightly put on the antenna vibrators. The antenna tube can be installed on a window frame, on a non-metallic mast, or in any other convenient location.
Simple coaxial collinear antenna
A simple collinear coaxial VHF antenna can be made of coaxial cable. A piece of water pipe can be used to protect this antenna from the weather as described in the previous paragraph. The design of a collinear coaxial VHF antenna is shown in Fig. eleven.
Figure 11. Simple collinear VHF antenna
The antenna provides a theoretical gain of at least 3 dB more than a quarter-wave vertical. It does not need counterweights for its operation (although their presence improves the performance of the antenna) and provides a flattened radiation pattern to the horizon. A description of such an antenna has repeatedly appeared on the pages of domestic and foreign radio amateur literature, but the most successful description was presented in the literature.
Antenna dimensions in fig. 11 are in centimeters for coaxial cable with a shortening factor of 0.66. Most coaxial cables with polyethylene insulation have such a factor of shortening. The dimensions of the matching loop are shown in Fig. 12. Without this loop, the VSWR of the antenna system can exceed 1.7. If the antenna turned out to be tuned below the 145 MHz range, it is necessary to slightly shorten the upper section, if higher, then lengthen it. Of course, the optimal tuning is possible by proportional shortening-lengthening of all parts of the antenna, but this is difficult to do in an amateur radio environment.
Figure 12. Dimensions of the matching loop
Despite the large size of the plastic pipe required to protect this antenna from weathering, the use of a collinear antenna of this design is quite reasonable. The antenna can be moved away from the building using wooden battens, as shown in fig. 13. The antenna can withstand significant power input to it up to 100 watts or more and can be used in conjunction with both stationary and portable VHF radio stations. The use of such an antenna in conjunction with low-power portable radio stations will give the greatest effect.
Figure 13. Installing a collinear antenna
Simple collinear antenna
This antenna was assembled by me similarly to the construction of a car remote antenna used in a cellular radiotelephone. To convert it to the amateur band of 145 MHz, I proportionally changed all dimensions of the "telephone" antenna. As a result of this, an antenna was obtained, the diagram of which is shown in Fig. 14. The antenna provides a horizontal radiation pattern and a theoretical gain of at least 2 dB over a simple quarter-wave rod. A coaxial cable with a characteristic impedance of 50 Ohm was used to power the antenna.
Figure 14. Simple collinear antenna
A practical antenna design is shown in Fig. 15. The antenna was made from a single piece of copper wire with a diameter of 1mm. Coil L1 contained 1 meter of this wire, wound on a mandrel with a diameter of 18 mm, the distance between the turns was 3 mm. When making a design exactly to the dimensions, the antenna practically does not require adjustment. It may be necessary to slightly tune the antenna by squeezing-stretching the turns of the coil to achieve a minimum SWR. The antenna was housed in a plastic water pipe. Inside the pipe, the antenna wire was fixed with pieces of foam. Four quarter-wave counterweights were installed at the lower end of the pipe. They were threaded and fastened to a plastic pipe with nuts. Counterweights can be 2-4 mm in diameter, depending on the ability to thread them. For their manufacture, you can use copper, brass, or bronze wire.
Figure 15. Construction of a simple collinear antenna
The antenna can be installed on wooden rails on the balcony (as shown in fig. 13). This antenna can withstand significant levels of power input.
This antenna can be thought of as a shortened HF antenna with a center extension coil. Indeed, the antenna resonance measured with a bridge resistance meter in the HF range turned out to be in the frequency region of 27.5 MHz. Obviously, by varying the diameter of the coil and its length, but at the same time maintaining the length of the winding wire, it is possible to ensure that the antenna works both in the VHF range of 145 MHz and in one of the HF ranges - 12 or 10 meters. To operate on HF bands, four L / 4 counterweights for the selected HF band must be connected to the antenna. This dual use of the antenna will make it even more versatile.
Experimental 5/8 wave antenna
When carrying out experiments with radio stations of the 145 MHz range, it is often necessary to connect the antenna under test to its output stage in order to check the operation of the radio station's receive path or to tune the output stage of the transmitter. For these purposes, I for a long time a simple 5/8 - wave VHF antenna is used, the description of which was given in the literature.
This antenna consists of a section of copper wire with a diameter of 3 mm, which is connected at one end to an extension coil and the other to a tuning section. At the end of the wire connected to the coil, a thread is cut, and at the other end, a tuning section made of copper wire with a diameter of 1 mm is soldered. The antenna is matched with a coaxial cable with a characteristic impedance of 50 or 75 Ohm by connecting to different turns of the coil, and may be a slight shortening of the tuning section. The antenna diagram is shown in Fig. 16. antenna design is shown in fig. 17.
Figure 16. Scheme of a simple 5/8 - wave VHF antenna
Figure 17. Construction of a simple 5/8 - wave VHF antenna
The coil is made on a plexiglass cylinder with a diameter of 19 mm and a length of 95 mm. At the ends of the cylinder, a thread is made into which the antenna vibrator is screwed on one side, and on the other side it is screwed to a piece of foil-clad fiberglass with dimensions of 20 * 30 cm, which serves as the “ground” of the antenna. On the back side, a magnet from an old speaker was glued to it, as a result of which the antenna can be attached to the windowsill, to the radiator, to other iron objects.
The coil contains 10.5 turns of wire with a diameter of 1 mm. The coil wire is evenly spaced over the frame. The coaxial cable is tapped from the fourth turn from the grounded end. The antenna vibrator is screwed into the coil, a contact lamella is inserted under it, to which the "hot" end of the extension coil is soldered. The lower end of the coil is soldered to the antenna ground foil. The antenna provides a SWR in the cable no worse than 1: 1.3. The antenna is tuned by shortening its upper part with pliers, which is initially slightly longer than necessary.
I have experimented with installing this antenna on a window pane. In this case, a vibrator with an original length of 125 centimeters made of aluminum foil was glued to the center of the window. The extension reel was used the same and was installed on the window frame. The counterweights were made of foil. The ends of the antenna and counterweights were curved slightly to fit on the window pane. Window 5/8 - wave VHF antenna is shown in Fig. 18. The antenna is easily tuned into resonance by gradually shortening the vibrator foil using a blade, and by gradually switching the coil turns to minimize SWR. The window antenna does not spoil the interior of the room and can be used as a permanent antenna for operating on the 145 MHz band from home or office.
Figure 18. Window 5/8 - wave VHF antenna
Efficient Portable Radio Antenna
In the event that communication using a standard "rubber band" is not possible, a half-wave antenna can be used. It does not require "ground" for its work and when working over long distances it gives a gain in comparison with a standard "rubber band" up to 10 dB. These are quite realistic numbers, given that the physical length of a half-wave antenna is almost 10 times longer than the "rubber band".
The half-wave antenna is supplied with voltage and has a high input impedance, which can reach 1000 ohms. Therefore, this antenna requires a matching device when used with a radio with a 50 ohm output. One of the variants of the P-loop matching device has already been described in this chapter. Therefore, for a change, for this antenna, we will consider the use of another matching device, made on a parallel circuit. In terms of their efficiency, these matching devices are approximately equal. A diagram of a half-wave VHF antenna together with a matching device on a parallel circuit is shown in Fig. nineteen.
Figure 19. Half-wave VHF antenna with a matching device
The coil of the loop contains 5 turns of copper silver-plated wire with a diameter of 0.8 mm, wound on a mandrel with a diameter of 7 mm along a length of 8 mm. Tuning the matching device consists in tuning the circuit L1C1 to resonance using a variable capacitor C1; using a variable capacitor C2, the connection of the circuit with the transmitter output is regulated. Initially, the capacitor is connected in the third turn of the coil from its grounded end. Variable capacitors C1 and C2 must be air dielectric.
It is advisable to use a telescopic antenna for the antenna vibrator. This will make it possible to carry the half-wave antenna in a compact folded state. It also makes it easier to tune the antenna together with a real transceiver. During the initial tuning of the antenna, its length is 100 cm. During the tuning process, this length can be slightly adjusted according to better work antennas. It is advisable to make appropriate marks on the antenna in order to subsequently install the antenna from its folded position to the resonant length immediately. The box where the matching device is located must be made of plastic, in order to reduce the capacity of the coil to "ground", it can be made of foil-coated fiberglass. This depends on the actual operating conditions of the antenna.
The antenna is tuned using a field strength indicator. With the help of the SWR meter, tuning the antenna is advisable only if it works not on the body of the radio station, but when using an extension coaxial cable together with it.
When the antenna is working twice on the radio station body and using an extension coaxial cable, two marks are made on the antenna pin, one corresponding to the maximum field strength level when the antenna is operating on the radio station body, and the other risk corresponds to the minimum SWR when using an extension coaxial cable in conjunction with the antenna. Usually these two marks are slightly different.
Vertical continuous gamma-matched antennas
Vertical antennas made of a whole vibrator are wind-resistant, easy to install, and take up little space. For their implementation, you can use copper tubes, aluminum power electrical wire with a diameter of 6-20 mm. These antennas can be easily matched with a coaxial cable with a characteristic impedance of both 50 and 75 ohms.
An inseparable half-wave VHF antenna, the design of which is shown in Fig. 20. Gamma matching is used to power it through the coaxial cable. The material from which the antenna vibrator is made and the gamma matching must be the same, for example, copper or aluminum. Due to mutual electrochemical corrosion of many pairs of materials, it is unacceptable to use different metals for antenna and gamma matching.
Figure 20. Continuous half-wave VHF antenna
If a bare copper tube is used to make the antenna, then it is advisable to adjust the antenna gamma matching using a closing jumper as shown in Fig. 21. In this case, the surface of the pin and the conductor of the gamma matching is carefully cleaned and using a clamp of bare wire as shown in fig. 21a achieve the minimum VSWR in the coaxial power cable of the antenna. Then, in this place, the gamma matching wire is slightly flattened, drilled and connected with a screw to the antenna sheet, as shown in Fig. 21b. Soldering is also possible.
Figure 21. Setting gamma - matching copper antenna
If an aluminum wire is used for the antenna from a power electrical cable in plastic insulation, then it is advisable to leave this insulation to prevent corrosion of the aluminum wire with acid rain, which is inevitable in urban conditions. In this case, the antenna gamma matching is adjusted using a variable capacitor, as shown in Fig. 22. This variable capacitor must be carefully protected from moisture. If it is not possible to achieve the SWR in the cable less than 1.5, then the length of the gamma matching must be reduced and the setting repeated again.
Figure 22. Adjusting the gamma - matching of the aluminum-copper antenna
With sufficient space and materials, a continuous VHF vertical wave antenna can be installed. The wave antenna works more efficiently than the half-wave antenna shown in Fig. 20. A wave antenna provides a more horizontal radiation pattern than a half-wave antenna. You can match the wave antenna using the methods shown in Fig. 21 and 22. The design of the wave antenna is shown in fig. 23.
Figure 23. Continuous vertical wave VHF antenna
When performing these antennas, it is desirable that the coaxial power cable is at least 2 meters perpendicular to the antenna. The use of a balun together with a continuous antenna will increase the efficiency of its operation. When using a balun, use symmetrical gamma matching. The balun connection is shown in fig. 24.
Figure 24. Connecting the balun to a continuous antenna
Any other known balun can also be used as the antenna balun. When placing the antenna near conductive objects, it may be necessary to slightly reduce the length of the antenna due to the influence of these objects on it.
Round VHF antenna
If the placement in space of the vertical antennas shown in Fig. 20 and fig. 23 in their traditional vertical position is difficult, then they can be placed by rolling the antenna web in a circle. The position of the half-wave antenna shown in Fig. 20 in the "round" version is shown in fig. 25, and the wave antenna shown in Fig. 23 in Fig. 26. In this position, the antenna provides combined vertical and horizontal polarization, which is favorable for communications with mobile and handheld radios. Although, theoretically, the level of vertical polarization will be higher with side feeding of circular VHF antennas, but in practice this difference is not very noticeable, and the side feeding of the antenna complicates its installation. The side power supply of the circular antenna is shown in Fig. 27.
Figure 25. Continuous round vertical half-wave VHF antenna
Figure 26. Continuous round vertical wave VHF antenna
Figure 27. Lateral power supply of circular VHF antennas
The round VHF antenna can be placed indoors, for example, between window frames, or outdoors, on a balcony or on a roof. When placing a circular antenna in the horizontal plane, we get a circular radiation pattern in the horizontal plane and the operation of an antenna with horizontal polarization. This may be necessary in some cases when conducting amateur radio communications.
Passive "amplifier" of the portable station
When testing portable radios or working with them, sometimes there is not enough power for reliable communication. I made a passive "amplifier" for portable VHF stations. A passive "amplifier" can add up to 2-3 dB to a radio station's signal on the air. This is often enough to reliably open the squelch of the correspondent station and ensure reliable operation. The design of the passive "amplifier" is shown in Fig. 28.
Figure 28. Passive "amplifier"
The passive "amplifier" is a large enough tinned coffee can (the bigger the better). A connector is inserted into the bottom of the can, similar to the antenna connector of a radio station, and a connector for connecting to the antenna jack is sealed into the lid of the can. 4 counterweights 48 cm long are soldered to the bank. When working with a radio station, this "amplifier" is switched on between the standard antenna and the radio station. Due to the more effective "ground" and there is an increase in the place of receiving the strength of the emitted signal. Other antennas can be used in conjunction with this "amplifier", for example, a L / 4 pin made of copper wire, simply inserted into the antenna socket.
Broadband survey antenna
Many imported portable radios provide reception not only in the amateur band of 145 MHz, but also in the survey ranges of 130-150 MHz or 140-160 MHz. In this case, for successful reception in survey bands, on which a twisted antenna tuned to 145 MHz does not work effectively, you can use a broadband VHF antenna. The antenna diagram is shown in Fig. 29 and dimensions for different ranges of operation are given in table. one.
Figure 29. Broadband VHF vibrator
| Range, MHz | 130-150 | 140-160 |
| Size A, cm | 26 | 24 |
| Size B, cm | 54 | 47 |
Table 1. Dimensions of broadband VHF antenna
To work with the antenna, you can use a coaxial cable with a characteristic impedance of 50 Ohm. The antenna can be made of foil and glued to the window. You can make the antenna fabric from an aluminum sheet, or by printing on a piece of foil-clad fiberglass of suitable dimensions. This antenna can transmit and receive in the specified frequency ranges with high efficiency.
Zigzag antenna
Some service VHF long-distance radio stations use antenna arrays consisting of zigzag antennas. Radio amateurs can also try to use elements of such an antenna system for their work. The view of an elementary zigzag antenna included in the design of a complex VHF antenna is shown in Fig. thirty.
Figure 30. Elementary zigzag antenna
The zigzag elementary antenna consists of a half-wave dipole antenna that supplies voltage to the half-wave vibrators. In real antennas, up to five such half-wave vibrators are used. Such an antenna has a narrow directional pattern pressed to the horizon. The type of polarization emitted by the antenna is combined - vertical and horizontal. It is advisable to use a balun for antenna operation.
In antennas used in service communication stations, a reflector made of a metal mesh is usually placed behind elementary zigzag antennas. The reflector provides one-way directivity of the antenna. Depending on the number of vibrators included in the antenna and the number of zigzag antennas included together, the required antenna gain can be obtained.
Radio amateurs practically do not use such antennas, although they are easy to perform for the amateur VHF bands of 145 and 430 MHz. For the manufacture of the antenna sheet, you can use an aluminum wire with a diameter of 4-12 mm from a power electrical cable. In the domestic literature, a description of such an antenna, for the web of which a rigid coaxial cable was used, was given in the literature.
Kharchenko antenna in the range of 145 MHz
Kharchenko's antenna is widely used in Russia for television reception and in service radio communications. But radio amateurs use it to work on the 145 MHz band. This antenna is one of the few that works very efficiently and requires little or no tuning. The diagram of Kharchenko's antenna is shown in Fig. 31.
Figure 31. Kharchenko's antenna
Both 50 and 75 ohm coaxial cables can be used for antenna operation. The antenna is broadband, operates in a frequency band of at least 10 MHz on a range of 145 MHz. To create a one-sided radiation pattern, a metal mesh is used behind the antenna, located at a distance of (0.17-0.22) L.
The Kharchenko antenna provides a width of the beam pattern in the vertical and horizontal planes close to 60 degrees. To further narrow the radiation pattern, passive elements in the form of vibrators with a length of 0.45L are used, located at a distance of 0.2L from the diagonal of the square of the frames. To create a narrow radiation pattern and increase the gain of the antenna system, several combined antennas are used.
Directional loop antennas in the range of 145 MHz
One of the most popular directional antennas for operation in the 145 MHz band are loop antennas. The most common two-element loop antennas on the 145 MHz band. In this case, an optimal cost / quality ratio is obtained. The diagram of a two-element loop antenna as well as the dimensions of the perimeter of the reflector and the active element are shown in Fig. 32.
Figure 32. VHF loop antenna
The antenna elements can be made not only in the form of a square, but also in the form of a circle, a delta. To increase the vertical component radiation, the antenna can be powered from the side. The input impedance of the dual element antenna is close to 60 ohms, and both 50 ohm and 75 ohm coaxial cables are suitable for operation. The gain of a two-element VHF loop antenna is at least 5 dB (above the dipole) and the ratio of radiation in the forward and reverse direction can reach 20 dB. When working with this antenna, it is useful to use a balun.
Circularly polarized loop antenna
An interesting design for a circularly polarized loop antenna has been proposed in the literature. Circularly polarized antennas are used for communication via satellites. The dual feed of the 90 degree phase shift loop antenna allows the synthesis of a circularly polarized radio wave. The loop antenna power circuit is shown in Fig. 33. When designing an antenna, it is necessary to take into account that the length L can be any reasonable, and the length L / 4 must correspond to the wavelength in the cable.
Figure 33. Circularly polarized loop antenna
To increase the gain, this antenna can be used in conjunction with a loop reflector and a director. The frame must be powered only through a balun. The simplest balun is shown in Fig. 34.
Figure 34. The simplest balun
Industrial antennas of the range of 145 MHz
Currently, you can find a large selection of branded antennas for the 145 MHz range on sale. If you have the money, of course, you can buy any of these antennas. It should be noted that it is advisable to purchase solid antennas already tuned to the 145 MHz range. The antenna must have a protective coating that protects it from corrosion by acid rain, which can fall in a modern city. Telescopic antennas are unreliable in urban environments and may fail over time.
When assembling antennas, it is necessary to strictly follow all instructions in the assembly instructions, and do not spare silicone grease for waterproofing connectors, telescopic joints and screw connections in matching devices.
Literature
- I. Grigorov (RK3ZK). Matching devices of the 144 MHz range // Radio amateur. KV and UKV-1997.-№ 12.-С.29.
- Barry Bootle. (W9YCW) Hairpin Match for the Collinear - Coaxial Arrau // QST.-1984.-October.-P.39.
- Doug DeMaw (W1FB) Build Your Own 5/8-Wave Antenna for 146 MHz // QST.-1979.-June.-P.15-16.
- S. Bunin. Antenna for communication through the satellite // Radio. - 1985. - No. 12. - P. twenty.
- D. S. Robertson, VK5RN The “Quadraquad” - Circular Polarization the Easy Way //QST.-April.-1984.-pages16-18.
To conduct local communications on VHF (including through repeaters), an antenna is needed with a circular radiation pattern and noticeable gain. In amateur radio practice, this problem is usually solved by using elongated vertical antennas, consisting of several radiators, which are fed through phasing two-wire lines. Many foreign companies produce very similar antenna models, and almost identical models are sometimes produced under different names. A typical antenna of this class (for example, the CUSHCRAFT model ARX-2B) has a gain of 7 dB and a VSWR at the resonant frequency no more than 1.2 (typical value). The bandwidth is about 3 MHz. In the horizontal plane, the antenna has a circular radiation pattern, in the vertical plane, the maximum radiation angle is 7 degrees. Usually, antennas have a certain margin for adjustments, so during installation, their operating frequency can be varied within wide limits (for example, for the above model - in the band from 135 to 160 MHz). Similar antennas can be made in an amateur environment.
The design of this type of antenna is shown in Fig. 1. It is made of thin-walled aluminum tubes and is installed through an insulator on a grounded metal mast (total antenna height is 4.3 m). Antenna dimensions are for the amateur band of 2 meters, with a center frequency of 145 MHz.
Element 1 is a tube 890 in length and 9 mm in diameter. A plug is installed in the upper part of element 1 to prevent moisture from entering the antenna. Element 3 is a tube 700 in length and 13 mm in diameter. Element 6 is a tube 530 in length and 13 mm in diameter. Element 7 is a pipe 380 long and 16 mm in diameter. Element 8 is a tube 1000 in length and 19 mm in diameter.
At the upper ends of the tubes 3, 7, 8, vertical cuts are made with a length of 30 mm, providing a tighter fit of the internal fixed elements. The fixation of the tubular elements is carried out using expanding clamps 2, a sketch of which is shown in Fig. 2. The design uses three clamps with internal diameters D = 13, 16 and 19 mm.
Elements 3 and 6 are electrically connected to each other through the phasing element 5. For this, an insulator is installed between elements 3 and 6, Fig. 3. The phasing element is a U-shaped bracket made of 6 mm diameter aluminum wire. At the ends of tubes 3 and 6, inserted into the insulator at a distance of 10 mm from the edge, holes with a diameter of 6 mm are drilled. With the help of M5 screws through the threaded holes in the insulator, elements 3, 5 and 6 are fastened together. The length of the phasing element 5 is set according to the dimensions shown in Fig. one.
The antenna is installed through an insulator 11 (Fig. 4) on a metal mast 17 with a diameter of 32 mm. At the upper end of the mast, a metal glass 16 with an internal diameter of 32 mm is fixed (welding or any other mechanical connection). An insulator 11 is placed in this glass. The depth of the glass 16 is chosen so that the insulator 11 protrudes from it by 30 mm.
Metal corners 13 are fixed to elements 8 and 16, as can be seen in Fig. 1, with screws 13. At the ends of the corners, remote from the antenna, one hole with a diameter of 127 mm is drilled from a copper wire with a diameter of 5 mm.
On the corner attached to part 16, closer to the antenna, a 50-ohm socket is installed so that its threaded or bayonet part faces downward towards the antenna base. A piece of copper wire 12 with a diameter of 5 and a length of 130 mm is soldered to the central terminal of the connector (Fig. 5). At one end, the wire is flattened and a hole is drilled in it equal to the diameter of the center terminal of the connector. The wire is bent so that it, without touching the antenna, lies with its opposite end on element 9. With the help of a metal bracket (detail 10, Fig. 6) and an M5 screw on the bracket, the end of wire 12 is fixed on element 9. В at the same time, this contact is movable and is used when tuning the antenna. Moving within some limits the bracket 10 around the circumference of the ring 9, choose such a position in which the antenna SWR is minimal.
Before its installation, a metal ring 18 is put on the antenna mast, made according to Fig. 7. Three aluminum counterweights 19, 521 in length and 6 mm in diameter, are screwed into this ring. At one end of the counterweights, a 20 mm long M6 thread is cut. Before installing the counterweights in place, locknuts are screwed onto the threads.
Angle 13 is attached to part 18 by means of a screw by analogy with part 16. Only the connector is installed here through the passage. A cable with connectors at the ends and a total length of 1272 mm is produced separately.
A ring 18 is installed on the length of the tensioned attached cable and, screwing in the counterweights to the stop, rigidly fix it on the antenna mast. After that, the locknuts are tightened.
The lengths of the tubes given in this article correspond to the version of the antenna that allows you to tune its operating frequency over a wide range. For an antenna with a range of 2 meters, the radiators can be non-composite, which will significantly simplify the antenna design.
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To conduct local communications on VHF (including through repeaters), an antenna is needed with a circular radiation pattern and noticeable gain. In amateur radio practice, this problem is usually solved by using elongated vertical antennas, consisting of several radiators, which are fed through phasing two-wire lines. Many foreign companies produce very similar antenna models, and almost identical models are sometimes produced under different names. A typical antenna of this class (for example, the CUSHCRAFT model ARX-2B) has a gain of 7 dB and a VSWR at the resonant frequency no more than 1.2 (typical value). The bandwidth is about 3 MHz. In the horizontal plane, the antenna has a circular radiation pattern, in the vertical plane, the maximum radiation angle is 7 degrees. Usually, antennas have a certain margin for adjustments, so during installation, their operating frequency can be varied within wide limits (for example, for the above model - in the band from 135 to 160 MHz). Similar antennas can be made in an amateur environment.
The design of this type of antenna is shown in Fig. 1. It is made of thin-walled aluminum tubes and is installed through an insulator on a grounded metal mast (total antenna height is 4.3 m). Antenna dimensions are for the amateur band of 2 meters, with a center frequency of 145 MHz.
Element 1 is a tube 890 in length and 9 mm in diameter. A plug is installed in the upper part of element 1 to prevent moisture from entering the antenna. Element 3 is a tube 700 in length and 13 mm in diameter. Element 6 is a tube 530 in length and 13 mm in diameter. Element 7 is a pipe 380 long and 16 mm in diameter. Element 8 is a tube 1000 in length and 19 mm in diameter.
At the upper ends of the tubes 3, 7, 8, vertical cuts are made with a length of 30 mm, providing a tighter fit of the internal fixed elements. The fixation of the tubular elements is carried out using expanding clamps 2, a sketch of which is shown in Fig. 2. The design uses three clamps with internal diameters D = 13, 16 and 19 mm.
Elements 3 and 6 are electrically connected to each other through the phasing element 5. For this, an insulator is installed between elements 3 and 6, Fig. 3. The phasing element is a U-shaped bracket made of 6 mm diameter aluminum wire. At the ends of tubes 3 and 6, inserted into the insulator at a distance of 10 mm from the edge, holes with a diameter of 6 mm are drilled. With the help of M5 screws through the threaded holes in the insulator, elements 3, 5 and 6 are fastened together. The length of the phasing element 5 is set according to the dimensions shown in Fig. one.
The antenna is installed through an insulator 11 (Fig. 4) on a metal mast 17 with a diameter of 32 mm. At the upper end of the mast, a metal glass 16 with an internal diameter of 32 mm is fixed (welding or any other mechanical connection). An insulator 11 is placed in this glass. The depth of the glass 16 is chosen so that the insulator 11 protrudes from it by 30 mm.
Metal corners 13 are fixed to elements 8 and 16, as can be seen in Fig. 1, with screws 13. At the ends of the corners, remote from the antenna, one hole with a diameter of 127 mm is drilled from a copper wire with a diameter of 5 mm.
On the corner attached to part 16, closer to the antenna, a 50-ohm socket is installed so that its threaded or bayonet part faces downward towards the antenna base. A piece of copper wire 12 with a diameter of 5 and a length of 130 mm is soldered to the central terminal of the connector (Fig. 5).
At one end, the wire is flattened and a hole is drilled in it equal to the diameter of the center terminal of the connector. The wire is bent so that it, without touching the antenna, lies with its opposite end on element 9. With the help of a metal bracket (detail 10, Fig. 6) and an M5 screw on the bracket, the end of wire 12 is fixed on element 9. В at the same time, this contact is movable and is used when tuning the antenna. Moving within some limits the bracket 10 around the circumference of the ring 9, choose such a position in which the antenna SWR is minimal.
Before its installation, a metal ring 18 is put on the antenna mast, made according to Fig. 7. Three aluminum counterweights 19, 521 in length and 6 mm in diameter, are screwed into this ring. At one end of the counterweights, a 20 mm long M6 thread is cut. Before installing the counterweights in place, locknuts are screwed onto the threads.