144 MHz portable radio antenna. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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144 MHz portable radio antenna. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / VHF antennas

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When using portable VHF radios, situations may arise when a broadcast VHF transmitter interferes with reception on the 2 meter band. The proposed antenna allows you to detune from interference, because it has additional selective properties.

In the amateur band of 2 meters, strong interference can occur on the image channel. This is due to the fact that the value of the first IF in many branded radio stations lies within 21 ... 23 MHz. If the local oscillator operates below the reception frequency, then the mirror reception channel falls on the VHF broadcasting range of 88 ... 108 MHz, where powerful radio stations operate. The interference is especially unpleasant if it falls on the calling frequency of this range (for FM - 145,5 MHz). For fixed radio stations, where effective narrow-band antennas are usually used, this interference is less noticeable. For portable radios, where the antennas are wider, the impact of such interference is more significant.

A similar situation arose in the city of Kursk when using the ALINKO DJ-191 radio station. It has a first IF value of 21,7 MHz, so the frequency of the mirror receive channel for a frequency of 145,5 MHz is 145,52x21,7 = 102,1 MHz. It is at this frequency that a commercial broadcasting radio station with considerable power operates within the city. Although ALINKO DJ-191 has a good three-circuit tunable preselector, it is not able to eliminate interference near a broadcasting radio station.

You can improve reception conditions by modifying the antenna. In my article "Antennas for portable VHF radio stations" ("Radio", 2001, No. 12 pp. 61, 62), simple antenna designs for portable VHF radio stations were described. The design of these antennas includes a matching inductor, with which they are tuned into resonance. However, due to the fact that the length of the antennas is small, their resistance is less than 50 ohms and therefore the matching is not optimal. In addition, they are relatively broadband and do not suppress the image channel signal very effectively.

It is possible to improve the matching and at the same time increase the selectivity of the antenna if a slightly more complex T-shaped matching device is used. The antenna circuit with this device is shown in fig. 1. The matching device includes two tuned capacitors C1 and C2 and an inductor L1. It allows you to almost perfectly match the antenna sheet (radiating element) with a length of several centimeters to several tens of centimeters and, in addition, performs the functions of a high-pass filter, additionally suppressing the signals of the image channel.

144 MHz handheld radio antenna

For the manufacture of such an antenna, you will need an RF cable of the brand RK-50-11, RK-50-13 or similar with internal insulation with a diameter of 4,5 ... 4 mm and an outer diameter of about 6 mm. As a radiating element, you can use either the central conductor, then the outer insulation and braid are removed, or the braid, then only the central conductor is removed.

The design of the antenna is shown in fig. 2. For the first option (Fig. 2, a), a piece of cable without braid and external insulation 1 is soldered in the connector plug 2 (SR-50-74PV). Then, at a distance of about 10 mm from the plug, half of the insulation is removed in a section 10 mm long and deep to the center conductor 3. The central conductor is cut and bent so that leads are 1 ... 2 mm long. Trimmer capacitors 4 (KT4-25 with a capacity of 4 ... 20 pF) are installed in the resulting groove and soldered to the terminals of the central conductor. Then the end of coil 5 is soldered to them and its turn to turn is wound as close as possible to the capacitors with a PEV-2 0,41 wire (11 turns). The second end of the coil is soldered to the plug body. Previously, even before assembly, it is advisable to solder a piece of wire of the same diameter to the plug, thus making a conclusion, and solder the coil to it.

Places of soldering, installation of capacitors, and the coil must be carefully covered with a thin layer of epoxy glue, giving rigidity to the structure. Then it is necessary to prepare a plastic protective casing 6 and, after the glue has hardened, adjust to the minimum SWR, maximum antenna current or field strength. Since there are already at least two tuned elements (C1 and C2), the tuning must be carried out more carefully. When using an SWR meter or a panoramic SWR indicator, the adjustment must be carried out in conditions close to reality. To do this, the antenna must be placed on a metal case, similar in size to the radio station. You need to rotate the rotors of the trimmer capacitors with a dielectric screwdriver, holding the case in your hand.

Ferrite rings with a diameter of 30 mm must be put on the wire connecting the measuring device to the antenna, passing two or three turns of the cable through them, so the cable must be of small diameter. Since the setting is "sharp", it can also be affected by the protective cover, which must be taken into account when setting. If, when adjusting the SWR, it does not turn out to be less than 1,1, the number of turns of the coil must be changed. After adjustment, the casing is glued with epoxy glue.

For the second design option (Fig. 2, b), a shielding braid is used as a radiating element. In this case, the outer insulation 7 and braid 8 are removed only in the place where the coil and trimmer capacitors are placed. The center conductor can be removed. The output of the capacitor C2, right according to the scheme, is soldered to the braid.

For a radio with a small size SMA connector, the first design option is suitable. In this case, you can use the SMA plug for cable installation by crimping, and install the cable segment in this plug by fusing.

In the author's version, antennas with a total length of 20 ... 22 cm were manufactured and tuned. By tuning, an SWR of less than 1,05 (at a frequency of 145,5 MHz) was obtained, but its value strongly depended (increased to 1,5) on the position radio stations relative to the operator. Therefore, tuning should be carried out in such a position of the radio station, which is most often used when making communications.

The use of this antenna immediately gave a noticeable gain in transmission and reception. In addition, tests were carried out using a selective microvoltmeter SMW8. The signal level of the interfering radio station was measured (frequency 102,1 MHz). Compared to the antennas described in the above article, the proposed version of the antenna provided additional suppression of the interfering signal by 18...20 dB.

Author: I. Nechaev (UA3WIA), Kursk

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