When using portable УSW radio stations can be situations when broadcasting УSW transmitter causes interference to the reception in a range of 2 meters. The proposed antenna allows one to check for interference, as it has additional selective properties.
In the Amateur band 2 meters may experience strong interference mirror channel. This is because the value of the first inverter in many brand the stations lies in the range 21...23 MHz. If the local oscillator operates at lower frequency reception, the mirror receive channel accounted for broadcasting УSW range 88...108 MHz, with the powerful radio station. The disturbance is especially unpleasant if gets to the ringing frequency of this range (for FM at 145.5 MHz). For stationary radio stations, where, as a rule, are applied effective narrowband antennas, the interference is less noticeable. Portable radios, where antenna more broadband, the impact of such interference significantly more.
A similar situation arose in Kursk when using radiostantsiiy ALINKO DJ-191. She has a first value if - 21,7 MHz, so the frequency of the mirror channel admission to the frequency at 145.5 MHz is 145,52x21,7 = 102,1 MHz. This frequency in the city operates commercial radio station broadcasting with a considerable power. Although ALINKO DJ-191 and has a good three-loop tunable preselector, near a broadcasting station and he is not able to resolve the hindrance.
To improve reception conditions can be a modification of the antenna. In my article "Antenna wearable УSW radio" ("Radio", 2001, No. 12, pp. 61, 62) have described a simple antenna design for portable radio УSW range. In the design of these the antenna includes a matching inductor, with which they tune into resonance. However, due to the fact that the length of the antenna is small, their the resistance is less than 50 Ohms and therefore approval is obtained sub-optimal. In addition, they are relatively broadband and inhibit the signal the mirror channel is not very efficient.
To improve coordination and to increase the selectivity of the antenna, if you apply some more complex T-shaped matching network. Scheme the antenna device shown in Fig. 1. The composition of the matching device includes two rigged capacitor C1 and C2 and an inductor L1. It allows almost matched perfectly with the painting of the antenna (radiating element) length from several centimeters to several tens of centimeters and, in also performs the functions of high-pass filter, further suppressing the signals of the mirror channel.
For the manufacture of such antennas need RF cable brand RK-50-11, RC-50-13 or similar with internal insulation diameter 4,5...4, 6 mm and external with a diameter of about 7.5 mm. as the radiating element can be used either the center conductor, then the outer insulation and braid removed, or braid, then remove only the center conductor.
The antenna design shown in Fig. 2. For the first variant (Fig. 2A) cut cable without braid and outside insulation 1 raspalaut in the plug connector 2 (SR-50-PW). Then at a distance of about 10 mm from the plug half of the insulation on the land length of 10 mm and a depth of the center conductor 3 is removed. The center conductor is cut and bend it to form conclusions length of 1...2 mm. In the resulting groove set trimmer capacitors 4 (CT-25 capacity 4...20 pF) and soldered them to the terminals of the center conductor. Then they solder the end of the coil 5 and wound it round as possible closer to the capacitors wire sew-2 0,41 (11 turns). The second end of the coil solder to the plug. Before Assembly, it is advisable to fork to solder a pigtail of the same diameter, thus making a conclusion, and by him to solder the coil.
The soldering points, installation of capacitors, and the coil should be carefully covered with a thin a layer of epoxy adhesive, giving the design stiffness. You then need to prepare plastic protective casing 6 and after curing of the adhesive to carry out the adjustment on the minimum VSWR, maximum antenna current, or field strength. Here already, at least two rigged element (C1 and C2), the setting must spending more carefully. When you use an SWR meter or panoramic the SWR meter setting should be conducted in conditions close to reality. For this antenna must be placed on a metal case, similar size radio station. To rotate the rotors of trimmer capacitors have dielectric screwdriver, holding the case in hand.
The wire connecting the meter with the antenna, it is necessary to put a ferrite rings with a diameter of 30 mm, passing through them two or three turns of the cable, so the cable must be of small diameter. Since the setting turns out to be "acute", can affect the protective cover that should be considered when setting up. If you configure The CWS does not work less than 1.1, it is necessary to change the number of turns of the coil. After setting up the casing is glued with epoxy glue.
For the second embodiment (Fig. 2, b) as the radiating element used braid. In this case, the outer insulation 7 and 8 braid removed only place where the coil and trimmer capacitors. The center conductor can be removed. Right circuit output capacitor C2 soldered to the braid.
For a station with a small SMA connector suitable first option design. You can use SMA plug for cable mounting method wire and cable cut to install a fork in this method the mould.
In the author's version were made and tuned antenna with a total length of 20...22 see POI setting was obtained VSWR is less than 1.05 x (at a frequency of at 145.5 MHz), but it the size was strongly correlated (increased to 1.5) from the provisions of the radio station with respect to the operator. Therefore, the setting should be in this position the radio station that is most often used while links.
The use of this antenna immediately gave a noticeable gain in transmission and reception. Except also, the tests were performed with the use of selective microvoltmeter SMW8. We measured the signal level of interfering radio frequency 102,1 MHz). On compared to the antennas described in the above article, offer the option antennas provide additional suppression of interfering signal 18...20 dB.
Author: I. Nechaev (UA3WIA), Kursk