Side mount antennas for trunk systems. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / VHF antennas
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Continuing the topic of building antenna systems for basic trunk communication repeaters, begun in the 11th and 12th issues of the Radio magazine for 1997, I would like to dwell in more detail on such an important aspect as beamforming (DN).
The use of vertical collinear antennas makes it easy to obtain a circular pattern. Such antennas are widely used in "SmarTrunk-H" systems. However, despite their simplicity, these antennas have a number of disadvantages inherent in cheap designs. These include low mechanical strength and low resistance to aggressive environments. In addition, trunk system designers often face the problem of installing base antennas on truss towers. Usually, all advantageous positions are already occupied by the antennas of departmental radio stations, and it is necessary to mount the antennas along the mast "one floor below" (Fig. 1).
It is contraindicated to position whip antennas in this way, as this will lead to crushing of the main lobe of the radiation pattern and disrupt the coordination with the feeder. For these cases, there are special side-mount antennas (offset). Usually these are several half-wave dipoles combined into an antenna array. The matching devices of such vibrators are pre-designed for the presence of a metal mast nearby. To improve the match, you can change the distance between the element and the mast.
By placing the active elements in one line, you can create a collinear antenna with a gain of 6 to 12 dBd (using two to eight elements). Strengthening will be achieved by narrowing the DN in the vertical plane. However, when installing the antenna on a massive metal truss, the RP will have noticeable dips in the horizontal plane (Fig. 2, a).
This is quite acceptable in cases where the failure falls geographically on the service area of another repeater or the system is designed only for priority areas (for example, in the MPT-1327 or LTR protocol system). This is not allowed for a SmarTrunk-II Radial Repeater. The problem can be solved by placing several sheets of vertical gratings along the perimeter of the truss, thereby providing an almost circular DN (Fig. 2, c). Small dips, of course, will remain due to the interference effect of neighboring elements, but they will be insignificant (about 1,5 ... 2 dB) Dips can be reduced by placing the elements further apart - by (2 ... 3)X. In the end, you can put up with this - after all, you have to put up with a signal attenuation of 6 ... 10 dB due to urban development!
Directional antennas can also be used as active elements, for example, a three-element Yagi - "wave channel". The width of the main lobe of the radiation pattern of such an antenna is about 90°, which makes it possible to create an antenna array with an almost circular pattern of only four Yagi sheets (Fig. 3). The gain of such an antenna system will reach 5...6 dBd.
In all the cases considered, to combine and phase the signal, adders from a coaxial cable based on quarter-wave lines can be used. All active elements will be powered in parallel. This method allows you to save a wide operating band of the antenna due to the minimum phase shift, which will be the same in all arms when detuned relative to the center frequency (Fig. 4).
See other articles Section VHF antennas.
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