Modeling of TV antennas. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Television antennas
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Radio amateurs know how difficult it is to "tune" an already made television antenna in order to achieve the desired results. It requires a painstaking change in the geometry of the vibrators, a careful selection of distances between the elements of a multivibrator design, and rather laborious measurements (by the way, many simply ignore them). Meanwhile, the MMANA program allows you to do this in a matter of minutes. It was decided to test the effectiveness of the program on a variant of an already developed amateur radio design.
For this purpose, as well as to confirm the provisions made by N. Turkin in his note "UHF Antenna" ("Radio", 2000, No. 11, p. 9), the described television antenna was modeled using the MMANA program.
First of all, it was not difficult to establish that the usual six-element antennas are "wave channel" ("Uda Yagi"), several descriptions of which; already available in the program library provide a gain (gain) G up to 11 dB in relation to an isotropic radiator with a forward / backward radiation ratio F / B up to 20 dB. The disadvantage is a small relative bandwidth, of the order of 3...4%, which allows them to be used in television only as single-channel antennas.
N. Turkin's antenna uses ring elements: three directors; reflector and double active, in which the rings are fed with a phase shift of 225 °, providing additional directivity and some broadening of the band.
Modeled fairly accurately as described, the antenna showed Ga and F/B of 11,33 and 19,4 dB, respectively, i.e. the same order of magnitude as a conventional six-element wave channel. However, its bandwidth turned out to be much wider - about 7% for SWR<3 and about 14% for the directivity pattern (Ga and F/B > 10 dB). The radiation patterns (RP) of the simulated antenna in the horizontal (top) and vertical planes for the center frequency are shown in Fig. 1, all levels are given in dB.
As the author rightly noted, the antenna needs to be tuned. During simulation, this is entrusted to the computer, which automatically executes the optimization programs. Optimization is a pleasure. The optimized antenna showed a slightly larger gain (11,63 dB), significantly better back lobe suppression (24 dB), and the SWR and DN passbands returned to normal (10% each), and matching with the feeder improved. DN are shown in fig. 2.
It is interesting to note that on the graphs of the input impedance of the antenna, the jX (reactance) curve in the passband crosses the zero mark three times, which provides a good match. The design of the modified antenna is shown in fig. 3. The perimeters of the elements and the distances between them are given in wavelengths.
The active element of the described antenna exactly repeats the construction of the "Swiss double square" (in this case a circle, but this is not so important), which is well-deservedly popular, known to shortwave radio amateurs. For the sake of interest in the description of the antenna, the directors and the reflector were removed. And what? The dual active element showed excellent results: Ga and F/B were 7,8 and 15,5 dB. Thus, taking into account the complexity of manufacturing and the resulting quality, he, perhaps, has no equal!
Author: V.Polyakov
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