N.I. Bobkov – Ph. D. (Eng.), Head of Research Sector, JSC «VNII «Gradient» (Rostov-on-Don)
D.D. Gabrielyan – Dr. Sc. (Eng.), Professor, Deputy Commander of Scientific and Technical Complex, Rostov-on-Don Research Institute of Radiocommunication
S.S. Ivakina – Part-programming Engineer, JSC «VNII «Gradient» (Rostov-on-Don)
V.V. Prozhivalsky – Engineer, JSC «VNII «Gradient» (Rostov-on-Don)
In some applications of high directivity aperture antennas it is an urgent problem to maintain the constant radiation pattern parameters in a wide frequency band. It is known that on a decreasing amplitude distribution, with introduction in the linear aperture a frequency-dependent phase field distribution, it is possible to compensate the radiation pattern width frequency dependence in a wide frequency band. However, in some cases, the relevant solution is a frequency-independent radiation patterns formation with uniform or near-uniform amplitude field distribution in the aperture. The proposed method of solving similar problems for antennas with a circular or elliptical uniformly excited aperture based on application of properties of the equivalent linear radiator, which amplitude distribution decreases to zero. Specifying a frequency-dependent phase distribution along one or both coordinates of a circular aperture, the radiation characteristics stabilization effect in one or both planes can be achieved.
The calculations, based on the use of synthesis methods, of optimal phase distributions providing radiation patterns forming with minimal deviation from defined parameters, showed that for the phase distribution «cosine to the power of 0.35» with initial dephasing of 200°, the radiation characteristics stabilization achieved in the frequency band with an overlap more than four octaves, with a maximum deviation of radiation pattern width of ± 10%.
To check the synthesis results, the reflector antenna simulation with circular aperture and a rectangular waveguide open end radiator, has been done. To create an approximate to the quadratic phase distribution, the radiator was set out of focus. Also experimental research of a reflector antenna mock-up with single-channel and four-channel waveguide radiators, confirming the possibility of the radiation patterns forming, weakly dependent on the frequency in the band overlap of 2.25:1, has been done. For defocused multibeam antenna mock-up with a four-channel waveguide radiator, in the band of 2.25:1 partial radiation patterns crossing level ranging from 1.8 dB to 4.8 dB in the E plane and from 2 dB to 5 dB in the H plane. The radiator defocusing leads to the gain reduction, however, due to the increase of the partial radiation patterns crossing level at higher frequencies, the gain in the equisignal direction remains close to the value for a focused antenna. The gain reduction value is comparable to the energy losses for the multibeam reflector antennas based on the amplitude stabilization techniques with frequency-dependent adjusting coatings at the working surface of a reflector. The experiment result analysis and close match with the calculations indicate the prospects of the proposed method of forming a frequency-independent radiation characteristics of antennas with a circular aperture.
For different types of aperture antennas, the questions of forming special phase distributions, by using whom beamforming of radiation patterns with minimal deviation from the set parameters in the bandwidth of a few octaves are achieved, has been considered.
The proposed method of forming a frequency-independent radiation characteristics of circular aperture antennas and results of a numerical and physical investigations can be applied in the practice of broadband antenna systems design.