© A.V. Negrobov, I.V. Popov, Yu.A. Rembovskiy
When ultrabroadband signals are used in radiolocation systems, high-speed data transmission systems and remote scanning, there appear a number of technical difficulties and some of them are related with their own emissions. Base ultrashort pulse antenna types are as follows: vibrator antennae, horn antennae and Vivaldi antennae. The purpose of this work is to determine the extent and reasons of ultrashort pulse deformation by these antennae.
It is known, that non-distorted signal emissions are possible when antenna system has a stable polarization, constant amplification factor and linear phase and frequency dependence. Using a finite integration method, we have calculated amplification factors for the antennae within a band between 0.1 and 1.5 GHz. According to that calculation, the TEM-horn and Vivaldi antenna amplification factor reaches its maximum in the band between 4 and 13 GHz subject to ±2 dB flatness. LPA amplification factor, when compared to those of antennae, was less and its most uniform section was at a lower frequency (1.8 – 11.8 HGz).
To compare the TEM-horn of log-periodical and Vivaldi antennae in terms of ultrashort pulse distortions we have conducted an electrodynamic calculation for EM-field in a far field zone. According to that calculation, LPA can be used for ultrashort pulse emissions and distortions will be small if the frequency ratio does not exceed 2.
To enable qualitative comparison between incoming and emitted signals form, their correlation factors were also calculated. According to the calculation, the minimum pulse distortions will be observed with Vivaldi antennae and the maximum distortions will pertain to log-periodic antennae. With a lower frequency ratio, the differences in the form and duration of incoming and emitted signal will be lower.
To enable qualitative evaluation of emitted signal duration against excitation pulse duration, a total delay time was calculated for each antenna. The calculation confirmed that the ultrashort pulse distortions, in terms of time, were minimal with Vivaldi antenna and maximal with LPA.
Amplitude spectra of the signals in question were determined via Fourier transformation. The amplitude spectra of the signals emitted by TEM horn are most indented. When notch antenna was used, the distortions were minimum.
To find the reason why ultrashort pulses are distorted when passing through antenna, a numerical experiment was conducted: the antenna in question was replaced by an equivalent four pole device with variable amplitude-frequency and phase-frequency parameters. An ultrashort pulse was sent through low pass filter to the antenna input. The maximum distortions of the ultrashort pulse were caused by phase distortions, not amplitude distortions. This phenomenon was observed for all the three antennae under study. However, the maximum phase impact was observed in the log periodic vibrator antenna: For ideal amplitude and frequency dependence and real phase and frequency dependence, the emitted pulse was almost similar to the pulse emitted by antenna with real parameters.
In the work it was demonstrated that among ultra broadband antennae, in terms of minimum distortions of ultrashort pulses, Vivaldi antenna was the best and LPA antenna was the worst due to a wide dispersion of its phase and frequency parameters. It was also demonstrated that simultaneous adjustment of frequency/amplitude and phase/frequency dependencies can minimize ultrashort pulse distortions and bring them almost to zero. e carried out.