Verification of an Antenna Phase Pattern
Abstract Generated abstract
This paper proposes an experimental method for verifying distortions in the phase pattern of an antenna caused by a capacitive horizontal load, a problem that is difficult to treat analytically for broadband phased radio systems. The method compares measurements from the complete antenna with those from a vertical radiator left at its center, using a stationary transponder that converts the received signal to an integral frequency ratio and reradiates it to a phase indicator. By subtracting the two phase-meter readings, the distance-dependent terms cancel, so the measured variation gives the phase-pattern distortion due to the horizontal part of the antenna without requiring exact range knowledge or strictly circular motion around the antenna.
Full Text
Verification of the Antenna Phase Pattern
G. F. Ignat'ev, Sh. I. Shchipnikov
(Presented by the scientific seminar of the Radio Engineering Faculty)
In the design of phased radio-engineering systems, particular attention is paid to the phase patterns of radiation. To provide radiation over a broad frequency range, from long to medium waves, it is necessary to create antenna systems with a large capacitive load; the geometrical dimensions of the antenna are then comparable with the wavelength at the upper frequencies of the range.
Calculation of the distortions of the phase pattern of an antenna with a capacitive load is extremely complicated; therefore there arises a need to develop experimental methods for determining the influence of the antenna’s capacitive load on its phase pattern.
Below a method is proposed for checking the influence of a capacitive load on the phase pattern. When this method is used, it is not necessary to know the exact distance from the antenna to the phase indicator, or to move around the antenna strictly along a circle. This circumstance is a major advantage of the proposed method in comparison with known methods; it makes it possible to check the influence of a capacitive load on the phase pattern of an antenna with an accuracy down to fractions of a degree.
The method consists in comparing the phase pattern of the antenna with the phase pattern of a vertical radiator. The antenna under investigation is constructed in such a way that it is possible quickly to remove its horizontal part, leaving a vertical radiator at the center (antennas of mobile systems usually satisfy this condition). The phase pattern of the antenna is compared with the phase pattern of this vertical radiator. The measurement scheme is shown in Fig. 1.
Initially the vertical radiator operates. Oscillations from it arrive at the point where the phase-measuring device is located with a phase (relative to the phase of the current in the antenna) equal to $\varphi_1 = - \dfrac{\omega}{c} r_1$, where $\omega$ is the angular frequency, $c$ is the velocity of propagation of the radio wave, $r_1$ is the distance from the antenna to the receiving point, and at the point where the reradi-
with phase
\[ \varphi_2=-\frac{\omega}{c}r_2, \]
where \(r_2\) is the distance from the antenna to the transponder. The transponder receives the oscillations, transforms their frequency (and phase) in an integral ratio, for example \(2:3\), and radiates them at the frequency \(\frac{2}{3}\omega\). Then the oscillations from the transponder will arrive at the receiving point with phase
\[ \varphi_3=\frac{2}{3}\varphi_2+\frac{2}{3}\frac{\omega}{c}r_3 =\frac{2}{3}\frac{\omega}{c}(r_2+r_3), \]
where \(r_3\) is the distance from the transponder to the receiving point.
Fig. 1. Measurement scheme. The transponder and the phase indicator are located in the wave zone.
The transponder provides checking and maintenance of the constancy of the phase shift introduced by the entire transponder path.
Thus, oscillations of two frequencies arrive at the receiving point: frequency \(\omega\) with phase \(\varphi_1=-\frac{\omega}{c}r_1\), and frequency \(\frac{2}{3}\omega\) with phase
\[ \varphi_3=\frac{2}{3}\frac{\omega}{c}(r_2+r_3). \]
The phase difference, after reduction to one frequency \(\omega\), will be
\[ \Delta\varphi=\varphi_1-\frac{3}{2}\varphi_3 =-\frac{\omega}{c}(r_1-r_2-r_3). \]
An analogous measurement is carried out with the antenna under investigation (i.e., with the horizontal part operating). In this case the oscillations from the antenna will arrive at the receiving point with phase
\[ \varphi'_1=-\frac{\omega}{c}r_1+\psi_1, \]
where \(\psi_1\) is the distortion of the phase pattern in the direction toward the receiving point. The oscillations will arrive at the location of the transponder with phase
\[ \varphi'_2=-\frac{\omega}{c}r_2+\psi_2, \]
where \(\psi_2\) is the distortion of the phase pattern in the direction toward the transponder. Accordingly,
\[ \varphi'_3=\frac{2}{3}\varphi'_2+\frac{2}{3}\frac{\omega}{c}r_3 =\frac{2}{3}\left[\frac{\omega}{c}(r_2+r_3)+\psi_2\right], \]
and the phase difference of the oscillations arriving from the antenna and from the transponder at the receiving point (after reduction to one frequency) will be
\[ \Delta\varphi'=\varphi'_1-\frac{3}{2}\varphi'_3 =-\frac{\omega}{c}(r_1-r_2-r_3)+(\psi_1-\psi_2). \]
The difference between the phase-meter readings in the first and in the second case is equal to
\[ \Delta\psi=\Delta\varphi'-\Delta\varphi=\psi_1-\psi_2. \]
By carrying out such measurements at points located in different directions relative to the antenna under investigation, one can obtain a plot of the phase distortions introduced by the influence of the horizontal part.
Since the reradiator is stationary and the phase pattern of its antenna is circular, $\psi_2 = \mathrm{const}$. Consequently, the change in the quantity $\Delta \psi$ gives the distortion introduced into the antenna phase pattern by its horizontal part.