A Practical Approach for Controlling the Shape of the Radiation Pattern of a Microwave Log-Periodic Antenna for Wideband Applications [Antenna Applications Corner]

The log-periodic antenna (LPA) is a wideband frequency-independent antenna, used in many applications including direction-finding (DF) receivers. The direction finding is an important parameter measured by a direction-finding receiver in electronic support measures (ESM) systems. Amplitude-comparison direction finding (ADF) is one of the widely used angle-of-arrival measurement techniques in electronic support measures systems across the world. Many EW engineers have derived the mathematical equation for the estimation of the angle of arrival based on this popular technique. The angle of arrival accuracy using this technique depends on the number of antennas used in the configuration, channel imbalances, the beamwidth, and the shape factor of the antennas. To obtain good angle-of-arrival accuracy with the amplitude-comparison technique, the log-periodic antenna is one suitable antenna. Normally, electronic support measures systems are wide open in frequency (the bandwidths of electronic support measures receivers are a few thousands of MHz), and hence the antennas exhibit nearly the same beam shape (beamwidth and shape factor) over these wide operating frequency ranges so as to have a uniform angle-of-arrival error distribution across the frequency spectrum. However, a log-periodic antenna offers some variations in half-power beamwidth over a wide frequency spectrum at microwave frequency ranges, which is reasonably good, but is not enough to obtain uniform angle-of-arrival error patterns throughout the operating frequency range. Similarly, a uniform shape factor is also important across the bandwidth. The radiation beam of a log-periodic antenna over a wide operating frequency range thus need to be controlled. A practical solution to this problem, along with experimental results, are presented in the current paper.