Field strength above 1 GHz: Measurement procedures for standard antennas

Abstract

To calibrate antennas for state-of-the-art field-strength measurements above 1 GHz, standard antennas are needed that have gain values known to within ±0.1 dB. Since this requirements exceeds the verified accuracy of calculated gain values, these standards must be established by making absolute gain measurements. The discussion primarily concerns absolute gain measurements for horn antennas by the two-antenna method. However, much of the discussion is pertinent to high-accuracy field-strength measurements in general. The two-antenna method is considered to be essentially an insertions-loss measurement (with many additional problems and sources of error), and this concept is used to derive a working formula that is suitable for high-accuracy gain measurements. The two most intractable problems-- insufficient antenna separation and multipath interference--are discussed in detail. Some important experimental details are included that have previously been overlooked or inadequately discussed, and it is concluded that previous error estimates of less than ±0.1 dB for horn-gain measurements have been somewhat optimistic. To facilitate the design and evaluation of high-accuracy gain measurements, some simple terms, concepts, and formulas are provided that are useful in analyzing multipath interference.