1 Antenna installation
1.1 Frequencies below 30 MHz
It is recommended that, for frequencies below 30 MHz, vertical or loop antennas be used. A vertical antenna shorter than one-quarter of a wavelength may be used, with a RF ground system consisting of either buried radial conductors at least twice the length of the antenna and spaced 30° or less apart, or an equivalent RF ground screen. An inverted cone type vertical antenna with a similar ground system may also be used with some advantage.
1.1.1 It is generally accepted that random variations in polarization of ionospheric waves are such that the vertically polarized component is, in general, substantially equal to the horizontal component.
1.1.2 The voltage developed at the output of a passive vertical antenna shorter than one quarter of a wavelength is substantially dependant on the frequency. Since the impedance of this antenna is capacitively reactive, the subsequent voltage response in a field-strength measuring instrument, when connected to a properly terminated transmission line, is essentially a direct function of frequency, resulting in a simple, relatively uniform calibration curve.
1.2 Frequencies between 30 MHz and 1 GHz
1.3 Frequencies above 1 GHz
Antennas for field-strength measurements at frequencies above 1 GHz are usually directional. Their antenna factor (k-factor, see § 1.4) can be determined with high accuracy. Due to the directional characteristics of the antennas the influence of the environment on the measurement accuracy is normally low.
1.4 Antenna factor