Design of electrically small broadband receiving antennas under consideration of nonlinear distortions in amplifier elements

The design of electrically small receiving antennas is not a question of the received signal power, impedance bandwidth or antenna losses but only of the signal-to-noise ratio (snr) at the receiver output. Snr is limited by the field-to-noise ratio at the location of the antenna. It is shown that with a given antenna height optimum snr is only obtained if the first amplifier stage is directly connected to the antenna element. With vertical rod monopoles the optimum location for the amplifier in the antenna with respect to snr is found at a certain height above the ground. An antenna of this kind is an ideal element for a direction-finder system. The broadband pick up of undesired signals requires high linear and low noise amplifiers with a wide linearity range in order to suppress nonlinear distortions, which could deterioriate the snr. It is found, that an optimum antenna height may be determined, which in the frequency range from 10 kHz to 30 MHz is not greater than 1 m. At the present time antennas of this height and operational frequency range, which stand an undesired field intensity of 40V/m for 20dB suppression of cross modulation are produced in large quantities and are in practical application especially in mobile communication systems.