Relation between minimum antenna height and bandwidth of the signal-to-noise ratio in a receiving system

The most important specification of an electrically small transmitting antenna is the ratio of the radiated power over the available power of a generator. This ratio and its bandwidth decrease considerably with decreasing antenna height. With a receiving antenna however, not the received power but the signal-to-noise ratio (snr) in the receiving system is of importance. Due to the received external noise at low frequencies the bandwidth of the snr of a receiving antenna with receiver circuit is many orders of magnitude greater than the power bandwidth that may be obtained with an antenna of equivalent height. This snr bandwidth will be evaluated quantitatively as a function of the antenna temperature, the noise characteristics of an amplifier network and of the radiator bandwidth. Antenna losses will be taken into account. With short resonant antennas maximum snr bandwidth is only obtained with a considerable noise mismatch at center frequency. Curves of minimum required antenna height versus the center frequency are evaluated for different values of snr bandwidth. For median values of antenna temperature and an snr bandwidth of not greater than 30% the required height of a rod antenna is at no frequency greater than 70cm in the total range from 10kHz to 1GHz. A comparison shows that minimum required height of a transmitting antenna at low frequencies is many orders of magnitude greater than of a receiving antenna. The common line between a passive rod antenna and the matching network in the receiver causes a considerable reduction of the achievable bandwidth. By means of an amplifier, being directly connected to a capacitive radiator, short and extremely broadband active receiving antennas can be realized, the snr/ -curve of which is of low-pass character.