VHF Loop Antenna Loss Measurements

The product of the loss ratios of two very small loop antennas, oriented such that their axes coincide, is given approximately by L1L2 = A(1.5)-2(1 + k2d2)-2(kd)6 where L1 and L2 are the dissipative power loss ratios of the loops, d is the spacing between the loops, and k = 2¿/wavelength. A is the insertion loss measured between the ports of the two loops including their matching networks. The loops are assumed to be impedance-matched to the generator and to the load, respectively. A relationship is developed which takes into account the effects of the receiving loop current on the transmitting loop mesh. This makes possible measurements with more closely spaced loops than otherwise, without incurring intolerable errors. The minimum useful spacing depends on the loss ratios, being greater for more efficient antennas. For small antennas with practical loss ratios, the measurements can be made on the laboratory bench or in production test without special antenna test facilities. A more nearly exact field expression due to F. M. Greene is utilized for loops of appreciable size.