CORROBORATION OF A MOMENT-METHOD CALCULATION OF THE MAXIMUM MUTUAL COUPLING BETWEEN 2 HF-ANTENNAS MOUNTED ON A HELICOPTER

A moment-method calculation is described of the maximum mutual coupling (or minimum isolation) between two loop antennas mounted on the tail-cone of a helicopter at the low-frequency end of the aircraft HF band. The wire-mesh mathematical model of the aircraft is constructed with a high sampling density in an attempt to produce estimates of the required near-field dependent parameters with sufficient precision to calculate the maximum mutual coupling with acceptable accuracy. The calculation is a fairly demanding test of the procedure adopted for the construction of the mathematical model, because it depends upon the accuracy with which both near-field and far-field parameters are determined. This mathematical model does not incorporate any empirically derived information, and the calculation was of necessity performed in advance of an aircraft being available for experimental purposes. The difficulty of assessing the credibility of calculated results under such circumstances is considered. NEC was used as the solution code for the wire-mesh model. An independent corroboration of the calculation was made possible by the siting and configuration of the proposed antennas. This allowed a simple equivalent circuit model of the arrangement to be constructed, values of the circuit elements to be estimated, and hence an independent approximate determination of the maximum coupling to be made. The degree of consistency between the results obtained by these two methods is considered to enhance the confidence which can be placed in the results obtained using moment-method mathematical models of this type when applied to near-field aircraft HF antenna problems.