* * * D R A F T * * * – a working document.
This article documents the selection of the trial loop in ground configuration as a development from the loop on ground antenna (KK5JY).
The baseline is a minor variation of a design by KK5JY, a 15′ square loop 20mm above average ground, with 9:1 transformer and 50Ω load middle of one side.
Above is a plot of feed point impedance when the loop is driven. At 3.6MHz, the source impedance for a rx system is 43+j852Ω, and the mismatch loss to a 450Ω load is 11.0dB, a direct contribution to Antenna Factor (AF).
Note that these values are quite dependent on model parameters such as wire diameter, height above ground, soil type etc. NEC-2 may have issues with some aspects of the model, so it may not produce similar results to the NEC-4 models
For excitation being a plane wave at elevation 45° from direction of maximum response, AF is calculated at the transformer secondary 50Ω load to be 22.7dB.
3m loop in ground
Above is a plot of feed point impedance when the loop is driven. At 3.6MHz, the source impedance for a rx system is 240+j136Ω, and the mismatch loss to a 200Ω load is 0.4dB, a direct contribution of Antenna Factor (AF).
For excitation being a plane wave at elevation 45°, AF is calculated at the transformer secondary 50Ω load to be 13.9dB.
The smaller shallow buried loop has AF 8.8dB better than the baseline configuration.
AF=13.9dB @ 3.6MHz implies Gain=-32.5dB (@45° elevation) which at first might seem unusable, but when the ambient noise figure (Fam) is more than 50dB, such a low gain antenna system still captures sufficient external noise to dominate receiver internal noise and there is very little degradation in achieved S/N.