* * * 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). Note that in practice source impedance depends on soil parameters, and can be expected to vary with moisture content and so the impedance matching solution is a very approximate solution for a untuned loop.
For excitation being a plane wave at elevation 45°, AF is calculated at the transformer secondary 50Ω load to be 13.9dB.
The models are sensitive to ground parameters.
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.