Feasibility study – loop in ground for rx only on low HF – signal / noise degradation (SND) – more detail

Feasibility study – loop in ground for rx only on low HF – signal / noise degradation (SND) gave the following graph.

This article explains a little of the detail behind the graph.

NEC

The graph is based on a series of NEC-4.2 models of the loop in ground antenna. Key model parameters are:

  • 3m a side;
  • ‘average’ soil (σ=0.005, εr=13);
  • depth=0.02m; and
  • frequency 0.5 to 10MHz in 0.1MHz increments.

The models were scripted by a PERL script, and the output parsed with a Python script to extract feed point Z, structure efficiency, and average power gain (corrected to 4πsr).

Spreadsheet

The summarised NEC data was imported into a spreadsheet and an approximate model of the system built, comprising:

  • Receiver input impedance 50+j0Ω;
  • a length of transmission line (10m of Belden 8215 RG6/U);
  • an ideal transformer (4:1);
  • source impedance derived from the NEC data.

Calculation includes:

  • transmission line loss and impedance transformation;
  • transformer assumed ideal plus an allowance for transformer loss (1dB);
  • mismatch loss; and
  • average antenna gain.

Above is an extract of the spreadsheet.

Mismatch loss

Mismatch loss is an important element of the system behavior. A convenient place at which to calculate mismatch loss is the feed point of the loop in ground.

Above is a plot of the loop feed point impedance, the source impedance in the receive scenario.

Above is a plot of the loop load impedance, the receiver impedance transformed by transmission line and transformer. The varying impedance is a result of using 75Ω line.

The combination of these allows us to calculate mismatch loss.

Above is a plot of the calculated mismatch loss which must be added in to the system gain model.

Signal / noise degradation (SND)

From the system model, and an estimate of ambient noise from ITU-R P.372-14, we can calculate SND.

Above is a plot of SND.

Note that P.372-14 is based on a survey with short vertical monopole antennas, so it is likely to overestimate noise received by a horizontally polarised antenna (and therefore the SND estimate will be low).

Antenna performance is sensitive to soil parameters, especially those close to the surface and subject to variation with recent rainfall etc.

Conclusions

This is after all a feasibility study, and within acceptable uncertainty, the antenna system would seem to be feasible for low HF and even 160m receive.