LF: 137.5kHz ERP calculation based on WSPR reports (G3XBM)

[Roger Lapthorn <rogerlapthorn@gmail.com>]

This morning M0BMU started listening on 137.5kHz WSPR again and I now have a bank of reports from Jim 69km away from me. Using this data I've attempted to make some revised ERP calculations based on some working assumptions.

Firstly, I've assumed a noise level at Jim's QTH of 3uV/m, this being the figure he gave me last winter for 500kHz.  Secondly, I've taken the best WSPR S/N reports received from Jim (-19dB S/N). So my signal level with Jim is around 0.36uV/m. During the day the signals have got somewhat worse and are now around -25dB S/N.  The distance between us is 69km.  So, putting the numbers into formula:

ERP = (Ed)^2/49  = [(0.36*10E-6 x 69*10E3)]^2 / 49 = (0.36*0.36*69*69)/49 uW = 12.6uW in Jim's direction.

Now, I'm using a loop antenna, and I believe Jim is not in the main line of fire, so another 3-6dB down on the power in the main direction. This would suggest that in the better directions off the end of the loop the ERP could be as high as 25-50uW.  Ground wave propagation losses may also mean the ERP is another 6dB higher (?) suggesting a possible ERP as high as 100-200uW although this seems optimistic with my modest loop and 5W RF driving it.

Have I got this right?

Incidentally, thank you for all the responses on "loops on the ground". I can understand what people are saying so will try to elevate the lower part of the loop when I get back from my holiday in a few weeks' time. Incidentally this means no LF work from Oct 3-19th is possible.

73s
Roger G3XBM


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