LF: 136kHz ERP calculation (G3XBM's earth electrode antenna)

[Roger Lapthorn <rogerlapthorn@gmail.com>]

Background from Alan Melia.....

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�Hi Roger you can calibrate your loop first using DCF39 on 138.83. This has a field strength in our area of 1mV/m. Then you could work back your own ERP by comparison...... 1W ERP produces 300mV/m at 1km, 100mV/m at 10km (from memory ....I think that is correct....10dB per decade after 1km). �Doing it that way removes as lot of the variables, and its a good thumbnail calculation.�

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So, this afternoon I measured the strength of DCF39 as -22.8dB using the same loop and receiver and Spectran settings, so -22.8dB is equivalent to 1mV/m field strength based on Alan�s assumptions.

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G3XBM�s signals at 14.9km measured -42.5dB which is approximately 100uV/m.� Assuming 100mV/m at 10km equals 1W ERP, then 64mV/m would equal 1W ERP at 14.9kms (approximately 4dB weaker signal from the CCIR curves). The ERP is lower by the ratio of 100uV/m to 64mV/m. So, my ERP in the best direction is 56dB down on 1W, which is 2.5uW ERP.

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If this calculation is correct then I�m surprised that such a low power (2W from PA) into such a strange antenna structure (20m separated ground rods connected by wire just 1.5m above ground for most of the run) can produce so much ERP.

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Please can you check these figures Alan (or others) and see if you agree?

73s
Roger G3XBM

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