23 Jan 2010

ERP on 500kHz (part 2)

Yesterday I did a recalculation of the G3XBM ERP based on the classic formula and re-measurements of my antenna current. My antenna is a 5m long vertical with a spiral top hat of about 7 turns about 1.5m x 1.5m. Not being clear how to model the top hat I assumed effective heights of 5, 4 or 3m. The measured antenna current is 0.12A. My results are as follows:

Assumed effective height 5m     ERP = 1.53mW
Assumed effective height 4m     ERP = 0.98mW
Assumed effective height 3m     ERP = 0.55mW

So, I believe my 1mW ERP previously calculated is not a bad estimate, and it may indeed be lower than this figure.

Rik ON7YD (OR7T) has just sent me this email:
"I simulated the antenna based on your description (with MMANA) and the current through the vertical wire seems to be quite constant. So the tophat works well and the effective height will be more or less the physical height. The simulation gives a radiation resistance of 0.074 Ohm, so with 0.12 A a radiated power just over 1mW EMRP (1.75mW ERP). In the real world it will be some dB's less (as Jim stated recently), probably something like 0,5 to 1mW ERP.  Very impressive that it covers over 1200km."

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