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

[Roger Lapthorn <rogerlapthorn@gmail.com>]

Thanks Jim.

I must admit that the 100uV/m result did seem high and I am certain that my ERP is nowhere near 40mW!  So, I'll have to try the steps you suggest to get a better approximation of my ERP.

BTW I use an E-field probe between the loop and the FT817 taking the input from the top of the loop and feeding the FT817 from the low impedance output. It works.

73s
Roger G3XBM

On 11 August 2010 17:49, James Moritz <james.moritz@btopenworld.com> wrote:

Dear Roger, LF Group,

The formula to calculate field strength from ERP and distance is:

E = 7 * sqrt(ERP) / d

Where E is FS in volts/metre, d is distance in metres. In which case, 1W ERP gives you 700uV/m FS at 10km distance, not a scorching 100mV/m!

Rearranging the formula to calculate ERP from field strength and distance is:

ERP = (E * d)^2 /49

With E = 100uV/m, d = 14km this works out to 40 milliwatts ERP - which would be quite impressive if it were really the case!

If your signal FS were really 100uV/m at 14.9km, it would be a strong signal by amateur standards that could be detected at much greater distances, certainly audible all over the UK in reasonable conditions. So where does the error occur? My guess would be in measuring the relative levels of your signal and DCF39. I don't think you can switch the AGC off on the FT817, so AGC action would have the effect of reducing the gain when receiving DCF39, making the relative level of your signal appear higher. The way round this would be to:

-Tune RX to DCF39

-Back off the RF gain control until the measured level of DCF39 on Spectran fell by some dB - that way the signal level at the RX output would be below the AGC threshold.

-Without touching the gain controls, re-tune to your signal and measure the relative level on Spectran. You will probably hear nothing at all on the speaker...

That way, the RX would be operating at fixed gain provided the measured signal level was below that of DCF39.

The other possibility springing to mind is antenna tuning - to get reasonable matching between a multi-turn loop like this and a low-impedance RX input implies your antenna has quite high Q tuning - in which case you will need to ensure the tuning is accurately peaked on whichever signal you are measuring at the time, otherwise several dB error would result.

Of course, I'm just trying to guess from limited information, so I could be entirely wrong!

Cheers, Jim Moritz
73 de M0BMU

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