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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