With several repeatable ground wave reports on 472kHz WSPR using the earth-electrode antenna, it is time for a recalculation of my ERP based on a formula (for 500kHz) supplied some years ago by
M0BMU. The assumption (from M0BMU, that I am not at all sure is right) is that the noise floor is around 3uV/m in a 2500Hz (WSPR) bandwidth. So, based on the WSPR S/N ratio the field strength when using the earth-electrodes can be calculated.
- G3ZJO at 79km gets me at around -19dB giving my field strength around 355nV/m.
- G3XDV at 61km he gets me around -12dB S/N giving my field strength around 810nV/m.
- M0BMU at 69km he gets me around -30dB S/N giving a field strength around 100nV/m
ERP Calculations
- ERP using G3ZJO's results = (E*d)^2 / 49; with E = 355nV, d = 79km ERP = 16uW
- ERP using G3XDV's results = (E*d)^2 / 49; with E = 810nV, d = 61km ERP = 49uW
- ERP using M0BMU's results = (E*d)^2 / 49; with E =100nV, d = 69km ERP = 1uW
Now, to me these figures seem
far too low, so perhaps the noise level is much higher than 3uV/m. Certainly that is the case here where I have an S4- S8 noise floor due to SMPSUs and other noise sources. I think these numbers can be doubled based on ground wave losses but, even so, the figures appear to be extremely low. My Marconi antenna is some 8dB (average) better than the earth-electrode antenna and I am sure the ERP is well above 1mW using this antenna.
Conclusions
I am totally puzzled: measuring antenna current on the Marconi I get ERPs that are an order of magnitude higher yet measured results with the earth-electrode antenna average just 8dB down on the Marconi. This suggests ERPs for the earth-electrode antenna in the low mW region, which I find much more believable.
Any thoughts please?
If you look at the WSPR source code, the S/N value is 'guestimated' and is seemingly rather arbritraty so I don't know if can be relied on. No doubt Joe K1JT put something approximately right in, and for comparison is OK, but I would not rely on it too much.
ReplyDeleteHugh G6AIG
Me Again.. Another comment, there are some interesting papers about ELF transmitters available now.. google Clam lake ELF antenna for example. It used exacly what you are doing with the earth probes.. in N-S and E-W ground dipoles, each 14 miles in length, Earthing 'wells' and a site specially chosen over low conductive granite, to maximise the size of the earth loop current!
ReplyDeleteThere is an power estimate: 1MW in, 2W radiated (76Hz).. Hugh G6AIG