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Message-ID: <470D43E18300477E9397AA79CA57BAB9@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
Date: Thu, 21 Jan 2010 20:29:16 -0000
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Subject: Re: LF: about guns, water pistols and dripping tabs
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Dear Roger, LF Group,

I tried sending this earlier, but it does not appear to have got through...

--- On Thu, 21/1/10, Roger Lapthorn <rogerlapthorn@gmail.com> wrote:
This is most interesting. I notice that M0BMU was running with 50mW ERP last 
night, so it might be worth plugging in his reports as well for last night. 
...

Mea Culpa... I seem to have left the WSPR power indicated as 50mW from some 
136kHz tests - in fact the 500kHz ERP from M0BMU was close to the normal 2W 
level last night.

Estimating transmitter ERP from received WSPR signal reports is prone to 
large inaccuracies, for at least 2 reasons:
 -You have to estimate the ground wave propagation loss, which depends on 
various "unknown factors" such as ground conductivity, terrain and so on, 
and for distances greater than a few 10s of km the uncertainty could easily 
be several dB or more.
 -To get received signal field strength from WSPR SNR, you have to assume a 
value for the noise field strength. Noise being noise, this is likely to 
have unpredictable variations of several dB too.

So the ERP estimated from the received signal is better than nothing, but at 
best is going to be an order-of-magnitude estimate rather than a precise 
figure with well-defined error limits. Ground features usually lead to 
higher ground wave losses, and noise levels tend to increase rather than 
decrease, so these ERP estimates are likely to be on the low side. A much 
better estimate of ERP can be got by measuring antenna current and 
calculating radiation resistance from the antenna geometry - then the main 
source of error is the "site loss" due to environmental effects on the 
transmitting antenna. But this seems to be usually in the range 0 - say, 6dB 
for most locations, so as Andy says, if you assume -3dB you will probably be 
quite close.

Cheers, Jim Moritz
73 de M0BMU