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Message-ID: <C60A421BC61F4935A7EEE0B506AD140E@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
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Date: Thu, 8 Oct 2009 17:13:16 +0100
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Subject: LF: Re: ERP at LF
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Dear Chris, LF Group,

G4AYT wrote:
...> comments referring to a stations ERP by various operators : 'Estimate', 
'Guess', 'Approx.', 'Reckon I'm fairly close to...', 'Approaching' etc. 
Nobody REALLY knows exactly! ...>

This is true of any measurement - nobody can measure ERP or anything else 
"exactly"; the issue is how much uncertainty exists in the measured result.

If you predict ERP based on antenna current and dimensions, my experience is 
that the "real" value from field strength measurements will be lower by 
about 0 - 6dB due to effects of the environment around the antenna. There is 
also the uncertainty in measurement of RF current, and the physical 
dimensions of the antenna, which might amount to a few dB, depending on how 
well it is done.

For field strength measurements, the measuring antenna and receiver 
calibration can be done with amateur facilities to within 1 - 2dB with care. 
I have found measuring LF/MF field strengths is repeatable within about 1dB 
with a given TX antenna, however the ERP calculated from individual 
measurements varies by +/- a few dB, depending on the location where the 
measurement was done. So it has been my practice to calculate the ERP as an 
average between a dozen or so locations.

This may sound vague, but it does mean being able to predict ERP and field 
strength over reasonable distances to within a few decibels, a fraction of 
an 'S' point. Compared with the way amateurs normally specify radiated 
signal strength, this is fantastic precision. In particular, the traditional 
way of stating TX output power and describing the antenna is quantitatively 
almost meaningless, especially where small antennas in less-than-ideal 
locations are concerned. Hence some stations "get out well" compared to 
other apparently similar stations using this type of description.

...>I don't know what the answer is, but it is clear that ERP at LF is far 
more complex than at VHF/UHF etc. where 'full size' antennas are generally 
used....>

I expect this is true up to a point, although somebody specifying the gain 
of an antenna to the nearest 0.1dB doesn't mean the gain and ERP is actually 
known with anything like that accuracy. I think there is major difference 
between LF and VHF/UHF. At VHF/UHF, you would consider a signal with a 
particular ERP radiated by an antenna being subject to separate effects such 
as absorbtion, reflection, re-radiation, etc. by objects in the environment 
nearby which would affect the field strength at a distant point. At LF/MF, 
the surrounding environment is so close to the antenna in terms of 
wavelengths that it affects the impedance and radiation resistance of the 
antenna, and has to be considered as part of the antenna, with a direct 
effect on the ERP.

Cheers, Jim Moritz
73 de M0BMU