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
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