Dear Andre, LF Group,
Regarding ERP and so on, significant points that can be drawn from European
experience on LF are:
Measurements by several people have shown that the efficiencies achieved by
typical amateur LF antennas are rarely better than 0.1%, and normally
rather less. So a 100W PEP TX power limit in practice would limit EIRP to
less than 0.1W for the vast majority of amateur stations - experience shows
that to get 1W EIRP with 100W transmit power probably requires a favourably
located antenna around 30m high. Even with unlimited transmitter power
available, EIRP greater than 1W would be difficult to achieve from many
amateur locations due to electrical breakdown of the antenna. Only a small
minority of European stations approach our 1W ERP limit, in spite of the
fact that transmitter powers are normally well over 100W. A TX power
limited to 100W seems unnecessarily restrictive. In Europe, the
availability of higher transmit powers means that less well-off amateurs
can generate usable signal levels without having to acquire large tracts of
real estate for antennas!
It is certainly possible to make consistent measurements of ERP, although
difficult for the average amateur station. However, an estimate can easily
be made from a knowledge of the antenna geometry (effective height or area)
and a measurement of the antenna current. The actual ERP determined by
field strength measurements is invariably lower than the figure given by
this estimate by an amount depending on environmental factors, usually in
the range 0-10dB. So a safely conservative (from the regulatory point of
view) estimate of ERP can be made from very simple measurements.
In the European context, signal levels are present in the range of
kilowatts to megawatts ERP from numerous broadcast and utility transmitters
throughout the LF frequency range. There seems to be little interference
caused to the operation of other electronic systems by breakthrough of
these high level signals, which are obviously vastly greater than any
feasible amateur signal level. Short range RF tagging systems using the
125kHz and 134kHz frequencies seem to be successfully in use over here, in
spite of these large signal levels, so amateur LF operation would seem to
pose little threat to these systems. I don't know if the power utilities in
Europe use the same type of LF PLC systems (perhaps that is what the
"Watford Whistle" carrier on 136.647kHz I D/F'ed to some power lines a
while ago is.)
The other side to the above is that, because they are relatively of
extremely low power, amateur LF signals experience high levels of
unintentional interference from numerous sources - for instance noise
sidebands and intermodulation from high power LF transmitters (especially
Loran), and switching noise from a wide range of mains driven appliances.
These are often a severe limitation on amateur operation at LF, and would
be considerably more of a problem with the proposed US power limits.
Do you think it would be useful to send this type of comment directly to
the FCC, or would it be better to do it via AMRAD/LWCA/ARRL etc?
Cheers, Jim Moritz
73 de M0BMU
|
