In a message dated 5/18/00 8:52:58 AM Eastern Daylight Time,
[email protected] writes:
<< ERP will be calculated on the basis of estimated radiation
resistance and measured antenna current for each antenna. For
the inv. L this should be about 20 milliohms and 2A, for the main
mast about 0.7 ohms and 0.34A, both giving about 80mW radiated
power in theory.
By definition, two transmitters/antennas giving the same ERP will
yield the same signal strengths at equal distances. But a number of
things may modify the actual ERP obtained ... >>
Please don't misunderstand my earlier inquiry, Jim. I think this experiment
is worthwhile and may yield much useful experience. I'm just concerned about
the seemingly rather casual use of the term ERP, and the tendency to equate
it with power dispersed in radiation resistance. These are very different
things.
ERP _is_ actual ERP, and can only be actual ERP. That is, ERP is determined
by actual field strength. It therefore already includes antenna and nearby
environmental losses; hence, equal field strength is the only way to actually
say the ERP of two systems are equal, not the power in the radiation
resistance. (Measuring the signal out of two different systems with equal
powers in the radiation resistance is instructive, of course, because it does
permit one to compare the interaction of two different radiators with their
environments. I take this to be your fundamental objective.)
Someonce commented earlier--I was thinking it was Rik, although I can't find
the post now--that there were those who believe that one watt ERP from one
kind of antenna is better than one watt ERP from another kind. I got a good
chuckle from that, but actually there is a certain grain of truth to the
concept...particularly when the radio authorities set a maximum ERP limit on
transmissions.
This means that the field strength in ANY direction or elevation cannot
exceed so many mv/m at so many km from the antenna. (Different
administrations may choose to define the distance differently for their own
regulatory purposes, and the corresponding field strength will vary inversely
with that part of the definition.) The key words in the definition of
maximum ERP, though, are usually "maximum field" in "any direction" from the
antenna. I supervise a number of FM broadcast stations, by way of showing
why this is significant. One has a maximum ERP of 100kw, yet covers less
than half the geographical area of another one that also has a maximum ERP of
100kw, over similar terrain. Why? Because the former uses a directional
antenna. The _maximum_ ERP is indeed 100kw in the main lobe, but is less in
all other directions around the antenna than its omnidirectional counterpart.
If any two given antennas have the same 1w ERP in the same direction, and it
happens to be the direction you need to reach, then all is well. You have
equal field strength...in that one direction. But, if that maximum 1w ERP
happens to be in a less useful direction from one antenna (say, straight up)
than the other, then yes, 1w ERP produced by one antenna can be different
from 1w ERP produced by another!
Neither any theory nor practice that I can find indicates two electrically
short verticals (one roughly .004 wavelength and the other .04 wavelength,
for instance) would produce any different radiation pattern. If you can
force enough power through them to achieve the same ERP, it will be achieved
in all directions, barring local obstructions.
Your experiment, however, is not so much about big versus small, but between
antennas of differing shapes. That's what I think is interesting: can
amateurs make up for terrible inefficiency by directing their energy with
other geometries? And if so, can it be done predictably, or will we always
be imprisoned by trial and error? I'll be watching with keen interest. Good
luck.
73,
John KD4IDY
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