Topband: receiving antennas
Wed, 18 Oct 2000 08:27:50 -0400
On receiving, within the limits of internal noise of a system, gain
does not matter. Neither does efficiency.
If two antennas are compared on the basis of gain, or take-off
angle, the result is basically meaningless.
None of the modeling programs we use will allow us to directly
compare (in numerical results) the relative usefulness of two
antennas....or different antenna lengths. None of us can look at a
"gain figure" and decide from that if one antenna is better than
another, because gain gives only the absolute signal level.
Gain does NOT give S/N ratio. Gain is almost always meaningless
for VHF and lower receiving antenna systems. The only exception
is if system internal noise approaches or exceeds absolute signal
level of the desired signal. On 160 meters, a one foot piece of wire
will have more "gain" than needed even in the quietest locations
when used with a good preamplifier!
If we want to compare two antennas, or optimize the length of an
antenna, we need to compare the sum of noise response from all
directions of noise arrival to the response in the direction (and at
the angle) of the desired signal! Pretty much nothing else matters.
The only way I'm aware of to easily compare antennas (without
trying them) is to print the pattern plots of the antennas and
compare the patterns. The antenna with the highest ratio of
response in the desired direction to responses in all other
directions will be the best receiving antenna, assuming the noise
arrives equally from all directions. If you have local noise, or noise
is "concentrated" in a definite direction(s) then you must compare
The person would could figure out how to use the pattern tables
that are exported by Eznec in a spreadsheet to determine S/N ratio
would be a top-band hero, because the data would express the
effectiveness of an antenna for receiving. We need directivity, not
Without that, all we can do is overlay the patterns (with no
attention to gain figures) and compared by eye.
73, Tom W8JI
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