Topband: receiving antennas

[Tom Rauch]

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 
those responses.

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 
gain.

Without that, all we can do is overlay the patterns (with no 
attention to gain figures) and compared by eye.

73, Tom W8JI
w8ji@contesting.com


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