[TowerTalk] Dipole gain?

[Jim Lux]

On 12/11/14, 3:35 AM, Stan Stockton wrote:
> I'm talking about 350 numbers averaged, for example covering the 20 meter band - not averaging two numbers like 10 and 20 dB and I'm suggesting that because an antenna can have in excess of 60 dB of front to back at a few frequencies that the F/R number is more important.  Unless you are a direction finding person or trying to null some particular source of QRM, front to rear is a better number to look at.   The thing to avoid is for one manufacturer to claim 35 dB of F/B when in reality that occurs at 14.287 and there is no place in the band where Front to Rear is even 20 dB while another antenna is overlooked because it is shown to have 18-20 dB of F/R across the band.
>
>

That's similar to typical kinds of specifications you see on radar antennas:
Integrated Sidelobe Level Ratio
Maximum Sidelobe Level Ratio

And other ones like "maximum sidelobe width".


There's already a similar ham statistic called "receiving directivity 
factor" (RDF)
Tom Rauch(W8JI) and Greg Ordy (W8WWV) have been advocates
http://www.seed-solutions.com/gregordy/Amateur%20Radio/Experimentation/RDFMetric.htm

ON4UN suggested a slightly different one he calls the Directivity Merit 
Figure.


Then, there's a whole issue of "what's important in a directive 
antenna".. Most HF antennas have pretty broad main beams: a 10 degree 
change in azimuth doesn't have a big change in gain.
Many HF antennas have fairly narrow nulls.  A 10 degree change in az 
might have a 10-20 dB change in gain.

it might be that what you're really looking for in a receive antenna is 
the ability to put a null on a strong interfering signal (be it 
atmospheric QRN, SW broadcasters, or the other hams in your town  in the 
pile up trying to work some ham in a rubber raft anchored over a rock 
that emerges above the surface for an hour a day).

In that case, the "best" antenna might be one that can "place a deep null".

As noted elsewhere, for a given physical size, the forward gain is 
pretty close among all the different designs, so for "squirting the 
maximum signal in a desired direction", I would posit that it doesn't 
much matter what antenna you use.  Make the decision based on mechanical 
features, ruggedness, ease of feeding/matching, or whatever is important 
to you.