To: <topband@contesting.com>
>Date: Sat, 08 Nov 1997 19:00:52 -0800
>From: Bill Hohnstein <k0ha@navix.net>
>
>Earl W Cunningham, K6SE wrote:
>>
>> ...This time I took greater care in determining the optimum
>> termination resistances...
>>
>> ... I ... viewed the elevation pattern to "determine" the best
>> termination for low-angle (groundwave) rejection to the rear...
>>
>
> Why is there so much concern on Beverages and Yagis on front-to-back
>ratio? I always design my Beverages and Yagis to have a small back
>lobe with two nulls on either side of it to increase the size of the
>null area...
>
>73, Bill K0HA
>
Bill,
A lot depends on what you call F/B ratio. Most of the modeling
programs simply take the ratio of the pattern max and the pattern
directly (exactly 180 degrees) behind the max. I am guilty of
talking about F/B using what I have in mind for it rather than
the strict ratio as defined above. I use something that is very
close to what you describe when I talk about F/B in the context
of Yagis. That is (for me at least) the ratio of the patterm max
to the largest sidelobe behind the first nulls. Designing
antennas to this criteria results in designs with patterns pretty
much as you describe. Its pretty amazing how much better
receiving antennas these are than ones designed just for maximum
gain. It is usually possible to keep all sidelobes 22 dB or
better below the main lobe without giving up more than a bit over
1 dB forward gain.
I'm not sure how you go about controlling sidelobes in a beverage
(other than changing its length). But to reduce unwanted
response from directions other than forward, it is necessary to
have the beverage correctly terminated.
73, Eric N7CL
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