My OB2-40 model has similar SWR dips around 7.1Mhz, 23MHz and 48MHz. I
have not done a SWR sweep on this beam and there are many antennas around
it that would likely alter the results. I may be modeling this
incorrectly, however I tried removing the stubs and I see no real
difference in the SWR dips. It would be interesting if I could use this
antenna on 6m!
John KK9A
To: towertalk@contesting.com
Subject: Re: [TowerTalk] 40m optibeam de-coupling stubs
From: David Gilbert <xdavid@cis-broadband.com>
Date: Sun, 15 Nov 2015 16:35:29 -0700
My OB2-40 isn't connected up right now so I can't scan it outside the
band, but I do have a pretty good EZNEC+ model of the antenna that I
worked up based upon actual physical measurements I took of everything ...
elements, coils, and the decoupling stubs. That EZNEC+ model gives SWR and
gain plots virtually identical to those supplied by DF2BO, as well as
identical to actual SWR and pattern plots I took once the antenna was on
the tower ... which surprised me a bit since NEC2 isn't known for modeling
closed spaced elements all that well.
Anyway, my model shows no resonances above 40m until a small dip at 29.5
with about a 12:1 SWR at that point. There is a rather strong resonance at
47.8 MHz (about 1.2:1 SWR) which matches my on-the-air experience that I
can get a decent SWR and slight directivity at the low end of 6m with the
OB2-40.
I'm not actually sure the antenna "has to resonate somewhere" near it's
third harmonic. It seems to me that the decoupling stubs could conceivably
kill the resonance there without shifting it.
For what it may be worth ...
73,
Dave AB7E
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