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
On 11/13/2015 9:20 AM, Jim Thomson wrote:
The 4 el 40m optibeam yagi uses de-coupling stubs on each ele half..on all 4 x
eles. This is done so the array doesn’t resonate on 15M.
Ok, so where does it resonate then? It has to resonate somehwere if not on
its 3rd harmonic. The F12 340N resonates around 24 mhz..and will screw up
a 12m yagi....
yet the 340n does not use the de-coupling stubs.
Has anybody with an optibeam 4 el 40m yagi swept it with a MFJ-259B or an
xcvr on low power..to see if and where it does resonate. ( freqs other than
40m obviously)
Tnx... Jim VE7RF
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