Steve,
Interesting.... my plans are for a 40M4LLDD mounted ~31’ above the top antenna
of a 2x6el stack on 15... when I model something similar (I happen to, at the
time, be looking at a moxon on 40) I am seeing essentially no change in the SWR
of the 15 and a reduction in the FB of about 3 db (of course at a particular
frequency – from 33 db to 30 db). No change in gain (actually a tiny increase
as is not unusual) ... I had modeled a 20 stack (my plans are for 2x5el – 20’s
and the top one is 11’ above the 40) using the 20 stack and a full size 2
element 40 and this shows virtually no interaction on 20 from the 40. I’d love
to model the 40M4LLDD directly but suspect it wouldn’t be correct using
EZNEC6/NEC2.
Bills question extended to 10 meters – I’d suspect there’d be little
interaction but it would be interesting to hear about this (my main tower won’t
have 10 on it) ... note as others I’m sure have discovered – if your SWR is
effected by proximity of antennas – your FB is WILDLY effected! Generally the
FB is the most sensitive. Modeling guys for example – the ARRL AB dimensions
for ‘safe lengths’ ... aren’t! They couple enough energy, I’ve tried a good
variance of lengths, even a good distance out from the tower, and they do
effect the FB. Likewise my latest modeling shows using only ~50’ of philly
nearest to the tower for the bottom 2 sets of guys and 70’ for the top set does
a great job with limiting interaction to about 2 db worse case with no change
in the gain or in the SWR.
Gary, K9RX
[still looking for R55 tower .... help?]
There is a big design difference between a linear loaded M2 and a High-Q
coil loaded OptiBeam with detuning stubs, however I think neither antenna
is resonant on 15m. I do not see any issued with the proposed layout. I
am sure that DF2BO would be very responsive if you asked OptiBeam this
question.
John KK9A
The M2 40M4LLDD is close to resonant on 15 meters. I have one, and the SWR is
about 1.6 across the entire 15 meter band. I have even used it as a 2nd radio
antenna on 15 meters.
I would be very cautious about putting it in close proximity to an Optibeam
16-3 or any other 15 meter antenna.
73,
Steve, N2IC
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