You make no mention of the 40/30 design, which will significantly impact
the interactions. About the only "for sure" interaction is 40m to 15m,
so expect the TH11 15m to be seriously disturbed, pattern, swr and F/B.
That alone probably requires the 40/30 dipole to parallel the TH11
boom. OTOH if any parts of the 40/30 have harmonic relationships to the
TH11 boom resonances given the loading from the elements, then there may
be problems with the 40/30 performance when boom parallel.
The other wisdom is more separation is better.
Re "in the same plane": Interlaced multiband antenna design (all on one
boom) is very difficult as I am finding with several of my efforts. By
"difficult", I mean preserving the performance of isolated single band
optimized same configuration designs. I have significntly increased
respect for those designers with proven interlaced designs. Much
experience and art is needed in addition to tools such as EZNEC Pro/4
and AC6LA's AutoEZ. You don't want to embark on such a design effort
without modeling automation and optimization, as many many design trials
will probably be needed. Of course, you will need a good TH11 model to
start. I've also found that 17m interacts significantly with other
bands, although I have no ready explanation for that.
My choice of tradeoffs - F/B, gain,and SWR would be different than yours
in the modeling of interactions. Allowing SWR to go from 1.1 to 1.6 is
relatively meaningless if F/B degrades by 6 db or gain by 2 db.
Unfortunately, the only easily measured implemented physical property is
SWR. Real pattern measurement is hard and gain measurement virtually
impossible for amateurs. That's the bad news. The good news is the
modeling programs are very good at predicting actual performance, if the
model is well put together and accurately reflects reality (i.e no
Q=5000 traps).
My advice is get at least EZNEC+ and build some models and learn from
them before cutting aluminum and climbing.
Grant KZ1W
On 5/10/2016 5:40 AM, N1BUG wrote:
I am sure this subject has been beaten to death many times but I am
not finding all of the wisdom I seek either on the bookshelf or in the
archives of this list.
I have a TH-11DX at 100 feet. I plan to build a 40/30 meter rotatable
dipole which I want to put above the TH-11. I assume (uh-oh) that with
the close frequency spacing of 20-17-15-12-10m it will be difficult or
impossible to get resonances of the 40-30m antenna far enough away
from all of those bands to get away with putting the elements in the
same plane. This assumption however leads to two questions:
1) I'm not sure this can be answered in general terms, but: How far
removed from a band does a resonance have to be in order to not cause
significant problems? I don't mind a little distortion of pattern or a
dB loss of gain but significant SWR changes are not to be tolerated.
2) Is there a simple way to measure said resonances of the 40/30 meter
dipole that will be good enough for this purpose? I have an MFJ
antenna analyzer at my disposal.
Assuming putting the 40/30 dipole in the same plane as the TH-11
elements is out brings me to my next question.
3) If mounted in the same plane Will the boom of the TH-11 cause
problems for the 40-30 meter dipole? How far above the boom should I
be trying to mount it for good performance? Four feet could be done
very easily but I worry that may be too close. I could get a few feet
more with extra work and expense. Anything beyond 8 feet is probably
not possible in this instance.
I would appreciate any comments or advice I can get before I get too
deep into this project.
Thanks,
Paul N1BUG
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