I'm posting this in the probably vain hope that one of you guys has seen
this situation before. I know that the 'real' answer is either a) model it
on a computer or b) experiment - but maybe I can short circuit what is
looking like a very annoying debugging session if one of you has experience
with this kind of thing... Here is the issue:
I have a single, free-standing Trylon tower, 72' tall. Sticking out of the
top is 16' of 2" mast. From top to bottom are:
Force 12 EF-180S @ 83'
H/B 2 ele. 40 meter yagi @ 80' (electrical mimic of Mag 240N, not resonant
on 15 meters)
Force 12 C-31XR @ 71'
160 meter inverted V apex at 68' (one leg is coiled up, the other leg is
extended)
Force 12 C-3E @ 40'
All the antennas are aligned - i.e. they all radiate the in the same
direction. The tri-banders are fed with 50 ohm coax, the 40 and 80 are fed
with 75 ohm hardline cut to the 'right' length.
The EF-180S has a relay that disconnects the two element halves so that it
doesn't interfere with the tri-banders on 20 meters when it isn't being
used.
The problem is that both tri-banders are messed up (SWR) on 10 and 15 meters
only. They are both 'fine', i.e. operate as specified, on 20 meters.
The C-31's upper 2:1 SWR points are at 28.400 and 21.200. On 15 meters, it
is resonant at around 20.300.
The C-3's upper 2:1 SWR points are at 28.400 and 21.270.
In each case, the 10 and 15 meter SWR curves are displaced low, as if the
antennas are coupling to something on those bands.
I am almost certain that both antennas are built properly. The C-3 was up
at another location and performed perfectly - the elements were not
disassembled when it was moved. The C-31 could be built wrong, but it is
the 3rd one that I have built, the other two are fine, and I don't think I
messed up.
I have played around some, trying to fix this.
On the C-31, I tried all the 'standard tricks'. I played with the 20 meter
beta coil. I played with the parallel feed wires that connect the 20 meter
DE and the 10 meter DE (spacing them closer together, or further apart). I
rotated the C-31 so that it was 90 degrees to everything else. All of these
things made 15 meters on the C-31 worse, i.e. moved the resonant point down.
The best case, i.e. the one I described above, is with the parallel wires as
close together as possible, and the beta coil compressed all the way, and
the antenna aligned with the others.
Any hints, suggestions, comments, tricks, etc?
My only guess is that something on the tower is coupling to the two
tri-banders. What I can't figure out is what could be effecting them both
(given their separation), and what it could be if rotating the C-31 makes it
worse, not better... Could it be the one leg of the 160 meter V? That is
the only thing that is close to both antennas, and long enough to possibly
be the problem.
Thanks!
***dan, K6IF
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