Do a simple check.
You can first install one antenna on the tower and run a SWR plot. Then
install the second antenna and run the plot again. If the SWR changes, you
have an interaction problem. The other way is to install both antennas at
one time, see if the SWR curves look like they should for where you have the
antenna tuned then rotate one antenna 90 degrees and check SWR again. If SWR
changes, you have interaction.
I have a F-12 C4SXL (C3S with 2 el 40M on same boom) and have installed two
different WARC beams (one home brew 2 el on 12/17 the other Cushcraft A3WS)
In both cases the results were the same. With both antennas aligned in the
same direction 15M on the C4 shot up to 3.5:1 with 10' seperation. When I
rotated the WARC beam 90 degs the SWR came back in line. I even pushed the
WARC beam to within 3 feet of the C4 and noticed no change when clocked 90
degrees. I tried the same test with the Cushcraft antenna and obtained the
same results. It appears the F-12 with fullsize non-loaded elements can be
tempermental to close proximities of other antennas. Im sure since the C4
works on 17M without being resnonant the frequencies are close enough for
good coupling (18.1 vs 21 mHz).
However, the F-12 40M is fairly narrow banded, linear loaded and has a tuned
feed design and may not resonant too well on 15M. You can use an antenna
bridge and check for 15M resonances on the 40M. I havnt done this but think
I will for curiosity sakes.
Ask F-12 what they think or try the above simple test.
I do have a friend who has a Wilson tri-bander with a "Shorty Fourty"
mounted 3' above it and swears there is no interaction! Go figure.
Good luck es 73
Chuck KR6C
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