James: I'm not sure if you are going to feed these antennas with a power
divider and one coax or separately. At my QTH I have a stack of 144 and 432 MHz
beacon antennas about 15~20 feet away from some chimney mounted yagis, both
vertically and horizontally. Out of curiousity one night I ran the 2M 10dbd
yagi and the pair of PAR loops with a T-connector in the shack. There were a
few cases where the combined pair was better than either individually on local
stations, although the F/B and F/S was obviously reduced to some what of a
squash plant pattern as far as I could tell. I did check the isolation between
the antennas individually. With about 10W ERP from the omnis, I had several
beam headings with a received beacon signal of -10dbm and one beam heading
around 0 dbm. Neither of these antennas are entirely clear of nearby trees, so
there are obvious near field reflections going on. I'd expect the inverse
square and cube laws to prevail for near field coupling (electromagnet
ic and inducation, respectively). If you run separate coax lines and terminate
the unused antenna into a good 50 ohm load the re-radiation should be limited
to the return loss of the termination (30, 40, 50, etc. db?? depends on what
you have). I'm not sure what effect on the re-radiation would be if you used
two coax lines and an A/B switch that grounded the coax on the unused port. You
would have to check that with a power meter and directional coupler in several
electrical length locations along the shorted coax to get a better
understanding of the isolation in that situation. 73's Mike wa3tts
I have a stack of 4 halos on 432 with a 15 db 432 Mhz yagi on the chimney. I
have not checked the isolation on that band with my spectrum analyzer, but
could do so if you think the results are of interest. Again, the antenna
separation is around 15 to 20 feet horizontal and vertical for my QTH.
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