[VHFcontesting] omni and yagi combo

[MICHAEL SAPP]

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 (electromagnetic 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.