[Moon-net] Effects of a metllic cross boom

[Joe Barger]

Fellow EMEers,

I tried adding a second horizontal 2M5WL using some thick walled 1 1/2" fiberglass tube as the horizontal cross-boom.  Even with a 8' solid fiberglass insert in the middle of the 14' span, the antennas droop and swing around way too much.

So, some basic questions about the need for non-metallic cross booms and potential alternatives.  The array is intended for primarily 144 MHz EME/MS and not terrestrial work.

What are the disadvantages of using vertical polarization for the antennas (besides loosing the 3 db ground gain when pointed at the horizon)?  Since most EME setups use horizontal polarization, will vertical polarization on my end result in a significant polarization mismatch?  Or is it all pretty much random polarization depending on polarizations changes along the EME path anyway?

Assuming there's a good reason to go with horizontal polarization, how much pattern distortion is introduced by a metallic cross-boom with horizontal antennas?  How far must horizontal antennas be from horizontal metallic objects to reduce the interactions?  For example, can the center of the cross-boom be metal and the last few feet be non-metallic and still avoid potential interactions?

And finally, assuming a horizontal antenna and non-metallic cross-boom of some sort, doesn't running the coax along the antenna boom and then over the non-metallic cross boom essentially nullify the value of a the non-metallic cross-boom?  It seems to me the coax shield is metallic and can potentially have similar effects on antenna pattern as a metallic cross-boom.

On the surface, the best solution for me would be to use vertical polarization on the antennas with a strong metallic cross-boom and run the coax along the cross-boom.  I suspect the optimum solution is horizontal polarization with a completely non-metallic cross-boom with the coax dropped vertically for some distance and then run to the tower.

This is probably EME 101 for most, but I appreciate the education from the experts in the group.

Thanks

joe N6KK