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Technical Topics => Equipment => Topic started by: i_hear_you on June 03, 2019, 2010 UTC

Title: Multiple dipole positioning
Post by: i_hear_you on June 03, 2019, 2010 UTC
Hey all,

Awhile back I purchased a four-way remote antenna switch kit from the bay and intend to hang several dipoles cut to frequencies of interest.  The idea is a long run of coax to the switch, with the shortest possible coax lengths to the antenna feed points.  What sorts of orientations would minimize negative interactions between four dipoles?
Title: Re: Multiple dipole positioning
Post by: ChrisSmolinski on June 03, 2019, 2026 UTC
In theory you want the dipoles to be perpendicular to each other, to minimize interactions. That said, assuming they are going to be relatively (in terms of wavelength) close to each other and the ground, you'll probably get some coupling effects no matter what you do. 

I have lots of dipoles (and other HF antennas) and some of them are relatively close to each other. I don't notice any ill effects (although maybe I don't know that I have any ill effects since I have not done before and after comparison tests, but I am generally happy with how they work).

I run all my feedlines into the shack, so I can switch between various antennas and radios. See my various posts here on the HFU for how I do this, with buried conduit and grounding blocks to run the coax, and a patch board for selecting what goes where. Coax loss at HF is incredibly low, so I am not worried about a few more feet of length. If you have only one receiver, then it might indeed be easier to only have one coax line enter the house.
Title: Re: Multiple dipole positioning
Post by: Josh on June 03, 2019, 2043 UTC
If they're all grounded together by the switch when not selected, they're participating in the pattern anyway so there's that to consider. The best thing about this single feedline paradigm is it keeps rf from coming back down a separate feedline to the shack. Some guys have popped front end parts in rigs by trusting a switch that should have grounded the antenna connected to that expensive rig but the 2kw coming back down that coax from the antenna/feedline/rig/amp in use done burnt things up.

I have commercial 80 and 40 dipoles laying around doing nothing and been mulling over connecting both to the same center support and coax, with the ends separated a few feet per a SRI research paper on fan dipoles. The internal tuners of most HAM rigs should be able to get a match on most any HAM band with that setup, and no risk of rf flowing back down into the shack from the unused dipole as in the case of separate dipoles and feedlines.

Title: Re: Multiple dipole positioning
Post by: i_hear_you on June 04, 2019, 2023 UTC
Maybe a 20/40 fan is a good bet for starters.

I'm running QRP so I am trying to be as efficient as possible, but it's sounding like the switch may be too much hassle.