How far off the deck? - AntennaRestrictions Forum at eHam.net

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eHam.net Forum : AntennaRestrictions : How far off the deck?

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How far off the deck?	Reply
by TACTIC on August 19, 2001	Mail this to a friend!
If I mounted a short (5-6') tower on a flat roof and mounted on that tower (stubby though it may be)say a 3 element yaga, beam or log type antenna. What sort of distance from the roof would it have to be? What sort of ground plane would I need if any? Does the hight of the deck need to be 1/4 - 1/2 wave? I may be able to mount such a tower on the flat roof of my building (14 floors) in the center so it can not be seen from GL. There are very high winds at such a hight.

RE: How far off the deck?	Reply
by WB2WIK on August 20, 2001	Mail this to a friend!
This depends on the frequency (MHz) and also the deck material. If the deck is non-conductive, height above it will not be nearly as critical as if it were conductive. For a conductive rooftop/deck, 1/2-wavelength above it, for the frequency intended, will allow the beam to begin working like a beam and provide approximately the pattern predicted by emulation software. This is obviously very easy to accomplish, at very low heights, for VHF-UHF work; not so easy to accomplish at, say, 20m, where 1/2-wavelength is 33-1/2 feet. Actual results may be accurately predicted by computation using any of the popular antenna modeling programs, e.g., EZNEC, a demo version of which is available for free. I've found a small log periodic dipole array that works extremely well on 14-18-21-24-28 MHz even at rather low installation heights is the Tennadyne model T-6. It's small and lightweight, will take very strong winds, and seemingly works while laying across a couple of saw horses in the backyard (essentially 3' above ground), as I had it during initial testing. You might consider one for your tripod rooftop application. 73 de Steve WB2WIK/6

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