[TowerTalk] Antenna Side-mounted to Tower
Dan Zimmerman N3OX
n3ox at n3ox.net
Thu Aug 31 01:55:45 EDT 2006
J-poles are somewhat sensitive to their surroundings and can be
sensitive to the feedline details... but if you're just looking for 2m
FM, as long as you can tweak it a bit on-tower you shouldn't have a
problem. You don't exactly have a high-performance application in
mind if you're doing verticals on 2m, I expect.
That said, if you're doing a side mounted collinear you could consider
something like a 1/2 over 1/2 over 1/2 wave fed in the center with a
balun and coax... or even better (as you can get more gain) a phased
array of dipoles like they do for broadcast and repeater work, where
each element in the collinear is a separate dipole. You can also put
them all around the tower and make your pattern more circular.
The question about how far away the antenna should be doesn't have a
single answer. I think whoever told you you can't stick a J-pole four
feet away from the tower was being a bit paranoid, though. I think
it's worth toting an antenna analyzer + balun up the tower so you can
tweak the coax tap points to give a good match at the antenna... but
don't sweat this one very much.
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