For a homebrew large beam I think it is easier to assemble with the
elements on top. It is easy to level the elements this way and connect the
coax to the DE. I use the DX Engineering element brackets on my 20m beams
and the bracket pitches the element up slightly which helps minimize
overall sag. If you use a boom support it will try and flip regardless of
how the elements are attached unless you take rigging precautions to
prevent this. Last week I assembled a Force 12 Tribander in St Croix USVI
and the elements were underneath the boom. It was a small antenna and
since the brackets were riveted to the boom so there was nothing to level.
Assembly was pretty easy on the lawn except I had to prop it up to attach
the feedline which was only easy because it was a light antenna.
John KK9A - WP2AA
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Booms
From: jimlux <jimlux@earthlink.net>
Date: Fri, 10 Mar 2017 19:55:19 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 3/10/17 7:12 PM, Ed Karl wrote:
Hey Troops-
I've been around for a while, always wondered about this. Looking at the
discussion
of square vs round booms. How come the elements are on top of the round
book?
Seems like less inclination to rotate out of alignment if they were
already on the bottom ...
Tradition?
Elements are on top with round booms too, probably because of assembly
processes If you're assembling it on sawhorses, it's easier to have the
boom sitting there, and then bring the elements over and mount them one by
one, on top of the boom. If they're on the bottom you have to thread them
around the sawhorse legs, etc.
And then once it's assembled, who wants to try and flip it over (although
it's not that hard, it is unwieldy.
I can't think of any *electrical* reason why you'd care. On some LPDAs,
the elements alternate top and bottom (both across and along the boom) if
the boom is the transmission line.
(and we'll leave aside any theories about the fact that the north facing
owl will tend to sit on the element rather than the boom, because it's
higher, and if you put the elements on the bottom, you'll have a 90 degree
pointing error...)
On VHF/UHF arrays with CP antennas, a common error is to not have all the
antennas oriented the same (there's this tendency to make it mirror
symmetric, because it "looks" more balanced). I may overstate, I've seen
this error once, but I've only seen an array being assembled a few times.
Generally by the time most people look at it, it's been fixed.
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