Hi Wilson -
A few comments on LPs -
I designed (electrical and mechanical), and have in service, a fairly large
7-60 Mhz LP. 15 elements on a 46 foot boom. The four lowest frequency
elements are 45 feet long with linear loading, with 1 5/8 OD max and up to
four wall plytubular construction. All is designed for 100 MPH wind with
1/4" ice loading. I designed it with a tapered boom feed line going to a 200
ohms feed point , 200 ohm feed line back to the mast and a 4:1 balun to the
50 ohm coax. All insulators are Lexan and hardware is stainless steel. I
wound up with Tau = .835 and Sigma = .042 which are definitely compromise
design figures although I did stretch out the spacig for the last two
elements to help out 40m operation. The resulting antenna has served me well
for ten years now and shows no signs of degregration. I love the way it
allows me to operate any of the 8 bands with only one feedline and one
rotator (a Prop Pitch Motor). Gain and F/B of course are not what you might
get from a yagi but I don't spend much time waiting in line, but then, I've
got a goodold 8877 and the LP is at 100 feet. My contention here is that
height/radiation angle and a good amp are as, if not more, important than
actual free space antenna gain until you get into the very top ranks of
competition. An LP allows you to get reasonably effective/effieient
radiation at whatever maximum height you have available. I have a
considerably rebuilt TH-6DX on a sidegate at 45 feet which is a very nice
contest supplement but certainly no competition for the LP.
Bottom line - In a well designed LP more elements probably would give you a
little more gain, perhaps a little better F/B and a better SWR curve, but
for a given number of elements a longer boom probably would probably serve
you better. From a budget standpoint I would say put your money into height
(particularly up to 70 or 80 feet) before more elements, or longer boom.
Good Luck with whatever you decide on !
Gene / W2LU
----- Original Message -----
From: "Wilson Lamb" <firstname.lastname@example.org>
Sent: Sunday, December 13, 2009 3:34 PM
Subject: [TowerTalk] Log Periodic Beam Performance
> I'd appreciate hearing of experience with LP beams, especially performance
> vs number of elements. The little T6 spec's are remarkably similar to the
> T10, but some experienced people are pretty adamant that six elements
> do a good job.
> Anything will be a lot better than the little low endfed I use now, but
> like to have some real idea of what to expect.
> The LP is great for getting me WARC coverage and avoiding traps!
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