|To:||email@example.com, "'Towertalk'" <Towertalk@contesting.com>|
|Subject:||RE: [TowerTalk] HF LOG-PERIODIC ANTENNAS Comments Please|
|From:||Pete Smith <firstname.lastname@example.org>|
|Date:||Mon, 21 Jun 2004 13:54:47 -0400|
At 11:16 AM 6/21/2004, Keith Dutson wrote:
Stacking LPAs is rare to my knowledge. The problem is in planning how far to separate them. For best 20-15-10 performance the distance should be about 33 feet. But this will not work well for 17 and 12, so you will get better performance working each antenna separately (unphased) on these bands.
I agree that the LPDA is not the right solution -- it is designed for acceptable swr and gain over a wide, continuous frequency range, at the cost of gain and F/B in the ham bands, weight and wind area. However, if you chose to go that route, I think that modeling can answer the question about stack spacing.
I have not modeled an LPDA stack, but short-boom tribanders generally require closer spacing than those with longer booms and/or more elements. Given conditions for the next few years, I would go for compromise spacing that prioritizes among 20, 17 and then 15 meters, letting 12 and 10 fare for themselves. Modeling might also disclose spacing that truly optimizes for one band -- for example my 2-high C-3E stack is currently at 28 feet spacing, but modeling shows that if you focus on 20 meters, spacing of 36 feet is a real sweet spot, yielding excellent gain and F/B ratio much better than either antenna separately. I may be moving my bottom antenna down in a year or so, just for that reason.
73, Pete N4ZR
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