Hi Tom,
Tell your friend to forget it. He will just screw something else
up. LPDAs do not operate in any sense like most hams are used to
thinking about. In particular, it is very important that all the
elements have the same geometrical relationship to one another.
But it is completely unimportant exactly where the element
resonant points actually fall within the frequency range. The
vast majority of the time he will be operating with _NO_ elements
at resonance (but one may be pretty close).
The SWR will be whatever it will be. It will _NEVER_ be high
enough that it constitutes a problem operationally. Especially
if his TX has a tube final or has a built in tuner. If he has a
solid state radio that can't handle 1.7 or 2:1 SWR, he may want
to get a good "T" network tuner. But operating the system with a
tuner in line at a 2:1 SWR costs nothing in terms of performance
unless the line losses are already excessive or the tuner is
junk.
There will be interactions when the resonant antenna elements
(not in the LPDA) are not separated enough from the LPDA and the
antennas are aligned with the elements in parallel.
Interestingly, the LPDA itself will still be working pretty well.
It somewhat self compensates for junk in its vicinity. But the
pattern will probably suffer due to reradiation from the
offending Yagi elements, and losses may go up. Pointing the LPDA
90 degrees relative to the other antennas will minimize this type
of interaction. Then it is down to praying that none of the
booms are resonant in ham bands between 20 and 10 meters.
73, Eric N7CL
To: <towertalk@contesting.com>
>Date: Thu, 07 Oct 1999 17:55:21 -0500
>From: Tom Hammond - N0SS <n0ss@earthlink.net>
>
>Hi Guys:
>
>I'm asking this question for a friend who has ordered a KLM LPDA
>for use the 'higher'bands, 10M-20M, or so, I think.
>
>He tells me that if the SWR doesn't fall into those (CW)
>portions of the WARC bands where he wants them to fall, it is
>his intention to 'tune' the element for that band so that the
>SWR is more acceptable.
>
>My concern is that in doing so, he may wind up screwing up the
>entire alignment of the array.
>
>Any comments?
>
>Finally, given that this will be an LPDA, and purportedly
>resonant on most of the bands (10M-20M, at least), what type of
>separation is he gonna have to have if he's gonna put it on a
>rotating tower along with monobanders for 40/30/20/15?
>
>Don't bother telling me to tell him to buy something else, or
>such, it's already on its way, or maybe already here. I just
>want to keep him from screwing it up, if that's possible.
>
>73 to all, and thanks to anyone who takes the time to reply.
--
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