[WWYC] Log Periodic

[Martin Monsalvo, LU5DX]

Are you feeding the antenna with the same transmission line used when it's
up the tower?
If so, seems like the tx line is misbehaving.
Have you tested the balun?
Is there an additional current choke balun at the feed point?
Vy 73.

Martin, LU5DX

On Fri, Sep 7, 2012 at 7:17 PM, Paul MJ0PMA <paul at jerseyars.com> wrote:

> Hi All,
>
> Im turning to you guys for help.  At GJ2A we have spent the summer
> replacing our damaged roof mounted versatower with a newer P40 which will
> put our main beam and LF dipoles at 60ft AGL.   All that is finished but
> the club treasurer donated his Titanex LP6 Log Periodic.  Its a very
> interesting antenna with 2 parallel 40mm diameter booms acting as a
> balanced transmission line to feed each element alternately.  I built it
> exactly to factory spec with element lengths and spacing measured to +/-
> 1mm but when it went up on the tower the SWR curve was all over the place
> with a good dip to around 1.2:1 around the 15m band and everything else
> nearly 3:1.  So we brought it down and discovered that the clamp which
> feeds the positive side of the boom was nearly open circuit.  So with that
> cleaned up we tested it again whilst resting on the concrete roof of the
> club.   The Curve was great with dips on all 5 bands down to 1.2:1.  They
> were just resonant 100k or so low which is expected wh
>  en close to ground and the tower.   However when it went back on to the
> tower the SWR curve returned to a mess with nothing below 2.5:1.
>
> I assumed it was an intermittent fault with the feedline so brought it
> back down and the plot looked pretty good again, although not as good as
> first time around, as soon it goes back up the tower it is detuned again.
> I'm completely at a loss now.  I have tried holding the coax clear of the
> metalwork, nudging the elements and boom with a pole, shorting the booms
> and all sorts but nothing improves the SWR.  There is also nothing shorting
> the boom to the stub mast or any metal except for the tower that could
> affect it.
>
> Any ideas or does anyone have one of these monsters?  Im very close to
> giving up and sticking a Cushcraft A3 up there to be done with it, but that
> will lose a lot of gain and the WARC bands!
>
>
> 73,
> - Paul Ahier, MJ0PMA
> Competition Secretary, Jersey Amateur Radio Society (GJ3DVC) & Jersey
> Contest Group (GJ2A/GH3IT)
> - www.JerseyARS.com <http://www.jerseyars.com/> -
>
>
>
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-- 
Martin Monsalvo, LU5DX
lu5dx at mail.ru
http://rurls.ru/lu5dx