[WWYC] Log Periodic

[Paul MJ0PMA]

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 when 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 -