[TowerTalk] Cushcraft D3W SWR Issue

[John]

Hi Serge,

 

Have just joined this reflector.  I have used a D3W for possibly 12 or more years.  It's stacked about one metre above the TH5 and about the same distance below the Create 6ele 6M beam.  Until about 5 years ago the D3W worked just fine with very low SWR on 10 and 18 MHz but not so well on 24MHz.  Then like your D3W it started to display poor SWR's.  I guessed the traps were zapped but then in the summer the SWR was generally OK - a fair weather antenna!  I had it down and checked it over, replaced the coax but it was just the same.  The poly support stay that is fixed between the 24 and 18MHz traps to hold the antenna in a horizontal position looked a bit dirty with moss, so I wondered if this, when it got wet was detuning the 24MHz elements/traps and therefore upsetting the whole antenna.  I changed it for 180lb fishing line and everything was again OK for a couple of years.  A few weeks back, it was back to square one.  The fishing line looked clean.  This time I have opened all the traps and can find no fault there. Cushcraft traps are easy to get into.  Just drill a 3mm hole where the punch marks are and slide the inner part out.  When reassembled put a few punch marks beside the holes you made and cover the holes with self amalgamating tape.   The antenna is on a 3 metre pole while I work on it - no poly element support stay and the SWR is again excellent on 10 and 18MHz.  However 24MHz is about 1.3:1 and adjusting that element will not improve it - 24MHz has a very broad bandwidth.  I would like to be able to measure the resonant frequency of the traps but cannot think how that may be done.  Ideally they should be just LF of the lower band edge but I note that on the A3WS beam, exactly the same traps (type TB1) are used for all three elements.  I would have expected there to be a difference.  Any advice on how to measure the resonant frequency of the traps would be welcome.  I have GDO, frequency counters etc.

 

I have not offered you any solution to this problem but you have an account of what I have done to try to overcome the problem that we both share.  I do still think that the support stay may be the culprit.  When the temperature rises above freezing I will pursue the matter further.

 

John

G3HTA