TopBand: Shunt-fed tower - The culprit !

[Ron Stone]

Following recent bulletins on this and tests after the WW contest the
culprit has been identified beyond doubt. 

It is the 40-2CD at the top of the tower. 

I have a coaxial relay at the top of the tower which can choose between
the 402CD and an unused port. Switching the 40m feedline (about 25 ft
between the relay and the yagi feedpoint) to either the yagi or the
unused port gives the heating / shifting SWR problem,. As several (N2IC
and others) hve suspected the balun is heating up and causing the SWR
drift. 

During the WW, I disconnected the feeder from the 40m yagi at the relay
box. No VSWR drift at all - and for those who heard me - for most of the
contest the vertical was being used. It worked well. 

So what do I do now chaps? I dont want to take the yagi down as it is
January (and it also works well). The ideal solution would of course be
to put a double pole relay at the feedpoint and take both sides of the
402CD driven ele down down to the boom near the feedpoint. I may do that
in the summer, but does anyone have a better idea that would not involve
remote relays and so on? I guess gamma matching the 402CD would also be
a neat idea, but are there any solutions I can implement without taking
the yagi down?

I havent tried earthing the inner of the 402CD while the outer is
connected to the relay box. May try that next. 

Any useful ideas appreciated !!

73

 
Ron Stone
Vine Antenna Products - for World-Class RF Communications
Telephone +44(0)1691 831111
Mobile          0585 785659
Facsimile +44(0)1691 831386
Mail - The Vine
       Llandrinio
       Powys SY22 6SH
       Wales, UK

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