Topband: Shunt Fed Tower SWR Troubles

[Jeff Blaine]

Had the same issue as you Lloyd here - in a prior antenna matching net.  
On 160 it would start to drift after calling CQ a few times.  I did not 
flip to vac caps.  But instead ended up using lower valued, paralleled 
doorknobs, so that the current was shared among multiple paralleled caps 
and as a result the drift was much less from what I assume was a larger 
surface area to dissipate whatever heat there was.  Could have been a 
lousy initial cap as well.  Hard to say.

But if the op has fixed pads in there, this is definitely a place to look.

73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com


On 8/27/20 2:23 PM, Lloyd - N9LB wrote:
> Hello Dale,
>
> I had this same problem.
> In my case I discovered that it was the "Doorknob" capacitors heating up and
> changing value.
> I had installed some fixed value doorknobs to "pad" the variable cap in the
> matching circuit.
> Changed to Vacuum Caps, no more problems.
>
> 73
>
> Lloyd - N9LB
>
>
> -----Original Message-----
> From: Topband [mailto:topband-bounces+lloydberg=tds.net at contesting.com] On
> Behalf Of Dale Drake
> Sent: Thursday, August 27, 2020 12:06 PM
> To: topband at contesting.com
> Subject: Topband: Shunt Fed Tower SWR Troubles
>
> Here's my problem. I'm experiencing SWR problems
> with 160M shunt feed on my tower.  Using a gamma
> match with HV air variable caps I am able adjust
> the tap point to get a very nice SWR curve using
> my AA-35 Zoom analyzer.  The problem I having is
> that when transmitting the SWR goes up with power
> and if I key-down for very long the SWR rises
> rapidly and takes off.
>
> My set up is 70ft Rohn 25 with about 22 feet of
> mast above the top of the tower.  At the top of
> the mast is a Diamond 2M/440 vertical.  At 15 feet
> above the tower top is an XM-240 and 2 feet above
> the tower top is a 4el SteppIR.  The elements of
> the SteppIR are fully retracted.  All of the coax
> and control lines are run from ground level inside
> the tower up to the service loop. On the mast
> above the tower the coax is tywrapped to the mast.
> There are 60 ground radials with an average length
> of 90ft.  The reflector of the XM-240 is insulated
> from the boom and the driven element is fed
> through a Comtek 1:1 balun. All of the coax
> shields are grounded at the tower base.
>
> I have an 80M gamma matched shunt on this tower
> that plays FB with no weird SWR stuff going on.
>
> I suspect that what is causing my 160M trouble is
> that RF is coupling through the 40M coax, through
> the balun and into the driven element. Somehow the
> coupling changes with power, which I don't
> understand.  I suspect that when I key-down and
> the SWR takes off, the core in the balun is
> getting hot and the magnetic properties are
> shifting until it cools off and returns to normal.
>
> My plan to try to correct this problem is to
> connect the center of the XM-240 reflector to the
> boom and to mount a box at the XM-240 feed point
> with 2 vacuum relays that I will use to connect
> the driven element to the boom when I'm on160.
> This will be a considerable effort and expense so
> I'm looking for the group's input as to the
> soundness of my plan or if there may be other
> approaches that would be less difficult to try
> first.
>
> Thanks,
>
> Dale, AA1QD
>
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