Topband: Shunt Fed Tower SWR Troubles

[Guy Olinger K2AV]

I agree about the padding caps, good addition. I know of instances of both,
fixed caps with insufficient current rating, and the balun insufficient for
160m blocking.

If the caps were starting cold, there would be some period where the SWR
was normal and then start climbing.

The 160m stressed balun would change SWR somewhat, but immediately with
increased power, indicating saturation. Then after a period of operating,
when the core reached the Curie temperature, it would fall off the table.

Once in a contest at a multi/two station, padding caps got hot and failed
in parallel with a vacuum cap being used to detune a tower. I walked
outside and was confronted by a strong acrid smell. Of such I informed the
owner who could actually see the padders being destroyed in the
dark, knowing where they were. I saw those caps a few days later, and it
was like the "Really Most Sincerely Dead" song in the Wizard of Oz.

73, Guy K2AV

On Thu, Aug 27, 2020 at 3:28 PM Jeff Blaine <KeepWalking188 at ac0c.com> wrote:

> 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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