Hi Dale,
The RF voltage is always looking at the far end (stuff on the end away from
the tower base) to jump onto.
*** Provide a deliberate, designed, long-term weather survivable metallic
path to the boom of the highest yagi and provide a low R&X path from the
boom to BOTH sides of at least one element at either END of the boom. This
will drastically reduce current via unintended paths.
You will have to retune the match.
This is much easier for a parasitic element, where the center of an
insulated element can be just strapped to the boom. But you should also
replace the balun. It will remain a source of RF loss, even if it doesn't
change value or burn up.
RF current heating of a "balun" comes about because an engineer is only
designing the balun for the performance at 40 or 20 meters. The R&X of such
a device on 1.8 MHz can be quite low. I have measured one that worked FB on
a tribander that was only 173 ohms on 160. That will get very hot at power
and its effect on the tuning at the tower base can be enormous. And it is a
guaranteed source of RF loss that throws away dB's of your amp's power. RF
heating of a ferrite device gets to a point where the R&X characteristics
suddenly fall off the table. 80 meters likely just doesn't quite get to
that temperature because the R&X of the balun is higher on 80m
You still may need to change that device for something with good stiff R&X
at 160, so it will not heat up and change. The 160 RF will still try to go
there. It just needs to be solidly blocked. See the Balun Designs 1116dx (x
is one of 6 or 7 hardware configurations, same innards). Or go to K9YC's
web site to make your own. Jim has some recent designs, excellent R&X for
160 based on a new monster #31 ferrite core & RG400.
Good luck, 73, and stay safe from that virus,
Guy K2AV
On Thu, Aug 27, 2020 at 1:06 PM Dale Drake <daleaa1qd@gmail.com> wrote:
> 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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