Topband: Shunt Fed Tower SWR Troubles

[wb6rse1 at mac.com]

I experienced wide spaced air variables flash over at moderate to high power in my shunt feed omega match. I changed to vac variables. That solved the problem with the additional benefit of easier tuning via the vac cap's verniers.

GL - Steve WB6RSE

> On Aug 27, 2020, at 10:58 AM, John Harden, D.M.D. <jhdmd at bellsouth.net> wrote:
> 
> I am wondering if the air variable is flashing over. I am willing to bet that a vacuum variable would correct the problem. I use two (2) Vacuum variables in an Omega match.
> 
> 73,
> 
> John, W4NU
> 
> 
> 
> 
> 
> 
> Sent from my iPhone
> 
>> On Aug 27, 2020, at 1:38 PM, Guy Olinger K2AV <k2av.guy at gmail.com> wrote:
>> 
>> 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 at 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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