Topband: Shunt Fed Tower SWR Troubles

[John Harden, D.M.D.]

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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