[TowerTalk] VSWR Changes with Power

[Kimberly Elmore]

 Congratulations on chasing this down. 

I struggled with something vaguely similar for the better part of a year. It was also a 90 deg connector, and was NOT an Amphenol. It got progressively worse until I decided I had no choice but to find it. I finally isolated to a particular 90 deg connector, it but it took forever and seemed to be different every time it showed up. I was curious enough that I carefully cut the offending connector open and found that the center conductor was made of two parts, threaded into each other. The threads had become corroded creating symptoms best described as weird. SWR varying with time, occasional dead receive cured by transmitting a single dit, strange intermittent received intermod... This took many hours for me to discover. After that, I now use only Amphenol.
Kim N5OP
    On Monday, October 21, 2019, 05:38:12 PM CDT, <ed_richardson at shaw.ca> wrote:  
 
 
I did some more testing today and found out the there are indeed out of band oscillations that are occurring.  They did not occur everywhere in the band, only when the antenna load was slightly inductive +9<X<+3. 

I swapped out the solid state amp and tried a different manufacturer's product. The problem only occurred with the original amp. 

The oscillations started when operating at about 21.070 and ended at 21.250. Antenna resonance is 21.280.  The oscillating frequencies vary during the transmission and  oscillation amplitude starts high and slowly drops during the transmission.

Switching an W3NQN bandpass filter into the amplifier input stopped the oscillation.
Putting the amp into full power operation also stopped the oscillations. The oscillations only were present when operating in half power mode.

During station construction I followed the Motorola R56 manual as well as Jim K9YC's excellent tutorials for everything bonding and grounding. I use Andrew F2A Sureflex jumpers for RF paths between the rig, filters, amp and antenna port. 7/8" heliax from the station to the top of the tower. Single point station ground, everything bonded etc.

The one exception was I used a right angle adapter on the amplifier input. I removed that when I switched in the other amp and hadn’t bothered to reinstalled it  With that connector out of circuit the oscillations did not occur. I reinstalled the adapter and the problem returns. I can not get the problem to resurface by manhandling the cables or connectors.

So possibly the adapter is the ingress point, however it doesn’t explain the frequency selectivity of the oscillations or the oscillations stopping when you increase the gain when going to full power mode. 

I am glad the results are repeatable. Just wish I knew the failure mode. Hopefully others can benefit from these lessons.

1. Erratic power measurements or High VSWR when running power could be due to parasitic oscillations.
2. Spending serious $ on station construction is worthless if you insert the flea market low quality connector/adapter. Insert your favorite "weakest link" adage.
3. Towertalk and its members have a wealth of invaluable knowledge and experience. 

Problem solved for now...just not fully understood yet

Ed VE4EAR

Ed Richardson

-----Original Message-----
From: Jim Brown <jim at audiosystemsgroup.com> 
Sent: October 21, 2019 1:16 AM
To: ed_richardson at shaw.ca; towertalk at contesting.com
Subject: Re: [TowerTalk] VSWR Changes with Power

On 10/20/2019 3:33 PM, ed_richardson at shaw.ca wrote:
>  The were down about 25 dB from the fundamental. They were ± about 
> 700 kHz from the fundamental but drifted I placed a 15m BPF at the 
> input and now they are gone

Which model of BPF?

I repeat my advice about coax shield defects and bonding.

73, Jim K9YC

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