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Re: [TowerTalk] VSWR Changes with Power

To: <towertalk@contesting.com>
Subject: Re: [TowerTalk] VSWR Changes with Power
From: <ed_richardson@shaw.ca>
Date: Mon, 21 Oct 2019 17:37:57 -0500
List-post: <mailto:towertalk@contesting.com>
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@audiosystemsgroup.com> 
Sent: October 21, 2019 1:16 AM
To: ed_richardson@shaw.ca; towertalk@contesting.com
Subject: Re: [TowerTalk] VSWR Changes with Power

On 10/20/2019 3:33 PM, ed_richardson@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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