It's possible the adapter isn't defective, but just changed the
impedance seen at the amplifier input enough to cause the instability.
The transmitter output impedance (NOT 50 Ohms!), reflected through the
cable and adapter, may have been just the wrong value. If so, a
resistive pad, maybe 1 dB, between transmitter and amplifier might solve
the problem.
73,
Scott K9MA
On 10/21/2019 17:37, ed_richardson@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@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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Scott K9MA
k9ma@sdellington.us
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