[TowerTalk] VSWR Changes with Power

[ed_richardson at shaw.ca]

Finally got around to making the long awaited antenna repairs. However I now
find myself with a strange problem.

 

On 15m, I have swept the antennas using 2 different handheld devices and
measured the VSWR in the shack using both the VSWR meter in the amp and the
standalone power master meter on the amp output.  All measurements are the
same when using any power less than 200 watts drive from the rig and with
the solid state amp in standby. The VSWR from 1:1 to 1.5:1 across the band
in a normal curve.

 

When I place the amp on and adjust the drive, the vswr measured on the amp
and the power master jump  to 2.3:1until I get to more than 600 watts output
when it suddenly drops back to the expected reading.  The abnormal high
reading is present even at power levels previously shown as normal when the
amp is offline and the rive is direct from the rig.  Ihe VSWR = 2.3:1  amp
on 200 watts forward. VSWR = 1.1:1 forward power  200 w direct from radio.

 

Increasing the power to >500 watts = 1.1:1 VSWR as expected.

 

Replacing inserting a 50 ohm load at the shack end of the antenna system
feedline results in normal performance regardless of the power output.

 

I thought I might have been seeing a balun saturation problem which is why I
am posting on this group. However getting different results with the same
power originating from the amp or the rig perplexes me.

 

Suggestions on where to look?

 

Ed VE4EAR