SWR change with amplifier inline vs in bypass.

[NB8I]

Hello:

I have an FT 950/ Ameritron AL 80A/ Triband Yagi tuned to center of band operation. A Palstar AT 2K power/swr crossneedle meter is post radio and amplifier in bypass.

The FT 950 with amplifier in bypass shows SWR of 1.5/1. This is reflected on the post amplifier Palstar metering.

With the amplifer engaged my radio SWR drops to 1.0 on its metering. Post amplifter metereing is a SWR of  1.7/1 .

Does the amplifier present itself as a 50 ohm load to the radio?  Here is a short video of the actual event as it unfolds.

http://www.youtube.com/watch?v=36HrKn7UXEA&feature=share&list=UUASJ7-S0woxcEvQgEdEyrXQ

Thanks for any opinions or comments.
Regards
Mark NB8I

[W6EM]

Mark:

It appears that yes, the input of your amplifier is roughly 50 ohms, as indicated by your radio having flat SWR of 1.0 to 1 when feeding the amp input.

In bypass mode, the radio SWR, if not the same as the post-amp Palstar SWR, might just be metering error or something wrong with the relay contacts or short length of coax used for the bypass.  On the other hand, if the Palstar SWR changes from what it is with the amp bypassed to a higher value when the amp is in-circuit, it could be that the amp output impedance is slightly different going into the Palstar.  Or, if you are using a ferrite/iron powder core balun, it might be from balun core saturation/heating at high power.

[WA3SKN]

Start with a known good dummy load and cable.
Connect directly to radio and measure.  Then connect through the amp in bypass mode and measure.  Then measure with amp turned on.
You can then ask why there might be a variation in the readings.  And you might want to try this a various frequencies also... it can change per band!
Hopefully there will not be too much variation.
73s.

-Mike.

[NK7Z]

Does the amplifier present itself as a 50 ohm load to the radio?

Yes, the amp has an input stage, which takes the signal from your radio, and conditions it for the power amplification portion of the amp to deal with, this input stage should look like 50 ohms to the radio...  This amp input stage effectively decouples the antenna from the radio itself.  The amp still sees the mismatch though on the output side of things...  You could be running a 5:1 SWR on the antenna, yet only see a 1:1 on the radio, so make sure your antenna is close to 50 Ohms, or 1:1.  I use an Amp, a tuner and a radio.  I monitor SWR in three places here...  Between the radio and the amp.  This lets me know if there is a problem on the amplifier input stage...  I use the radio SWR meter for this...  Between the amp and the tuner, I use the tuner SWR for this, this lets me see what the tuner is presenting the amp with, and finally between the tuner and the antenna, this lets me see what the actual antenna SWR is, this lets me see if anything is broken antenna wise...