Gate to Source Voltage Shift

I have an LDMOS amplifier. I am testing it using a power meter. When the second port of the power meter is connected to standard 50 ohm termination, everything goes well.
However when I connect it to my load which shows 50 ohm on VNA, the Gate to Source Voltage increases and there is an abrupt increase in output power. I am unable find out the reason. Any help will be appreciated.

Do they have the same ground voltage?

How do you measure the power when you plug your load to the amplifier ?

I have a power meter between the amplifier and the load. The output of the amplifier is connected to the first port of the power meter and the second port of the power meter is connected to the load.



By 'they' do you mean the amplifier and the load?
Yes, they do have the same RF ground.

By 'they' do you mean the amplifier and the load?
Yes, they do have the same RF ground.[/QUOTE]



I mean the 50 ohm resistor ground and the VNA ground.

I mean the 50 ohm resistor ground and the VNA ground.[/QUOTE]

Oh yes, they do.

Sounds like an oscillation due to an out-of-band mismatch. Or possibly feedback (is your load some sort of antenna?)

Are you able to determine the frequency of the oscillation?