[Elecraft] Amplifier Input Impedance

[AD6XY]

I would say something slightly different. Firstly I am assuming the PA is on.

It might help to measure the input, especially if it is a valve amplifier.
If it is solid state the input match is more likely to be RF power
dependent, but if the match is really bad at low power it is not likely to
get better at high power and such an amplifier would not be linear. A better
way would be to measure at the power you intend to use but that requires a
directional coupler. It is not ideal but an SWR meter connected with a very
short lead would probably do.

If the amplifier oscillates then there is a very high chance of damaging the
analyzer, but if that were the case, then at least it did not destroy the
rig. Make sure the amplifier output has a wideband matched load - definitely
NOT an antenna. 

I don't think a blocking capacitor would be any use unless there is DC on
the amplifier input line. If there is, there is probably a problem.


Note: You can use an antenna analyser to initially measure matching circuits
of a high power valve amplifier, but only when it is off. You need to load
the anode with an appropriate impedance equal to the operational anode
output load and tune and load for a good match at the output. It will not be
spot on but it should be a good starting point. It is very hard to optimise
the tank circuit in a valve PA any other way because of the high voltages
both DC and RF. It does not tend to work so well with transistors as the
impedances are so low and often dominated by the device capacitance.

Mike



WILLIS COOKE wrote:
> 
> I would say no to all three questions.  The input impedance to the amp
> will not be the same when it is not powered and driven.  The SWR
> indication on the driver will give you some idea of the input impedance. 
> It should not hurt the antenna analyzer if the amplifier is not powered,
> but I don't think it will tell you anything useful.  With power on the
> amp, it might.
> 
> Willis 'Cookie' Cooke 
> K5EWJ
> 
> 

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