[TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?

[Jim Lux]

On 12/14/13 1:28 PM, Michael Tope wrote:
> On 12/14/2013 11:18 AM, Jim Lux wrote:
>> Interestingly, the output Z of your amplifier is probably not 50 ohms
>> resistive in any case, but most autotuners don't "match to 50 ohms",
>> they "match to minimum reflected power" or "match to minimum VSWR", so
>> they take out the effect of changing amplifier Z too.
>>
>
> Jim, I am not sure I agree with your statement, at least not in the case
> of standalone auto tuner. While I am not familiar with ins and outs of
> practical auto tuner design, I do presume they use a 50 ohm directional
> coupler at the RF input to compute the VSWR presented to the RF source
> device. The tuning algorithm presumably aims to minimize this 50 ohm
> referenced VSWR (again presuming the coupler is 50 ohms) in the same way
> an operator aims to minimize VSWR on his VSWR meter when adjusting a
> manual tuner. I would argue that is for all practical purposes "matching
> to 50 ohms" or perhaps more correctly "matching as close as practically
> possible to 50 ohms". This WOULD NOT take out the effect of the changing
> output impedance of the amplifier.
>
> If the auto tuner were part of the amplifier, then it would be possible
> for the tuner to using additional information (like drive power and DC
> current) to account for the changing amplifier output impedance.
>

FWIW, it would be easy to test, but you need a fair amount of power to 
get it to tune: the 20dBm from a signal generator isn't quite enough.

But, say you took your ham rig, ran it through a 10dB pad, giving you a 
pretty close to 50 ohm source, regardless of what the rig does.  Then, 
put a resistor in series or parallel to make it a 25, 75 or 100 ohm 
source.  Put a dummy load on the other side and let it tune.