[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.

No, they use some sort of transformer SWR bridge, with diode detectors 
(and some use a phase detector too(SGC), to know which direction to 
tune).  It's not a directional coupler in the sense that there's a 
sufficiently long line in terms of wavelengths and you couple off the 
traveling wave like you do in microwaves.

More like a clever transformer scheme as seen in a raft of SWR meters. 
  These things measure forward and reverse power, but don't really have 
a characteristic impedance in and of themselves.  What's 50 ohms about 
them is that the "calibration" is correct for 50 ohms.

Think about it.. how are you going to build a real directional coupler 
that's 1" long for 160 meters?  What you can do is build a form of 
hybrid which has two outputs which are related to the forward and 
reverse power.

http://www.sgcworld.com/Publications/Manuals/239man.pdf
has the schematic on page 26

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.

Sort of.. They have a routine that takes the ADC values for Fwd and Rev 
and turns that into a number that is similar to VSWR, and they minimize 
that. I don't have the listing handy, but it's soemthing like 
(Vfwd-Vreverse)/fwd, and drive it to zero.  The scaling from Vfwd to 
actual forward power may change, but optimizing this will get you to a 
match.

  Since they never need to display actual VSWR, they don't bother to 
normalize it or anything.  They just have a known threshold for when to 
stop adjusting.



>
> 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.
>
> 73, Mike W4EF............
>
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>