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

[Jim Lux]

On 12/14/13 3:33 PM, Michael Tope wrote:
> On 12/14/2013 2:24 PM, Jim Lux wrote:


>
>> 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.
>
> That depends on what you consider a "real" directional coupler. You can
> build very small "directional couplers" that work across a broad
> frequency range with ferrite transformers. If I gave you the
> S-parameters for one of those and S-parameters for a parallel
> transmission line coupler, you would be hard pressed to tell which one
> was "real" and which one was "fake".

Yes, but I don't think the things in the tuners are that sophisticated. 
They're typically a single transformer, and some resistors and diodes, 
with a couple pots.


>
>> 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.
>
> Yes, but it is normalized to a specific impedance in the sense that the
> VSWR bridge is calibrated for a certain Zo. If I put 100 ohm
> transmission lines on either side of a VSWR bridge calibrated for 50
> ohms and I terminate the load side 100 ohm transmission line with 100
> ohms and drive the source side with a 100 ohm source, the bridge will
> still read 2:1 VSWR even though both the input and output lines are
> matched (i.e. no standing wave on the line).


Would it... I'm not sure about that with these bridges.  I'd like to try 
the experiment (or maybe this is a good reason to get MacSpice working)




  A travelling wave 50 ohm
> coupler would do the same (i.e. show a ~10% power reflection on the
> reverse port) if I installed it in place of the VSWR bridge in the 100
> ohm matched system.
>