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

[Roger (K8RI) on TT]

On 12/15/2013 7:35 PM, Jim Lux wrote:
> On 12/15/13 4:21 PM, Richard (Rick) Karlquist wrote:

One simple question. Can the amp match the impedance, or do you have a 
tuner in the line that can.  If the answer to either is yes, it doesn't 
matter what the impedance is as long as it's some where in the ball 
park.  Even with a 3:1 SWR on the coax, if the amp is matched the 
additional loss is slight according to the ARRL Handbook

Revert to the old way, Put it up, hook it up, and see what you get. 
Prune if necessary.

73

Roger (K8RI)
>>
>>
>> On 12/13/2013 3:32 PM, kr2q at optimum.net wrote:
>>
>>> Contributor B says, "What if the impedance at the antenna is only 33
>>> ohms....
>>> and you have a quarter wavelength of 75 coax."  Then...Oh horrors.
>>>
>>> Well, don't look now, but if the feedpoint is 33 and the coax is 50, I
>>> calculate that to be the
>>> same VSWR as if the feedpoint were 50 and the coax were 75. No?
>>> Nobody worries about
>>> the former, do they?
>>
>> Let me clarify this.  If you have a 50 ohm transmitter and use 50 ohm
>> coax to drive a 75 ohm load, the SWR is indeed 1.5:1.  OTOH, if you
>> have a 50 ohm transmitter and use 75 ohm coax to drive a 50 ohm load,
>> the transmission line mismatch is 1.5:1, but the SWR as far as the
>> 50 ohm transmitter is concerned is 2.25:1 in the worst case of an odd
>> multiple of 1/4 wl.  Thus a load mismatch is not as bad as a coax
>> impedance mismatch.
>>>
>>> How likely is it to get your coax to be in odd multiples of quarter
>>> lambda?  I am guessing, "Not too likely."
>>> And if it is, well, just add some more coax (if you care about SWR).
>>> Would that not work?
>>
>> If you are driving a multiband antenna, it is actually
>> quite likely to have near a 1/4 wl on some frequency especially
>> if you have a few hundred feet of coax.
>>
>> Remember, the original poster asked "does it matter"?
>> In general it might matter, but you might be able to work
>> around it.  But you shouldn't just assume it doesn't
>> matter at all and ignore it.  The one case where it
>> doesn't matter is when the coax is close to a multiple
>> of a half wave.  Everyone knows that principle.
>> I highly recommend that, if you are able to implement it.
>>
>> Also to clarify some questions about directional couplers:
>> the concept would appear to be associated with
>> traveling waves in transmission lines, but
>> it is possible to make audio directional couplers
>> out of ordinary transformers, such as in legacy
>> phone patches.  Everything is essentially of
>> zero size relative to a wavelength.  You can
>> also make a directional coupler entirely from resistors.
>> The bandwidth of such a coupler is essentially
>> "DC to daylight" if you use very small resistors.
>> "directivity" is just a mathematical concept.
>>
>
>
> I think the real question is whether a  such a hybrid/coupler/line 
> sampler would have a characteristic impedance? That is, if I have a 75 
> ohm source and a 75 ohm load and I put a line sampler (designed for 50 
> ohm systems) in between, will the 50 ohm source see a mismatch.  I 
> think not.
>
> Taking the telephone hybrid for example.. the nominal impedance of the 
> 1:1 transformers is 600 ohms (which, obviously is only at one 
> frequency), but the impedance that a source on one side of the 
> transformer sees is pretty much the impedance that is connected to the 
> other side.
>
> looking at it another way, such a measurement system can be a series 
> current sensor with a parallel voltage sensor, and by measuring the 
> phase difference between the current sample and voltage sample (and 
> their levels) you can tell which way power is flowing.
>
> I suppose such a widget will have some series R (probably pretty 
> small), and some shunt R (probably pretty large), and you can pick the 
> values so that when you hook a 50 ohm resistor on one side, you 
> measure 50 ohms from the other.
>
> 50 = Rseries + (Rshunt || 50)
>
> And, were you to hook up 75 or 100 ohm resistors, this relationship 
> wouldn't hold.
>
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