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

[Michael Tope]

On 12/15/2013 5:40 PM, Hans Hammarquist wrote:
> -----Original Message-----
> From: Jim Lux <jimlux at earthlink.net>
> To: towertalk <towertalk at contesting.com>
> Sent: Sun, Dec 15, 2013 7:41 pm
> Subject: Re: [TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?
>
>
> 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.
> Wrong! If you put a "line sample, made for 50 ohm, in a matched 75 ohms line, the sampler will show a 1.5;1 SWR. It doesn't care what source you have, only what impedance the load have.
>
> You will find some type of referrence inside every SWR meter, may it be a transmission line with a characteristic impedance or a resistor in a bridge. As you might remember the "old" phone patches used a combination of resistors and capacitors for the referrence in order to achieve a good isolation between the incoming and the outgoing audio.
> I know you can buy SWR meters made for 50 or 75 ohms but I haven't seen anything for ladder lines (300 - 600 ohms). Maybe because nobody is looking it. Who cares what SWR you have on a ladder line, you always connect that to a tuner anyhow.
> Hans - N2JFS
>

Yes, that is correct, Hans. The termination of the forward coupled port 
in the transformer based coupler determines the impedance where the 
reflected port will show a reflected power null. The directional 
couplers used in the CATV business use the two transformer technique. At 
work when I couldn't find a "black box" directional coupler system model 
in the Microwave Office simulation program I created one a quick and 
dirty one from a pair of the program's ideal transformer models.

73, Mike W4EF.............