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

[Hans Hammarquist]

-----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


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