On 12/15/2013 5:40 PM, Hans Hammarquist wrote:
-----Original Message-----
From: Jim Lux <jimlux@earthlink.net>
To: towertalk <towertalk@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.............
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