Towertalk
[Top] [All Lists]

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

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Fwd: RG-149: 50 ohm/70 ohm - does it matter?
From: Michael Tope <W4EF@ca.rr.com>
Date: Sun, 15 Dec 2013 20:50:27 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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.............

_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk

<Prev in Thread] Current Thread [Next in Thread>