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

[Jim Lux]

On 12/15/13 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.
>

I'm not so sure.  If I make a short length of 75 ohm transmission line
(or, for that matter, use a PL-259/SO-239 barrel, which is NOT 50 ohms)
in a typical 50 ohm system.  So it's, say, 10 feet of 50 ohm line, 2" of
75 ohms, and 20 feet of 50 ohms, terminated in a perfect 50 ohm load.

I doubt that if you hooked up an SWR meter it's going to show anything
remotely like 1.5:1.

Sure, if I hook up 10 feet of 50 ohm line, then 20 feet of 75 ohm line,
terminated in a 75 ohm resistor, THEN the SWR meter is going to read 1.5:1.

(neglecting the loss in the line in all cases)

Here's why..

Call the starting point A, the transition from 50 to 75 point B, the
transition from 75 to 50 (2" later) point C, and the ending load point D.

My power starts at A propagating down the line.  It hits point B, and
some power reflects back.. it continues on and hits point C, also
reflecting some power back.  The amount of power is almost exactly the
same, and because the distance from B to C is so short, the phase
difference is negligible, except that the sign is flipped.  So both
those reflected waves essentially cancel, and the *net* reflected power
is very low.

This is exactly what happens when you have non-constant-impedance
connectors in the line: something widely acknowledged to not cause SWR
bumps, loss, etc. (yes, you can detect it with a time domain
reflectometer or a swept frequency measurement, but it's still pretty
darn small)

I contend that it is EXACTLY the same if the BIRD or whatever has a 50
ohm line inside, and you're operating it in a 75 ohm or 92 ohm or
whatever system.  The meter will show forward and reflected readings,
but they won't be numerically correct (because the meter is calibrated
for a 50 ohm system), but the ratio is probably right..





> 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.

Yes, but the "bridge" or "line" doesn't have to be exactly the same as 
the system impedance.  It can be close, and even if not close, the 
numbers can probably be calculated for a new system.

This is really nothing different from taking a VNA and using a set of 75 
ohm standards to calibrate it instead of 50 ohm standards.  The couplers 
inside the VNA don't change, just the relation between "voltage on the 
detector" and "power" changes.


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.

Yes.. trying to balance the nominal 600 ohm line impedance of the phone 
line.


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

And I think the difference in 50 and 75 ohm meters is more in the dial 
calibration than in the actual wiring inside.