Hi Jim,
-----Original Message-----
From: Jim Lux <jimlux@earthlink.net>
To: towertalk <towertalk@contesting.com>
Sent: Mon, Dec 16, 2013 12:07 am
Subject: Re: [TowerTalk] Fwd: RG-149: 50 ohm/70 ohm - does it matter?
>On 12/15/13 5:40 PM, Hans Hammarquist wrote:
>
>> 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.
Correct, an SWR meter (made for 50 ohm) would show 1:1 but your 75 ohms line is
now not matched as it sees a 50 ohms load. The short 75 ohms line does not
“upset” your set-up. That's why it is called “short”.
>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)
In the above set-up it doesn't matter the length of the lines (if we can
neglect the losses)
>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..
I suggest you try it. Forward and reflected power is just one way of dealing
with SWR. The “true way” is actually to measure the voltage along the line,
take note of the highest and the lowest reading. The SWR is the highest divided
by the lowest reading. This is transmission-line-independent way of measure
SWR. (It is easy to understand why we don't do that though.) The distance
between the high and the low reading is, by-the-way, 1/4 wave-length.
>> 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.
No, I didn't say that. E.g. you can measure a 15 kOhm resistor in a bridge
without using 15kOhm anywhere else in a Wheatstone bridge. If you care, you can
build a transmission line SWR meter using any type of transmission line as long
as you balance it for the desired impedance.
>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.
When you “calibrate” you do the balancing. If you calibrate for 75 ohms you set
up the system for 75 ohms. If you now try to use it (without recalibration) in
a 50 ohms system you will get the wrong readings.
Hans - N2JFS
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