On 12/16/13 7:07 AM, Hans Hammarquist wrote:
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”.
Exactly.. (because we've all done this)
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.
Yes, the slotted line for microwaves. Or you can use a open wire line.
I've seen some nice lecture demos of this using open wire line in the
2m band (so the transmission line is a practical length..)
It will take a bit of room to do it for 160m<grin>
You can also just take voltage samples at several places, and figure out
where the max and min would be (what a 6 port magnitude only
reflectometer does, basically).
But that doesn't get back to the original question..
If I have a 75 ohm transmitter, 75 ohm coax, and a 75 ohm load, and I
put a 50 ohm Bird in the middle, what does it read. I think 1:1 (or
FWD = something, REV = low)
Or, getting back to the "what's the transmitter output Z" question.. if
I have the Bird in the line, and I adjust the tuning networks (wherever
they are) to minimum reflected power (or lowest Fwd+ref/Fwd), is that
the "well matched" condition, even if it happens NOT to be 50 ohms.
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