Cecil Moore wrote:
Cecil Moore wrote:

wrote:
It was stipulated in the original post that the swr was being
measured at the source end for the 50 ohm line. If all one changes is
the impedance of a 1/2 electrical wavelength line, and nothing else,
the answer is 1:1.

Nope, not if a 72 ohm SWR meter is being used. An SWR
meter calibrated for the transmission line Z0 of 72 ohms
will read 1.44:1 even if the 50 ohm transmitter is happy
with the 50+j0 ohm virtual impedance being presented to it.
I have SWR meters calibrated for 50, 75, 300, 450, and 600
ohms - doesn't everybody?

Please note that it was ***NOT*** stipulated in the original
post that the SWR meter was calibrated for 50 ohms. "What is
the SWR?" was the question.
--
73, Cecil http://www.qsl.net/w5dxp


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SWR meter reads 1:1 at the source with 50 ohm line, that will tell
you that the swr bridge Z and the antenna Z are the same initially (1/2
wave line). Changing to 72 ohm line will still be 1:1.
Gary N4AST

wrote:

Cecil Moore wrote:

Fred W4JLE wrote:

Assuming I have an antenna that is perfect on 3.8 MHz. Perfect being
defined, as I am feeding it with exactly 1/2 electrical wave length of 50
Ohm feedline and it is 1:1 SWR measured at the source end.

What would the SWR be if I substituted the 50 Ohm feedline with a 1/2
wavelength of 72 Ohm feedline?

The SWR on the 72 ohm feedline would measure 1.44:1. You do
have an SWR meter calibrated for 72 ohms, don't you? :-)
Your 50 ohm SWR meter will measure 1:1, but as Reg says,
it is merely measuring the degree of match to your
transmitter designed for 50 ohm loads.
--
73, Cecil http://www.qsl.net/w5dxp


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It was stipulated in the original post that the swr was being
measured at the source end for the 50 ohm line. If all one changes is
the impedance of a 1/2 electrical wavelength line, and nothing else,
the answer is 1:1. Did not say anything about measuring the swr
anywhere but at the source. Whatever the antenna Z is in the 50 ohm
example that gives a 1:1 match, determines the swr bridge
characteristics. If the antenna Z was 72 ohms, then the bridge is 72
ohms for a 1:1 match. Change the transmission line to 72 ohms, still a
1:1.
Gary N4AST


VSWR stands for Voltage Standing Wave Ratio. It's supposed to be the
ratio of the greatest voltage on a transmission line to the least
voltage on the same line. On the line with a 72 ohm Z0, and a 50 ohm
load, there exists a standing wave, and the ratio of maximum to minimum
is 1.44 whether you measure it with a 50 ohm bridge at the beginning
or not. If you define SWR as what a 50 ohm SWR bridge measures, you
haven't quite grasped the concept.
73,
Tom Donaly, KA6RUH

It was stipulated in the original post that the swr was being
measured at the source end for the 50 ohm line.

He said, "WHAT WOULD THE SWR BE ...?" not what would a
50 ohm SWR meter measure. The SWR would be 1.44:1 on
the 72 ohm feedline no matter what the 50 ohm SWR meter
erroneously measured.

If all one changes is
the impedance of a 1/2 electrical wavelength line, and nothing else,
the answer is 1:1. Did not say anything about measuring the swr
anywhere but at the source.

Go back and read the question again, Tom. He asked:
"What would the SWR be if I substituted the 50 Ohm feedline
with a 1/2 wavelength of 72 Ohm feedline?"

Did he say, "What SWR would be measured?" NO!!!

Did he say, "What would the SWR be ...?" YES!!!

***THE SWR WOULD BE 1.44:1*** on the 72 ohm feedline.
You guys are not answering the question that he asked.
--
73, Cecil http://www.qsl.net/w5dxp

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Cecil Moore wrote:
Go back and read the question again, Tom. He asked:

Sorry, Tom, somehow I screwed up the attributes.
I don't think you posted what I responded to.
--
73, Cecil http://www.qsl.net/w5dxp

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Tom Donaly wrote:
VSWR stands for Voltage Standing Wave Ratio. It's supposed to be the
ratio of the greatest voltage on a transmission line to the least
voltage on the same line. On the line with a 72 ohm Z0, and a 50 ohm
load, there exists a standing wave, and the ratio of maximum to minimum
is 1.44 whether you measure it with a 50 ohm bridge at the beginning or
not. If you define SWR as what a 50 ohm SWR bridge measures, you haven't
quite grasped the concept.

And Fred didn't ask what an SWR bridge would measure.
He asked: "What would the SWR be ... ?" You're right.
The SWR would be 1.44:1.
--
73, Cecil http://www.qsl.net/w5dxp

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OK CECIL!!!

Leave it up to you to pull the rug out from under me...
If I have a transmitter that has 50 ohm out, and it is going to hook to
a 50 ohm cable (and I can't see how this coax is terminated) why would I
ever choose anything other than a 50 ohm calibrated swr meter to measure
it with?

Is that what I have seen on FS meters before (a rise in apparent
radiation from the coax shield--and yet match looks good) and the 1/2
wave coax is really now part of the antenna?

And, if the meter didn't give me the right reading, and cooked my "BEEG
LEENEAIR" could I sue the manufacturer, buy a yacht and live in the
Bahamas, drinking Peter Vella Merlot? grin

Warmest regards,
John

"Cecil Moore" wrote in message
...
Tom Donaly wrote:
VSWR stands for Voltage Standing Wave Ratio. It's supposed to be the
ratio of the greatest voltage on a transmission line to the least
voltage on the same line. On the line with a 72 ohm Z0, and a 50 ohm
load, there exists a standing wave, and the ratio of maximum to
minimum is 1.44 whether you measure it with a 50 ohm bridge at the
beginning or not. If you define SWR as what a 50 ohm SWR bridge
measures, you haven't quite grasped the concept.

And Fred didn't ask what an SWR bridge would measure.
He asked: "What would the SWR be ... ?" You're right.
The SWR would be 1.44:1.
--
73, Cecil http://www.qsl.net/w5dxp

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On Mon, 30 May 2005 16:12:44 -0500, Cecil Moore
wrote:

Fred W4JLE wrote:
Assuming I have an antenna that is perfect on 3.8 MHz. Perfect being
defined, as I am feeding it with exactly 1/2 electrical wave length of 50
Ohm feedline and it is 1:1 SWR measured at the source end.

What would the SWR be if I substituted the 50 Ohm feedline with a 1/2
wavelength of 72 Ohm feedline?

The SWR on the 72 ohm feedline would measure 1.44:1. You do
have an SWR meter calibrated for 72 ohms, don't you? :-)
Your 50 ohm SWR meter will measure 1:1, but as Reg says,
it is merely measuring the degree of match to your
transmitter designed for 50 ohm loads.


Gentlemen, I know most of you are much more knowledgeable in some of
this, but with all your nit-picking over the impedances, etc., may I
interject that something is being overlooked?

The antenna feedline has some loss, however little, that may affect
the swr at the "source", which I interpret to be at the radio.

100 watts may be transmitted, 90 watts received at the antenna, 9
watts reflected, and 8.1 watts received back at the rig for reflected
power. My numbers are hypothetical, but you get the idea.

However, your discussions concerning SWR bridge impedance have led me
to wonder how accurate the swr meter in my rig is concerning the
antennas I use. I feed a dipole with 72 ohm into my rig, an IC-706
MKII which expects a 50 ohm load.

My attitude is that it must be ok if the rig sees it as low as it
should measure it as it sees it.


--
73 for now
Buck
N4PGW