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SWR and VSWR

 

OK, for the youngsters in the group a bit of history.
It's over 100 years ago.Coax has not been invented yet.There are no SWR metersThere are no VNA's.? ?(Well, an RF Bridge and a lot of math will get much the same answer)
So you would walk along the parallel transmission lines between the transmitter and the antenna with a voltmeter.
At one point along the line you measured 50 volts, at another point you measured 25 volts.
Voltage Standing Wave Ratio is 50/25? or 2
These peak and nulls were waves of voltage that pretty much stayed in the same place, thus were called? "Standing Waves"
If at some point you got a ZERO voltage, this usually happened when there was an open or a short in the system, then.
VSWR was 50/0 and was just called "Infinite"
Now if you walked along the line and got 50 - 50 - 50 - 50 - 50
Then VSWR was 50/50 or 1And they liked to say the SWR was "FLAT"
Hope this helps, Kent
For the other old farts in the group, yep, still have a slotted line in the back garage.? Think I still know how to use it, but the HP-8510C does such a better job. hihi? ? Oh yea, the Nano's do a pretty good job and weight 200 lbs less!

On Wednesday, August 10, 2022 at 11:45:22 AM CDT, KENT BRITAIN <wa5vjb@...> wrote:

VSWR
Voltage Standing Wave Ratio

The cause is the impedance mismatch.
The measurement is in volts.???

Kent
 

Thankyou Kent, from someone who can still use a Leacher Line :-) 73 de GK8NXD

On 10/08/2022 19:10, KENT BRITAIN wrote:
OK, for the youngsters in the group a bit of history.
It's over 100 years ago.Coax has not been invented yet.There are no SWR metersThere are no VNA's.? ?(Well, an RF Bridge and a lot of math will get much the same answer)
So you would walk along the parallel transmission lines between the transmitter and the antenna with a voltmeter.
At one point along the line you measured 50 volts, at another point you measured 25 volts.
Voltage Standing Wave Ratio is 50/25? or 2
These peak and nulls were waves of voltage that pretty much stayed in the same place, thus were called? "Standing Waves"
If at some point you got a ZERO voltage, this usually happened when there was an open or a short in the system, then.
VSWR was 50/0 and was just called "Infinite"
Now if you walked along the line and got 50 - 50 - 50 - 50 - 50
Then VSWR was 50/50 or 1And they liked to say the SWR was "FLAT"
Hope this helps, Kent
For the other old farts in the group, yep, still have a slotted line in the back garage.? Think I still know how to use it, but the HP-8510C does such a better job. hihi? ? Oh yea, the Nano's do a pretty good job and weight 200 lbs less!


On Wednesday, August 10, 2022 at 11:45:22 AM CDT, KENT BRITAIN <wa5vjb@...> wrote:
VSWR
Voltage Standing Wave Ratio

The cause is the impedance mismatch.
The measurement is in volts.

Kent
 

Good, now take a current probe and do the same and you will get and ISWR. They are reciprocals. Once upon a time there was a myth that you must measure your antennas swr at the feedpoint. Not so. In a 50 ohm system you can measure swr with a bridge or the nano at any point between tx and the antenna.
F1AMM
 

Not so. In a 50 ohm system you can measure swr with a bridge or the nano at
any point between tx and the antenna.
It's true for the SWR

This is true at a fixed frequency but the nanaoVNA works at a variable frequency; it follows that the shape on the Smith is not the same over the length of the coaxial. It comes from the fact that all the points do not turn at the same angle since the wavelength is not the same for each of the points.

The modification is all the more important as the strip is wide and the coaxial long.

Is this correct?
--
Fran?ois

-----Message d'origine-----
De la part de tuckvk3cca
jeudi 11 ao?t 2022 11:01
 

SWR measurement can be done at any point of the line, even over a wide frequency range, and will be correct except for effects produced by line loss. The impedance instead will vary according to line length, or to the location of the measurement.

This is the basic concept that needs to be remembered: Impedance (R and X) varies along a mismatched transmission line, while the SWR stays constant except for parasitic effects like line loss.

Manfred
 

Addition to my previous post:

Whether you display the results on a Smith chart, or a standard XY chart, or a table, or whatever, doesn't matter at all! What matters is what you are displaying: SWR, or impedance.

A Smith chart allows to see both of them. Depending on where you place the NanoVNA in the line, the trace will appear rotated due to the impedance changes, but with the same shape and center, indicating same SWR per frequency. Diehard Smith chart users, please correct me if I'm wrong in that, because I'm only an occasional Smith chart user!
 

Yes, that is the application of SWR circles on the Smith Chart. For
example, any combination of R and ¡ÀjX so long as it lies on that 2:1 circle
(real radius intersects at 25 and 100-ohms), the SWR / VSWR / ISWR will be
2:1. Same for any SWR circle on the chart.

Dave - W?LEV

On Thu, Aug 11, 2022 at 4:28 PM Manfred Mornhinweg <manfred@...>
wrote:

Addition to my previous post:

Whether you display the results on a Smith chart, or a standard XY chart,
or a table, or whatever, doesn't matter at all! What matters is what you
are displaying: SWR, or impedance.

A Smith chart allows to see both of them. Depending on where you place the
NanoVNA in the line, the trace will appear rotated due to the impedance
changes, but with the same shape and center, indicating same SWR per
frequency. Diehard Smith chart users, please correct me if I'm wrong in
that, because I'm only an occasional Smith chart user!







--
*Dave - W?LEV*
*Just Let Darwin Work*


--
Dave - W?LEV
 

I used one in college..45 years ago :-)
 

Correct. The Smith chart provides a very clear visualization of impedance transformations which are rotations in Argand space. Well worth learning to use it.
 

It¡¯s fun when you bridge a lightbulb across it.

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