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Author Topic: Why Refer to SWR as VSWR?  (Read 734 times)
K0OD
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Posts: 2533




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« on: August 15, 2009, 08:17:49 AM »

The overwhelming majority of hams (what about engineers?) simply use the term SWR. I've always understand that SWR, VSWR and the rarely used ISWR are  the same number and are interchangeable.

So why do some hams use the longer VSWR and speak of Viz-swur?   (and why doesn't anyone say Eye-swur)
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W6RMK
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« Reply #1 on: August 15, 2009, 08:45:07 AM »

Why *V*SWR as opposed to something else?  Probably historical. In the microwave area, you used to directly measure VSWR with a slotted line, which measures the voltage along the transmission line.  Measuring current along the transmission line is a real pain.

These days, return loss or S11/S22 might be used more, although you still see VSWR specs on things like connectors (as a short hand.. people *know* what a VSWR of 1:1.2, 1:1.5, or 1:2 is like)

If you're into precision measurements, reflection coefficient turns up a lot (because a lot of times, that's what the actual box measures.. and then it's converted to RL, S11, or VSWR by some calibration and calculation process)
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K4JSR
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Posts: 513




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« Reply #2 on: August 15, 2009, 09:38:39 AM »

W6RMK has it right on!
I spent many years working in a microwave house.
All we ever called it was VSWR, but more frequently,
Return Loss expressed in dB.  You can convert between the two, but that would give you "A TOUGH RHO TO HOE"!!  That would cause a poor lab tech to kick over
his network analyzer and SCATTER PARAMETERS everywhere!!! (To the VECTORS go the spoils!)   This would be as bad as going into a computer lab and kicking over the bit bucket!

And yes, it is more convention/tradition to call it
VSWR.  Ask any old nun; Old HABITS are difficult to
part with!

My nurse is coming with my meds, I'll go peacefully now.   73,  Cal  K4JSR
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K4JSR
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Posts: 513




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« Reply #3 on: August 15, 2009, 09:40:42 AM »

W6RMK has it right on!
I spent many years working in a microwave house.
All we ever called it was VSWR, but more frequently,
Return Loss expressed in dB.  You can convert between the two, but that would give you "A TOUGH RHO TO HOE"!!  That would cause a poor lab tech to kick over
his network analyzer and SCATTER PARAMETERS everywhere!!! (To the VECTORS go the spoils!)   This would be as bad as going into a computer lab and kicking over the bit bucket!

And yes, it is more convention/tradition to call it
VSWR.  Ask any old nun; Old HABITS are difficult to
part with!

My nurse is coming with my meds, I'll go peacefully now.   73,  Cal  K4JSR
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W5DXP
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« Reply #4 on: August 16, 2009, 11:30:22 AM »

Voltmeters were historically more popular and available than current meters. Early RF engineers actually slid an RF voltmeter along their parallel transmission lines to measure the voltage maximum and minimum.
--
73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WA7NCL
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« Reply #5 on: August 17, 2009, 08:55:32 AM »

Cuz when y'all on 75 meters y'all gotta gud antenna when the viz war is 1 tuh 1.  It wouldn't sound as good as S war or S W R.
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WA3SKN
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« Reply #6 on: August 17, 2009, 11:00:51 AM »

Because most of the simple meters out there measure in terms of voltage!  It was a simple and inexpensive check to make, and made a difference when coax was becoming popular.  You could also ask "Why do you want to shorten it to SWR?", after all... you are measuring voltage.
73s.

-Mike.

PS: I do not miss working with waveguide!
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KC8VWM
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Posts: 3124




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« Reply #7 on: August 18, 2009, 12:54:55 AM »

Voltmeters were historically more popular and available than current meters. Early RF engineers actually slid an RF voltmeter along their parallel transmission lines to measure the voltage maximum and minimum.

-------

Yup I have actually done that in the past to find the "sweet spot" in the feedline / antenna system equation. Would you believe this method actually works?

I often refer to it as VSWR because there is more to the equation than a simple "checking the air tire pressure" approach when dealing with "antenna systems."

Antenna measuring devices are not intended to check "air in a tire."

VSWR measurements are only an indicator of the overall antenna efficiency factor. In fact, there are many antenna's which exhibit a poor VSWR that actually perform very well.  


Charles - KC8VWM
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W5DXP
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« Reply #8 on: August 18, 2009, 05:01:07 AM »

> KC8VWM wrote: Would you believe this method actually works? <

It even works with twinlead/ladder-line using a pickup loop. That's the way I researched my notuner antenna - by sliding a pickup loop up and down the ladder-line until I located the current maximum and minimum points from which I could calculate the SWR.
--
73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
K0BG
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Posts: 9845


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« Reply #9 on: August 18, 2009, 06:13:23 AM »

I suppose it is just to late to start calling it Reflection Coefficient?

One thing is for sure; way too much emphasis is placed on getting a low SWR, rather than a proper conjugate match. This is especially true in tuning mobile antennas.

The real surprise is, we have inexpensive tools now, like the MFJ-259B, which give us fairly accurate readouts of the reactive and the resistive values. Yet, far too many just look at the SWR readout, and assume that it is the only value which counts. Taint so MaGee!

Alan, KØBG
www.k0bg.com
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K4JSR
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Posts: 513




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« Reply #10 on: August 18, 2009, 01:42:10 PM »

Feedline enthusiasts like to sing "p p p Your Boat"!

That would be "rho  rho  rho Your Boat"!

    0   0
      ^
    \___/
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W5DXP
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« Reply #11 on: August 18, 2009, 01:56:22 PM »

> K0BG wrote: I suppose it is just to late to start calling it Reflection Coefficient? <

We could certainly convert over to talking about rho instead of SWR. How many people know the answer to the following question?

"The rho at my antenna is 0.3333 at 33 degrees. Do I need a tuner?" :-)
--
73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
KB1LKR
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Posts: 1899




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« Reply #12 on: August 18, 2009, 02:46:10 PM »

So: Z = 84+j55 ohms & VSWR = 2.0:1 -- right?

Would make a good starting point to generate a few variants of a question for the Extra exam.
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KASSY
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Posts: 165




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« Reply #13 on: August 18, 2009, 04:57:03 PM »

I think I'm familiar with conjugate match, but what's wrong with simply wanting a 1:1 VSWR?

Alan, can you give me an example where a 1:1 VSWR is achieved without a conjugate match also being achieved?  I may have missed something in my reading!

- k
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K1CJS
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« Reply #14 on: August 18, 2009, 05:45:34 PM »

Cal--what happened?  Didn't your nurse show up?  ;-)
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