[AMPS] Hold on - SWR meter placement

[Jon Ogden]

Larry Molitor wrote:

>Have you ever done slotted line measurements? As I recall from years ago,
>they sorta depended on the VSWR being the same all along the transmission
>line! Of course, all our "modern SWR bridges" and power meters all sense
>current. Think about that for a bit. Steve said it much better then I ever
>could in hie post.

Yes.  It's been about 10 to 12 years ago in a measurements class in 
college.  Back then the 8510B was the best network analyzer HP made.  We 
began the class with the concept of impedance and VSWR measurements using 
the old fashioned sig gens and slotted lines.  Then we gradually moved up 
the food chain until at the end of the semester we were using the 8510B.  
Could the prof had taught us the 8510 first off?  Sure he could have.  I 
have never used a slotted line or and HP8410 (I think that was it) since 
then.  However, I developed an appreciation for the technology at hand.  
I also learned Rocky Mountain Basic at the same time which proved a 
valuable lesson!

But yes, the most accurate way to measure "Voltage" standing wave ratio 
is to measure the RF voltage.

And yes, the VSWR would have to be constant cause we used a Vector 
voltmeter to measure the complex RF voltage.  You would first search for 
a peak voltage along the line and then for a minumum voltage.  From those 
two you could calculate the VSWR and the load impedance, etc.

Here's another interesting fact:

The voltage minimums and maximums occur at different points on the 
transmission line.  If the VSWR varied down the line  how could you ever 
measure it?  Think about it!  You voltage maximum may be one inch from 
the load.  But your voltage minimum could be 4 inches from the load.  So 
what's your VSWR if you believe that VSWR varies as you go down a 
transmission line.  Anyone who says that VSWR varies while going down a 
transmission line knows not the first thing about transmission line 
theory.  Note, that I am talking about INSIDE a transmission line where 
you have a consistent and constant impedance.  Any variation outside of 
that throws in a whole new set of boundary conditions.

Now that I think about the slotted line example, it just reinforces in my 
mind what I and others had said!

>
>Way back in the dark ages, my novice class peer group was divide into two
>camps. Those with enough money to buy a "SWR METER" and those who couldn't.
>Those without used bed springs, screen doors, porch rails or anything else
>you could get an alligator clip on for antennas and made lots contacts. The
>SWR meter crowd pruned dipoles to the Nth degree and never made QSO's
>because they *knew* you couldn't get out with a 1.5 to 1 SWR. This SWR
>business is a perfect example of witchcraft and superstition winning out
>over science and technology.

Good story!  Well, I like to get that low VSWR as seen on my meter.  
However, changing the match with my tuner does nothing to eliminate the 
VSWR on the line after the tuner.  It is still there.  The trasceiver 
doesn't see it.  In some cases, particularly on Solid state rigs, it is 
critical that VSWR be kept to a minimum.

Oh well......fun stuff.

73,

Jon
KE9NA



-------------------------------------
Jon Ogden
KE9NA

http://www.qsl.net/ke9na    <--- CHECK IT OUT!  It's been updated!!!!!


"A life lived in fear is a life half lived."


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