For those who have forgotten how or have never measured SWR.

Two separate voltage measurements are needed at two different places along
the line.
Slide the voltmeter along the line until a maximum is found.
Remember the reading, Vmax.
Slide the meter along the line again until a minimum is found.
Remember the reading, Vmin.
Before you forget, divide Vmax by Vmin.
You are left with a single number.
It has no dimensions.
It is the TRUE swr.

NOTE: In the above description and calculation there is no mention of Zo,
terminating impedance, source impedance, reflection coefficient, forward
power, reflected power, reflected volts, reflected current, Smith charts, or
conjugate matches. All these things are superflous to the determination. No
information other than the two voltage measurements is needed.

All other methods which purport to measure swr require injection of
additional information. And assumptions form an essential part of the
process. They can hardly be called swr measurements. Particularly when
they can indicate it on non-existent lines.

Reg Edwards wrote:
For those who have forgotten how or have never measured SWR.


Hang on, Reg - didn't you spend your career working on VLF cables that
went under the ocean?


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White, G3SEK wrote:

Reg Edwards wrote:

For those who have forgotten how or have never measured SWR.


Hang on, Reg - didn't you spend your career working on VLF cables that
went under the ocean?


How did you keep the water out of the slotted cable? And how far did you
have to swim between Vmax and Vmin?

Roy Lewallen, W7EL

Ian White, G3SEK wrote:

Reg Edwards wrote:

For those who have forgotten how or have never measured SWR.


Hang on, Reg - didn't you spend your career working on VLF cables that
went under the ocean?


How did you keep the water out of the slotted cable? And how far did you
have to swim between Vmax and Vmin?

Roy Lewallen, W7EL
===============================

Water can be kept out of slotted cables by the ship's radio operator who
never has anything else to do. We used to toss him overboard with a ladle
and pump. On occasions we used the ship's doctor when he wasn't propping up
the bar boozing duty-free scotch.

Didn't have to swim anywhere. The propagation velocity is so low at ELF in
sea water it is necessary only to sit on a stool in a diving suit and wait
for the max's and min's to pass by.
---
Reg.

Reg Edwards wrote:
All other methods which purport to measure swr require injection of
additional information. And assumptions form an essential part of the
process. They can hardly be called swr measurements.

They can be called indirect (calculated) SWR measurements
and assumptions indeed do form an essential part of the
process. That's not at all unusual for indirect measurements.

Particularly when they can indicate it on non-existent lines.

One of the assumptions is that a transmission line exists. If
a transmission line doesn't exist, the measurement conditional
assumptions are violated, and the actual values may not be
the desired or expected results. Happens all the time with
various measuring instruments.
--
73, Cecil http://www.qsl.net/w5dxp



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On Sat, 4 Sep 2004 14:08:53 +0000 (UTC), "Reg Edwards"
wrote:

NOTE: In the above description and calculation there is no mention of Zo,
terminating impedance, source impedance, reflection coefficient, forward
power, reflected power, reflected volts, reflected current, Smith charts, or
conjugate matches. All these things are superflous to the determination. No
information other than the two voltage measurements is needed.

Hi All,

No mention merely means there is no offer of accuracy (not very
important, eh what?). These two measurements (repeated at intervals)
can reveal a SWR that varies along the length of the line like a snake
- UNLESS of course, you DO observe unmentionables like Load reflection
co-efficients and Source reflection co-efficients. As such, a
description of how not to measure SWR, but rather how to exhibit error
if you perchance have the misfortune of having a transmitter that is
unmatched to a 50 Ohm transmission system whose load is in fact
mismatched also.

Need I point out that if both ends are matched - what's the point in
measuring SWR? ;-)

73's
Richard Clark, KB7QHC