Whoa, I did that experiment for Slick a month ago. I replaced the short
piece of LMR240 between the transmitter and Kenwood SW2000 meter with a
precisely measured 1/4 wavelength piece of RG59 75 Ohm coax. The loading
changed, as expected, but the SWR reading did NOT change for SWR values of
1:1 and 1.6:1. The transmitter now saw a load of 112.5 Ohms. The Kenwood now
saw a different source impedance.

I don't see any inconsistency. 100W into 112.5 Ohms requires a voltage of
106V RMS. In a transmitter with a fixed ratio output transformer that may
not be doable. It is designed to put out 70.7V RMS into 50 Ohms, with some
margin.

Tam/WB2TT
"Reg Edwards" wrote in message
...
To anybody interested.

We have a HF Transmitter + 50-ohm coax + SWR meter + Tuner + Feedline of
any
Zo + Antenna.


Suppose it is all tuned-up and ready to go. The transmitter is loaded
with
exactly 50-ohms resistive.


Now change the 50-ohm coax to shorter length of 75-ohm Zo.


As everybody agrees (after perhaps a little meter recalibration) the SWR
meter indication will not change. BUT THE TRANSMITTER WILL NOW BE
INCORRECTLY LOADED.


Where does the inconistency lie ? Does it lie in the change in effective
source impedance?

Ian said -

No, it isn't!

===============

Yes, it is!
---
Reg. ;o)

The inconsistency is that the power level changed, the load on the
transmitter changed, but the SWR meter gave no indication of it.

What's wrong ?

Richard Clark wrote:

On Sat, 18 Oct 2003 13:09:42 -0500, Cecil Moore
wrote:

The guys over on sci.physics.electromag said that two feet of
50 ohm coax guarantees a 50 ohm environment for a wattmeter.

At DC? 100Hz? 10KHz? ...
... 10GHz? 1THz?
Poor bounding as usual.

The frequency discussed was 10 MHz. Shall I add my family tree
to the context?
--
73, Cecil http://www.qsl.net/w5dxp



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Reg Edwards wrote:

The inconsistency is that the power level changed, the load on the
transmitter changed, but the SWR meter gave no indication of it.

What's wrong ?

Nothing's wrong with the equipment. Something's wrong with the operator.
If you want to make the problem even worse, remove the coax entirely.
--
73, Cecil http://www.qsl.net/w5dxp



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Reg Edwards wrote at 01:18:

Dear Ian, please forgive me. The twists and contortions in your use of
the English language are too involved for me unravel. No useful purpose
would be served.

Try reading it again, preferably at the other one o'clock...

--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

On Sat, 18 Oct 2003 22:40:57 -0500, Cecil Moore
wrote:
The frequency discussed was 10 MHz. Shall I add my family tree
to the context?
Odd that you feel it necessary, but if serves bounding then, yes,
perhaps you need to do that more often.

73's
Richard Clark, KB7QHC

The only "inconsistency" is that an SWR meter is obviously NOT a "TLI",
or transmitter loading indicator, as you've been so kind as to
graphically point out.

Roy Lewallen, W7EL

Reg Edwards wrote:
To anybody interested.

We have a HF Transmitter + 50-ohm coax + SWR meter + Tuner + Feedline of any
Zo + Antenna.


Suppose it is all tuned-up and ready to go. The transmitter is loaded with
exactly 50-ohms resistive.


Now change the 50-ohm coax to shorter length of 75-ohm Zo.


As everybody agrees (after perhaps a little meter recalibration) the SWR
meter indication will not change. BUT THE TRANSMITTER WILL NOW BE
INCORRECTLY LOADED.


Where does the inconistency lie ? Does it lie in the change in effective
source impedance?

The only "inconsistency" is that an SWR meter is obviously NOT a "TLI",
or transmitter loading indicator, as you've been so kind as to
graphically point out.
=============================

It would be more true if you had said the SWR meter is not being used as an
SWR meter because there's no SWR for it to measure.

There's an inconsistency because the line between transmitter and meter is
not 50 ohms. The Tx is incorrectly loaded but neither meter scale gives any
indication of it to the user. It could be a serious matter but there's no
warning. The operator is allowed to believe he has set up the equipment
correctly.

A TLI suffers from the same disadvantage as an SWR meter - it gives the
correct answers only when making measurements on 50-ohm lines. This should
not be surprising. They have identical circuits.

But when there is no line of any impedance, just a few inches of wire, the
TLI indicates correctly. Whereas the SWR meter requires at least a
1/4-wavelength of 50-ohm line before it stops being dishonest. And it
doesn't stop telling white lies even on longer lengths.

Reg Edwards wrote:
Whereas the SWR meter requires at least a
1/4-wavelength of 50-ohm line before it stops being dishonest.

I asked that question over on sci.physics.electromag. A formula was
provided. Using the dimensions of RG-213, two feet is long enough
to establish the 50 ohm environment at 10 MHz.

SWR = [sqrt(Pfwd)+sqrt(Pref)]/[sqrt(Pfwd)-sqrt(Pref)]
--
73, Cecil http://www.qsl.net/w5dxp



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