Reg Edwards wrote:
Dear Roy, a display of annoyance signifies weakness of argument.

Reg, do you ever display annoyance? :-)
--
73, Cecil http://www.qsl.net/w5dxp



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Reg Edwards wrote:
Where does the inconistency lie ? Does it lie in the change in effective
source impedance?

It lies in using the wrong coax Z0 for the situation. Assuming it's
long enough to develop 75 ohms, it will cause 4% of the forward power
to be reflected back toward the source at the meter. Guess you can't
call it a TLI after all since it is faithfully reporting the 50 ohm
SWR and not making the transmitter happy. :-)
--
73, Cecil http://www.qsl.net/w5dxp



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"Cecil Moore" wrote
Dear Roy, a display of annoyance signifies weakness of argument.

Reg, do you ever display annoyance? :-)
================================

Yes Cec, but you can't hear the sound of banging doors and breaking glass
over the internet.

Reg.

Richard Clark wrote:
Observe how in another
thread, a quarterwave dipole will exhibit no voltage drop because it
is metallic along its length (never mind the inline radiation loss).

Consider that if it were terminated with resistors that cause no
reflections, it would be a traveling wave antenna and the voltage drop
would indeed be very low in spite of the radiation.
--
73, Cecil http://www.qsl.net/w5dxp



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On Sat, 18 Oct 2003 13:57:22 -0500, Cecil Moore
wrote:
Consider that if
How charming....

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.

"Cecil Moore" wrote Reg Edwards wrote:
Where does the inconistency lie ? Does it lie in the change in
effective
source impedance?

It lies in using the wrong coax Z0 for the situation. Assuming it's
long enough to develop 75 ohms, it will cause 4% of the forward power
to be reflected back toward the source at the meter. Guess you can't
call it a TLI after all since it is faithfully reporting the 50 ohm
SWR and not making the transmitter happy. :-)
--
===================================
Cec, wrong! It is true the very incorrect use of 75-ohm line by the system
designer is the reason for the UNDETECTED deviation from 50 ohms of the Tx
load.

The reason it is undetected is because the SWR meter is in error. Although
the meter indication has not changed since the 50-ohm cable was in situ, it
is now in error because it can measure correctly only on 50-ohm lines but
the actual line being measured is 75-ohms.

So the meter can still be called a TLI. The moral of the story is "If the
connection between transmitter and SWR meter or TLI is of appreciable
length then always use 50-ohm coax."

Good to see you are still taking an interest. ;o)
----
Reg, G4FGQ

Reg Edwards wrote:
"Ian White, G3SEK" wrote -
No - I meant exactly what I said. The meter can only indicate the
rho/SWR of whatever is connected downstream (load side) of the meter
itself.

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

We're agreed, down to and including point 5.

(1) I'm sure we are agreed our meters will correctly indicate Rho and SWR
only on 50-ohm lines.

(2) Insofar as the meter is concerned the transmitter's load impedance is
the input impedance of the transmission line between the meter and the
antenna. There may or may not be an intervening Z-match network.

(3) Insofar as the transmitter is concerned the line between meter and
antenna can be ANY impedance. It is desirable only that line length with its
Zo transform the antenna input impedance to somewhere near to 50 ohms (Like
a G5RV on 14.15 MHz). If things become difficult then a Z-match can be
inserted. Once we have selected Zo for this line we are no longer
interested in its SWR. And if we WERE interested a 50-ohm SWR meter would
be incapable of correctly measuring it.

(4) Note that when a Z-match is located at the transmitter end of this
feedline, and varied, the actual SWR on this line cannot change - yet the
SWR meter responds readily to the Z-match settings.

(5) The only line which, ideally, MUST be 50 ohms coax and have a small
SWR, certainly if it is of appreciable length, is that between the meter and
the transmitter. Otherwise the load directly presented to the transmitter
would not be 50 ohms. Any other impedance would transform the 50 ohms seen
immediately on the antenna side of the meter to some other value.

.... so we're OK as far as here.

The reasons you give why the impedance of that connecting line between
the transmitter and the meter must be 50 ohms are all correct. But
there's another reason - or at least, another way of looking at the same
situation: because the SWR meter has been calibrated for a 50 ohm system
reference impedance, it will not correctly indicate a 50 ohm load to the
transmitter unless the connecting line is also of 50 ohms impedance (or
is too short to matter).

(6) It is the SWR on this line which the meter indicates.

No, it isn't! The value of rho or SWR obtained from the forward and
reflected readings of the meter is the rho/SWR of the *load* connected
to the meter's *output* terminals.

Where the confusion arises is because the SWR on that 50 ohm connecting
line will be numerically equal to the SWR indicated by the meter - but
that's only because they are both 50-ohm devices. It is *not* because
the meter is reacting to the standing waves on its input side.

(If you change the impedance of that connecting line, then as you say in
(5) above, the load impedance presented to the transmitter will change,
so the power output will change. However, the forward and reflected
signals from the meter will both be affected in the same proportion, so
the rho/SWR result will not change.)

Or look at it another way: think of the meter as a bridge, fed by that
same connecting line - after all, some SWR meters are literally
three-resistor bridges with the unknown load impedance forming the
fourth arm. The bridge balance is not affected in any way by the length
or impedance of the line that is merely energizing the bridge. Only the
load impedance affects the bridge balance.


(If this line is
NOT 50-ohms then the meter incorrectly indicates the standing waves on it.
Which is what I said before and a lot of people disagreed.

And they were right, because the rho/SWR reading will *not* change.

What's incorrect is to believe that the rho/SWR meter is measuring
"standing waves" OR ANYTHING ELSE to do with its input line. It ain't.


Not that a false
indication is of great consequence when it is the incorrect choice of line
Zo where the problem arises.)

That contradicts what you said in (5) above. If the meter falsely
indicates correct transmitter loading, that certainly could be of
consequence.


(7) In practice at HF the length of this 50-ohm coax is often negligible.
The meter is often inside the transmitter box. As the misleading idea of
standingwaves on this short, even zero-length coax is nonsense

Reg, you must be about the only person in the world who worries about
that. The term "SWR" has almost completely freed itself from its
original literal associations with standing waves. Everyone else accepts
SWR as just one among the many mathematically interchangeable ways of
expressing the quality of an impedance match or mismatch. None of them
is either more or less valid than any of the others.

the name of
the instrument should be changed to TLI. (Transmitter Loading Indicator).
Which is all that it is!

It is indeed, so the very best of luck with your campaign to change its
name... ;-)


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

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 ?

The inconsistency lies in failing to meet the requirement that:

1. the correct load impedance for the transmitter

*and*

2. the system reference impedance for which the SWR meter was calibrated

*and*

3. the impedance of that connecting line

....must all be the same.

See the longer reply to your other message. (This is yet another way of
saying the same things.)


Does it lie in the change in effective
source impedance?

That's the one place where the inconsistency definitely does *not* come
from.


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

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.


However, between us, you HAVE contributed to an excellent demonstration of
the nonsensical mess which occurs when standing waves, reflection
coefficients, conjugate matches, virtual thingammies, Cecil's guaranteed
environments, etc, etc, are dragged in in futile attempts to explain what
goes on on that most simple imaginable of all connections between the
transmitter output and that so-called SWR meter.


It is not, as you imply, a trivial derisory matter to get yourself off the
hook. However you are in good company. For many years it has mis-informed
and confused newcomers, CB-ers, genuine students, and has obstructed
education in general about understanding the operation and adjustment of
antenna systems. And the eminent 'guru's' have managed successfully to
mis-inform and confuse themselves as evidenced by the incessant squabbling
on this newsgroup.


Fortunately, with radio, any bloody thing will work, even if you DON'T, as
you should, use 50-ohm coax between Tx and TLI. It's not likely to blow up
your transistorised PA. ;o)


Incidentally, if you DO use 75-ohm coax because you just happen to have a
reel of it lying around, then do as I did with one of my ancient home-brew
transeivers and use a 75-ohm TLI. Just change the setting of one preset
resistor or capacitor in a 50-ohm model. As you say, it's only a 3-arm
bridge !


Swap the connections to the little toroidal current transformer and you have
a crude thru-power meter PROVIDED the Tx load really is 50-ohms.. Forget
about reflected power - which nobody wants to know anyway - its scale has
no more use or meaning than that for the defunct SWR. Try the typist's
white stuff.


"Carry on London. Sweet violets."
----
Regards from a Californian-Wine-imbibing Italian Clown. ;o) ;o) ;o) ;o)
;o) ;o)
----
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