Richard Clark wrote:

Cecil Moore wrote:
Ever notice how many SWR meters have 3.0 at half scale?

Amateur grade - 1 in 4;

Even my double-needle meters have 3:1 as a vertical line
at the center. All of my single-needle SWR meters have 3.0
at half scale since they are actually reading |rho|.

Check out this humoungous one. Click on the small meter
picture for a full-size picture.

http://www.mfjenterprises.com/produc...prodid=MFJ-868
--
73, Cecil http://www.qsl.net/w5dxp

But how does the attenuation length of non-TEM modes relate to the Zo of
a transmission line?

ac6xg

Cecil Moore wrote:

Actually, it is called an argumentum ad verecundiam, an appeal
to authority - a technical authority in this case. I don't know
Kevin G. Rhodes at Dartmouth. He merely answered my question on
s.p.e. that didn't find an answer on this newsgroup. Exactly what
did you find technically wrong with the following evidence?

****Quote****
Newsgroups: sci.physics.electromag
From: "Kevin G. Rhoads"
Date: Tue, 07 Oct 2003 12:49:14 -0400
Subject: Transmission Line Question

For 10 MHz I would expect that all other modes
would be non-propagating (i.e., evanescent) even though RG-213
is a large coax (improved RG-8 apparently). The speed of propagation
is listed as 66%, so the nominal wavelength is 3/2 times the free
space wavelength for the TEM mode. 3/2 x 30m = 45m, which implies
the decay rate in space for non-TEM modes is going to be large
as the cable diameter is .405" (jacket) which implies the
spacing from inner to outer conductors will be less than .203".
For order of magnitude estimate, assuming the lowest non-TEM mode
can be approximated using a characteristic equation that really
is only applicable in Cartesian geometries:
(1/45m)**2 = (1/.203")**2 + kz**2
Clearly, kz must be imaginary to make this work. thus an
evanescant, non-propagating wave:
kz**2 = (1/45m)**2 - (1/.203")**2
To the accuracy used to date, the first term on the right
is negligible, so the decay rate, alpha, can be estimated:
alpha**2 = - (kz)**2 = (1/2.03")**2
Or, the lowest order undesired mode should reduce intensity
by a factor of 1/e (0.37) in about 2.03"; power will reduce
by that factor squared in the same distance (.135). In
about four inches, undesired mode power is down to about
0.018, in six inches, .00248, and after a foot, 6.14x10-6

You should double check my algebra, but I think the estimate
is reasonable. To put it into other terms, since the wavelength
in the coax dielectric is 45m and the conductor to conductor
spacing is about 2", any non-TEM mode will suffer attenuation
in E-field intensity with a space-rate constant rounghly
equal to the conductor to conductor spacing. INtensity
drops by 1/e = 1/2.71828 every 2 inches. Power availalbe
drops faster, being square of intensity.

So unless almost all the power diverts into an undesireable
mode (by a factor of more than a million to one), one foot
of cable should see pure TEM at the end.
***End Quote***

On Tue, 11 Oct 2005 16:57:51 GMT, Cecil Moore wrote:
I dug up some calculations from sci.physics.electromag
which you recite here; then in sci.physics.electromag you can quote
their use by authorities (sic both times) in
rec.radio.amateur.antenna....
This appeal is called a circle of friendship - not evidence.
Actually, it is called an argumentum ad verecundiam, an appeal
to authority - a technical authority in this case.

Actually - you have said nothing that removes this from a circle of
old wives sitting around the stove in the kitchen.

Name dropping is not an appeal to authority, nor are third hand
quotes.

On Tue, 11 Oct 2005 17:11:15 GMT, Cecil Moore wrote:
Even my double-needle meters have 3:1 as a vertical line
You have a rather limited exposure to the field of instrumentation.

Jim Kelley wrote:

But how does the attenuation length of non-TEM modes relate to the Zo of
a transmission line?

And to whether it's parallel or coaxial? Good question.
Kevin Rhodes' email address can be had from Google.
--
73, Cecil http://www.qsl.net/w5dxp

Richard Clark wrote:
Name dropping is not an appeal to authority, nor are third hand
quotes.

Exactly what did you find technically wrong with that
third hand quote?
--
73, Cecil http://www.qsl.net/w5dxp

I would then assume you disregard anything written in books as it falls in
the same category.

"Richard Clark" wrote in message
...
On Tue, 11 Oct 2005 16:57:51 GMT, Cecil Moore

Name dropping is not an appeal to authority, nor are third hand
quotes.

"Cecil Moore" wrote
Reg, I dug up some calculations from sci.physics.electromag
from about a year ago that indicate one foot of 50 ohm coax
on each side of the Bird is enough to make the line real,
i.e. not imaginary, and that's a conservative estimate.
===========================================

Cec, I still have a 30-watt, 160m, portable transceiver which I made
about 20 years back. It's in an aluminium attache case and still
works. In my travelling days I used to toss a wire out of the hotel
bedroom window.

Lift the lid and on the front panel is a 1.5"-square moving-coil
meter. It is used as a TLI on transmit and as an S-meter on receive.

The meter scale is calibrated 0-500 microamps.

But my imagination doesn't fool ME.

On receive, when the meter indicates 50% of full-scale deflection I
know that the meter is actually measuring 250 microamps.

And on transmit, when the meter indicates 90% of full scale deflection
I know that the meter is actually measuring 450 microamps.

Let this little anecdote be a friendly warning to they who use meters
with a 0 to infinity SWR scale, or scaled in terms of forward and
reverse power.
----
Reg, G4FGQ.

Richard Clark wrote:
On Tue, 11 Oct 2005 09:11:19 +0100, Ian White G/GM3SEK
wrote:

The subsequent conversion to VSWR is a mathematical relationship only.

Hi Ian,

This seems to be a particularly notable difference - to which
absolutely NO ONE has ever deviated from in ANY determination of SWR!

That is to say, this "mathematical" distinction that some rely on to
differentiate their arguments has not got one scintilla of difference
over any other method.

The only way to claim you "directly" measure SWR is to find some way
to place two probes of a meter along the line such that one probe goes
into the trough and the other into the peak and the meter reads SWR
directly. Unfortunately for rhetoric's sake, this STILL renders the
determination in terms of a mathematical relationship. It cannot be
escaped.

Thank you, you're right. The key difference between direct and indirect
measurements is not about the need for mathematics; it's about the need
for additional input from theory.

What I should have said is:

When you calculate the VSWR from measurements of maximum and minimum
voltage on the line, that simple division formula comes directly from
the definition of VSWR. The measurement is direct and completely
self-contained, needing no further input from transmission-line theory.

But you cannot calculate VSWR from a single-point measurement of
reflection coefficient unless without some additional input from
transmission-line theory which connects them together. This dependence
on additional theory is what makes the measurement an indirect one.


Why this keeps on being revisited must be to allow the new lurkers to
observe my correction.

Yes, by all means.


--
73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Reg Edwards wrote:


Let this little anecdote be a friendly warning to they who use meters
with a 0 to infinity SWR scale, or scaled in terms of forward and
reverse power.

This lesson is easier to remember if your first transmitter had an anode
current meter scaled in "Gallons Gone".


--
73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek