Roy Lewallen wrote:

Dr. Slick wrote:
Agreed. Then any mismatch loss from PA to the 50 ohm coax is
never measured at the meter. So we never really measure the reflected
power coming right out of the PA, even if we attach the meter directly
to it's output.

No "reflected power" comes out of a PA.

By definition. I wonder who invented that definition? It seems pretty
obvious that not all PA's are Z0-matched so they will always re-reflect
100% of the incident reflected power. But that is exactly what that
definition implies.
--
73, Cecil http://www.qsl.net/w5dxp



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Dear all, Here's a useful tip -

The loss along any sort of HF transmission line, SWR or not, increases with
line temperature according to just ONE HALF of the resistance-temperature
coefficient of the conductors. Why ? It's something to do with skin
effect.

RTC of copper is near enough 0.4 percent per degree C.

So loss along any line, in dB or nepers, increases by 0.2 percent per degree
C.

Now you may not think this matters very much. But if you consider a seasonal
change in temperature on the ocean bottom of only 2 degree C along a
2000-mile transatlantic cable which has an attenuation at 5 MHz of 1.6 dB
per mile, total attenuation = 3,200 dB, then you will appreciate the
responsibilty laid on the shoulders of the design engineers of the first
oceanic telephone cable systems.

An uncertainty of 0.4 percent of 3,200 dB = 13 dB which is enough to wreck
system operation. Omission of a submerged repeater allows signals to fall
below noise level at the last repeater. Addition of one more repeater
overloads the last repeater causing cross-modulation, cross-talk and noise.
Bear in mind repeaters are both-direction amplifiers and the lowest speech
channels are at 60 KHz where overall attenuation over the same distance is
only about 350 dB.

Repeater power is fedover the the inner cable conductor from constant
current sources at both ends, maximum voltage = 10 KV. +ve from one end, -ve
from the other. During magnetic storms and other disturbances the potential
difference between ground electrodes in N.America and W.Europe can rise to
several thousand volts. Although the last time I measured it on an AVO-8 it
was only 1.3 volts. I did, of course, make use of the safety grounding
stick before using the crocodile clips on the ends of the meter leads.
Depended on the tides and the flow of the Gulf Stream across the Earth's
magnetic line of force.

It always struck me as being highly incongruous, even absurd, that in normal
operation, cables of the highest possible quality materials, manufactured by
automatically controlled, specially-designed precision machinery, laid at
great expense by an 8000-ton, specially-design ship over thousands of miles,
should end up by being terminated with a foot or so of soldered, screwed-up,
cotton-covered 22-gauge wire rescued from the terminal-station scrap bin.
This is true. I have seen both ends with my own eyes. On one occasion I even
did the soldering after completing overall tests! But I was careful to use
a fairly straight length of wire with sufficient sag to eliminate any
possible tension beyond that due to its own weight.

Dear readers, believe me, there's no time to worry about SWR when loss in
revenue amounts to $100,000 per hour + repair-ship expenses every time a
flatfish trawler scoops up a cable in its net, cuts it free with an axe, and
the skipper sneaks away at top speed without telling anybody in case he has
to pay for the damage.

Coax cable Zo = 43 ohms. Diameter over polyethylene = 1 inch. Inner
conductor = longitudinal overlapping crimped copper tape, laid over the
cable's principal strength-member of a number of high-tensile steel wires,
overall inner diameter about 1/3 inch. Outer conductor = 6, touching,
longitudinal aluminium tapes with a small spiral lay. Sheath = 0.1-inch
thick extrusion of black polyethylene if I remember correctly. For shallow
water and continental shelves there was a number of protective heavy iron
wires laid on a bed of tarred hessian as had been used for 100 years on the
first of the Atlantic telegraph cables.

I sometimes think of (the later) Lord Kelvin who followed his calculations
with the recommendation to investors "Go ahead, make and lay the bloody
stuff". But it was Heaviside, a generation later, a genius who died of
neglect, who eventually described how the "bloody stuff" and radio
propagation really worked.

Folks, just a little light-hearted digression, a respite from so-called SWR
meters. Please continue with your discussion. ;o)
----
Reg, G4FGQ

Someone sed:

"The SWR is based on the ratio of the forward to the reflected power."

But this isn't the *definition* of SWR.

Trivia question: What is the definition of SWR?

73 de Jack, K9CUN

(Tom Bruhns) wrote in message om...
....
The resistance of the copper in the transmission line changes with
temperature. If ambient is 20C and you put in enough power to heat up
the line (center conductor) to 70C, that's a 50C change, and will
result in about an 18% increase in resistance. So if you had a line
which had 3dB loss at 20C, it would increase to about 3.5dB at 70C.
If the load end has a 2:1 SWR, then the sending end will have about
1.40:1 SWR at 20C and about 1.35:1 at 70C. It's not a _big_ change,
but it should be observable on an SWR meter that is accurate over a
wide range of powers.

Correction... First, thanks to Reg for his posting on the same
general topic. He pointed out that the RF resistances changes as the
square root of the DC resistance, and though I know that perfectly
well, had not put that into the above statements. With that
correction, I believe the result will be more like a change of 8% in
RF resistance, a loss change from 3.00dB to 3.26dB, and a change from
about 1.40 to 1.37 SWR. The point is the same; it would be observable
on a good SWR meter, though not distinguishable from other effects on
a typical cheap one.

Cheers,
Tom

There is NO definition of SWR!!

There is a definition for VSWR!

There is a definition of ISWR!

But, unfortunately, there is NO definition for SWR!! Why don't you
offer one?

Deacon Dave, W1MCE

JDer8745 wrote:
Someone sed:

"The SWR is based on the ratio of the forward to the reflected power."

But this isn't the *definition* of SWR.

Trivia question: What is the definition of SWR?

73 de Jack, K9CUN

You must not have seen my posting yesterday on this thread, where I gave
the definition. Do I need to post it again?

Roy Lewallen, W7EL

JDer8745 wrote:
Someone sed:

"The SWR is based on the ratio of the forward to the reflected power."

But this isn't the *definition* of SWR.

Trivia question: What is the definition of SWR?

73 de Jack, K9CUN

On Thu, 14 Aug 2003 08:30:20 +0100, "Ian White, G3SEK"
wrote:


The subject is why SWR meters might read differently with different
lengths of coax. Your statements about mismatch uncertainty are true,
but not relevant.


Hi Ian,

Look at the subject line above. Everything reported by me responds
directly to it. I notice you have nothing relevant to add in that
regard.

To this point NO ONE has responded to the data, nor to the
authoritative citations. It would be more useful for you folks to
point out error rather than simply arm-chair your way through this
with denial. To this point NO ONE has offered any synopsis of my
error in method. Such close examination appears to be confined to
re-evaluations of the CFA which lack both, and thus make a more
amusing target that does not test anyone's skill beyond debate. In
the words of Ho Chi Minh: "Paper Tigers."

Also to this point, the only "critics" have been those with a voiced
interest in not assigning a value to Source Z, but again demonstrating
a more than ample capacity to arm-chair their way through byzantine
explanations of what it is "not." Bench work seems to be anathema,
however I do enjoy the zen-cartwheels being performed, thanks.

If I were to offer this with the infinite regress of Cecil's logic,
hide my data, and embellish my method, you would all be compounding
this thread to 600 entries. I don't play that game, sorry to
disappoint you fellows, but I don't write to entertain (even if I can
do it better than you without really trying - sometimes, like now,
opportunity begs). The repetition of data, like denial, is not
debate, so I am satisfied to post real bench work once, and pull paper
tails until that goes stale.

I am particularly amused by protestations that examination of data is
not worth anyone's time, but offering editorialization is. :-)
Clearly most of you should take more pleasure in your amusing
recreations.

You guys worry this out of all proportion, you act like this is
especially important and it certainly seems to set your teeth on edge.
As many would point out, this is only a hobby; it's not like your job
is on the line, or that you have to meet a customer's expectation. :-)

73's
Richard Clark, KB7QHC

Richard Clark wrote:
I am particularly amused by protestations that examination of data is
not worth anyone's time, but offering editorialization is. :-)

For those of us who missed the data, what date and title does
that posting possess?
--
73, Cecil, W5DXP

On 14 Aug 2003 08:30:53 -0700, (Tom Bruhns) wrote:

"Ian White, G3SEK" wrote in message ...
...
The subject is why SWR meters might read differently with different
lengths of coax. Your statements about mismatch uncertainty are true,
but not relevant.

Somewhere in all this mess, I believe RC posted that the OP's
originally posted observations supported his claim (that source
impedance affects line SWR), but yet when I go back and read the OP's
observations, they are that the POWER delivered to the load changes
significantly with line length and the measured line SWR DOES NOT
change noticably.

It seems to me that the OP's observations directly support that the
line SWR does not change with changes in source impedance: if the
transmitter output impedance is not equal to the line impedance, the
additional line can be though of as lumped with the transmitter and
where it connects to the original length would be the point where the
source impedance would be measured...and it would have changed
significantly with added line, while the SWR did not.

Cheers,
Tom

Hi Tom,

Thanks for actually looking. You are indeed correct in your statement
about the "small" variation of SWR:
SWR DOES NOT change noticably.
This, too, is consistent with the data I have offered. Casual
benchwork is as likely to reveal that as not. This is to say that no
real examination was originally performed, it was discovered by
accident (the portents of the variation of Power were more
distracting) and the readings were discarded as, perhaps, simple
statistical fluctuation. If through the sheer chance of opportunistic
luck in drawing different line lengths into the original posting,
those values may have been far more significant. If I had trimmed my
data to a very short interval, it would have matched his. The
variation follows a sine curve shape (if anyone had actually viewed
the data, this would be obvious) and you can take several readings
along the slope with alarm, or several along the crest to
indifference. This simply reveals that the original posting lacked
many essential details, but contained many confirmatory ones.

This does not change the fact that as a subset of my data, that it
lacks supporting my results simply because it does not go to the
breadth nor depth of my study. I in fact offered three significant
quotes of his original observations made that are classic indicators
of Mismatch Uncertainty. The simple point of that issue is that you
cannot measure Power with any promise of accuracy when placed between
two discontinuities. Unless you can describe all paths. This is a
staple of simple wave interference. It necessarily follows that if
you cannot determine Power due to this indeterminacy, you have no
chance of determining SWR (you can certainly offer a reading and go on
about life blithely unaware).

Simply put, I don't see anyone here willing to put their "faith" to
the test; and I am the only one here with my skin in the game. I am
far more likely to accept error through honest effort equal to mine
that demonstrates it. But critics to this point have not only refused
to step up to the bench, but they have also denied the argument
through aggressively ignoring my methods and their results. I observe
far more reading of my comments here, than with that one posting. As
such, it is more than obvious that entertainment is the driving force
for this criticism, not technical review.

73's
Richard Clark, KB7QHC

(JDer8745) wrote in message ...

Trivia question: What is the definition of SWR?

Oh, Jack, that's not a _trivia_ question, that's a _TRICK_ question! :-)

Cheers,
Tom