And for anyone doubting this dual definition of SWR, just take a look at
Agilent, Narda, or similar web sites, and notice that their high priced
and precision terminations all come with an SWR specification. You'll
also find SWR specs for test equipment inputs and many other devices not
containing transmission lines. These are clear examples of Definition 2.

Roy Lewallen, W7EL

Ian White, G3SEK wrote:
Reg Edwards wrote:


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.

In that case, calling the meter a "Transmitter Loading Indicator" will
only *add* to the confusion!

The rho/SWR meter has only ever been claimed to measure conditions on
the *load* side of itself. It knows nothing about conditions on the
transmitter side; and until Reg came along, nobody ever claimed that it
could.

Now here comes Reg, saying "Let's all call it a Transmitter Loading
Indicator!" Later, he realises that his "TLI" will only work correctly
if the connecting line between the transmitter and the meter is (a) of
impedance Zo, or (b) vanishingly short.

And his reaction to that: blame the meter! Me, I'd go looking for that
bloke who invented "TLI"...


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.



What Reg refuses to accept is that there are two alternative definitions
and usages of the term "SWR":

Definition 1. Given an impedance Z at any single point in any circuit,
and a system reference impedance Zo, then SWR is a *mathematical*
function of Z and Zo that tells you how close together the two
impedances are.



Definition 2. Given a transmission line that is terminated in an
impedance Z that is different from the line's own characteristic
impedance Z(line), then a standing wave of varying voltage and current
will appear on the line. If the line is long enough to identify a
voltage maximum and a voltage minimum (an electrical quarter-wavelength
away) then SWR is defined as the ratio Vmax/Vmin.

That was the original definition of SWR, but it has severe limitations.
For a lossless line with a characteristic impedance the same as Zo, the
two definitions of SWR are related by a simple, fixed mathematical
formula. But that's a special case; in all other cases, definition 2 has
to be applied with care (or you may even judge that it isn't valid at all).


Definition 1 is by far the most common definition and usage of "SWR" -
it's just one of several alternative measures of impedance match (others
including rho, return loss, S11 etc). All these different alternatives
are inter-related by defined conversion formulae. RF/microwave engineers
move freely between all of them, using whichever term is most convenient
at the time, or more normal in that particular area of electronics.

It's no big deal. Life is full of examples where the same word is used
to mean many subtly different things. The whole of the
electronics/RF/microwave engineering profession is very comfortable with
"SWR" meaning more than one thing. Most amateurs never need to think
about that difference; but if they do, they don't find it difficult.

AFAIK, Reg is the only person on the planet who understands all of this
perfectly, but refuses point-blank to accept it.

"Roy Lewallen" wrote And for anyone doubting this dual definition of SWR,
just take a look at
Agilent, Narda, or similar web sites, and notice that their high priced
and precision terminations all come with an SWR specification. You'll
also find SWR specs for test equipment inputs and many other devices not
containing transmission lines. These are clear examples of Definition 2.

====================================
Roy,

When trying to sell high-priced instruments, including those which are not
associated with transmission lines, manufacturers would be stupid not to
communicate with prospective customers in terms which customers imagine they
understand.

Mention of SWR no doubt sounds highly technical, but is confusing, perhaps
beyond comprehension, to people who have no connection with lines.

Manufacturers are not paid to educate their customers. They may claim it's a
part of their job in the sales blurb but we are familiar with sales-blurb
writers standards of education.

For example, only suckers accept antenna performance specifications as being
definitions of anything.

Roy, you really are scraping the bottom of the barrel to find some support
for the nonsense and confusing-to-learners waffle which fogs and enshrouds
what goes on on that short length of wire between HF transmitter and
so-called SWR meter.

Old habits die hard.
---
Reg, G4FGQ

Reg Edwards wrote:
"Roy Lewallen" wrote And for anyone doubting this dual definition of SWR,
just take a look at
Agilent, Narda, or similar web sites, and notice that their high priced
and precision terminations all come with an SWR specification. You'll
also find SWR specs for test equipment inputs and many other devices not
containing transmission lines. These are clear examples of Definition 2.

====================================
Roy,

When trying to sell high-priced instruments, including those which are not
associated with transmission lines, manufacturers would be stupid not to
communicate with prospective customers in terms which customers imagine they
understand.

Mention of SWR no doubt sounds highly technical, but is confusing, perhaps
beyond comprehension, to people who have no connection with lines.

Manufacturers are not paid to educate their customers. They may claim it's a
part of their job in the sales blurb but we are familiar with sales-blurb
writers standards of education.

For example, only suckers accept antenna performance specifications as being
definitions of anything.

Roy, you really are scraping the bottom of the barrel to find some support
for the nonsense and confusing-to-learners waffle which fogs and enshrouds
what goes on on that short length of wire between HF transmitter and
so-called SWR meter.

Old habits die hard.

If we could wipe the slate clean and start again, RF engineering could
probably manage quite well without the term "SWR".

But we can't. In the real world of radio communication, "SWR" is used
everywhere. Even absolute beginners have already heard about it. Nothing
will change that.

Roy and I are starting from where people actually a they've already
heard about SWR, so now they need to know what it means. And in the end,
it isn't all that hard to understand.


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

Ian, misguiding learners, undermining the foundations of radio education, is
my main objection to the ill-educated do-gooder discussions which take place
on this newsgroup.

Although at first I really did suspect your typist had made a translation
error between your dictation and her keyboard, I now see you insist you were
right first time.

You said -

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.


Ian, this statement will mislead and undermine students of radio for the
remainder of their careers. (You were one yourself.)

For their (and your) benefit it must be stated the so-called SWR/Rho meter
will indicate NOTHING about what lies on the antenna side of it EXCEPT,
indirectly, the magnitude RELATIVE to 50 ohms of the INPUT IMPEDANCE the
meter immediately sees looking towards the antenna.

Obviously, the impedance seen by the meter is that which, at HF, terminates
the other length of line, if one exists, between the transmitter and meter.

If, AND ONLY IF, the transmitter-to-meter line is 50-ohms then the 50-ohm
meter will indicate |Rho| relative to 50-ohms at the meter end of that line.
If the line is not 50-ohms then, although the meter may indicate SWR =1 or
Rho = TLI = 0, the transmitter will be loaded with something different from
50 ohms. Highly unsatisfactory!

The impedance of this line must NOT be neglected, as somebody said, on the
grounds that it forms part of the source impedance as seen by the meter, and
can be treated like the internal resistance of the transmitter as
irrelevant.

Alternatively, if the 50-ohm Tx-to-Meter line is long enough then that's the
line the meter indicates the SWR on. It is common knowledge SWR/Rho
determinations MUST begin by determining the value of the reflection
coefficient at the point where the reflection originates. ie., where the
meter is located along the uniform length of line.

The G5RV demonstrates the meter is ignorant of what may be happening on the
antenna side of the meter. Whatever the meter indicates it will NOT
correspond to |Rho| or SWR on the antenna's high-Zo balanced feedline. But
that's OK. Nobody wants to know anyway.

(Sooner or later someone will re-discover the special case that when the
transmission system is 50 ohms all the way from Tx to antenna, the meter
appears to measure |Rho| in both directions. This is because |Rho| is
sensibly constant in magnitude along the whole line. Actually |Rho| is
indicated looking back towards the transmitter and, by reversing the meter
in the line, thru-power is indicated looking towards the antenna. This
special case is the norm at UHF where line and connector dimensions are
critical and engineering economics force simplifications, including
simplification of underlying formulae. Chains of matrix algebra fit very
nicely into number-crunching computer software.

Now expect sombody who is still missing the point to say "not if the line is
1/2-wavelength long." But who wants a half-wavelength of line on 1.9 MHz
between the transmitter and swaarrr meter.
----
Reg, G4FGQ

We should embrace your definitions and perceptions rather than "scraping
the bottom of the barrel" by seeing how terms are used by manufacturers
of professional test equipment? Sorry about the blow to your ego, Reg,
but I do put more weight on how Agilent uses technical terms than how
Reg does. I guess Agilent has their audience and you have yours. Each to
his own.

And, um, Agilent (formerly HP) and Narda do sell products which are
"associated with transmission lines".

Roy Lewallen, W7EL

Reg Edwards wrote:
"Roy Lewallen" wrote And for anyone doubting this dual definition of SWR,
just take a look at

Agilent, Narda, or similar web sites, and notice that their high priced
and precision terminations all come with an SWR specification. You'll
also find SWR specs for test equipment inputs and many other devices not
containing transmission lines. These are clear examples of Definition 2.


====================================
Roy,

When trying to sell high-priced instruments, including those which are not
associated with transmission lines, manufacturers would be stupid not to
communicate with prospective customers in terms which customers imagine they
understand.

Mention of SWR no doubt sounds highly technical, but is confusing, perhaps
beyond comprehension, to people who have no connection with lines.

Manufacturers are not paid to educate their customers. They may claim it's a
part of their job in the sales blurb but we are familiar with sales-blurb
writers standards of education.

For example, only suckers accept antenna performance specifications as being
definitions of anything.

Roy, you really are scraping the bottom of the barrel to find some support
for the nonsense and confusing-to-learners waffle which fogs and enshrouds
what goes on on that short length of wire between HF transmitter and
so-called SWR meter.

Old habits die hard.
---
Reg, G4FGQ

Roy Lewallen wrote:
I guess Agilent has their audience and you have yours.

Agilent has some very good papers on s-parameters.
--
73, Cecil http://www.qsl.net/w5dxp



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