Owen Duffy wrote:
On Tue, 27 Sep 2005 01:36:15 +0000 (UTC),
wrote:

Owen Duffy wrote:
On Mon, 26 Sep 2005 21:39:27 +0000 (UTC),
wrote:


Jim, that seems inconsistent with your earlier statemetn "No, the SWR
being measured is on the load side of the meter."

The load side is the side with the load, i.e. the antenna, on it.

In the example you quoted with a 100 ohm load on a 100 ohm line, were
the line loss low, and the line long enough to be sure to sample a
fully developed voltage maximum and voltage minimum it would be found
that the VSWR was 1:1.

Not for a 50 Ohm system, i.e. a transmitter expecting 50 Ohms and a
meter calibrated for a 50 Ohm system.

I am sorry Jim, the VSWR is a property of the transmission line and
its termination, and the VSWR on that 100 ohm line with a 100 ohm
termination is 1:1. The VSWR could be *MEASURED* on that line by
sampling the magnitude of the voltage at different points on the line
and it would be found that the magnitude of the voltage was constant,
which means VSWR=1:1.

No, the measured SWR is relative to the design impedance of the SWR
meter which is normally 50 Ohms.

The SWR on the line depends on the characteristic impedance of the
line and the impedance of the termination of the line. 50 ohms does
not come into it.

We are not talking about SWR on the line, we are talking about SWR
at the input END of the line; big difference.

The SWR on your proposed 100 ohm line with a 100 ohm termination is
1:1. If your measurement indicates anything else, then you need to
consider your measurement as invalid.

Once again, we are not talking about SWR *ON* the line, we are talking about
SWR as seen at the input *END* of the line; big difference.

Furthermore, the SWR *ANYWHERE* on the line is *NOT* 1:1 for a 50 Ohm
reference.

Try hooking a length of 93 Ohm line (which is easier to get than 100 Ohm
line) terminated with a 93 Ohm resistor to any 50 Ohm SWR meter of any type.

Then hook just the 93 Ohme resistor to the meter and tell me what the
difference is in the readings.

Owen
--

If anything is misnamed it is the term SWR.

SWR is nothing more than a dimensionless impedance ratio.

You do NOT need a transmission line to have SWR in spite of the W in
SWR standing for 'wave'.

--
Jim Pennino

Remove .spam.sux to reply.

On Tue, 27 Sep 2005 02:25:11 +0000 (UTC),
wrote:


SWR is nothing more than a dimensionless impedance ratio.

Is that so... Owen
--

Owen Duffy wrote:
On Tue, 27 Sep 2005 02:25:11 +0000 (UTC),
wrote:


SWR is nothing more than a dimensionless impedance ratio.

Is that so... Owen
--

Yes, it is so and it is equal to:

SWR = (A + B)/(A - B)

Whe

A = sqrt ( (R + Z)^2 + X^2 )
B = sqrt ( (R - Z)^2 + X^2 )

R = resistive component of load impedance in Ohms.
X = reactive component of load impedance in Ohms.
Z = reference impedance (purely resistive) in Ohms

If you don't believe it, get some resistors, capacitors and a half
way decent SWR meter and do some experiments; no transmission line
required.

--
Jim Pennino

Remove .spam.sux to reply.

On Tue, 27 Sep 2005 02:54:31 +0000 (UTC),
wrote:

Owen Duffy wrote:
On Tue, 27 Sep 2005 02:25:11 +0000 (UTC),
wrote:


SWR is nothing more than a dimensionless impedance ratio.

The fundamental definition of SWR flows from the behaviour and
properties of RF transmission lines.

When a transmission line is terminated in an impedance other than its
characteristic impedance, there will be both a forward wave and a
reflected wave of such magnitude to resolve the conditions that must
apply at the termination.

The forward wave and the reflected wave sum at all points along the
line having regard for their magnitudes and relative phase to produce
a "standing wave". The Standing Wave Ratio (SWR or VSWR) is defined to
mean the ratio of the maximum to the minimum of the magnitude of the
standing wave voltage pattern along the line.

The SWR on a lossless line can be calculated knowing the complex
characteristic impedance of the line and the complex load impedance.

The SWR on the line does not depend in any way on some unrelated
independent reference resistance as you suggest in your formula.

You seem to be suggesting that your redefined SWR is a really good
(obscure) way to talk about an impedance (independently of a
transmission line) in terms of some standardised reference value, and
you can throw away the fundamental meaning of SWR to support your
SWR(50) concept. In your terms (independently of a transmission line),
for instance, a Z of 60+j10 would be SWR(50)=1.299, and so would an
infinite number of other Zs have SWR(50)=1.299... how is that of
value. To know Z is 60+j10 is to know more than to know SWR(50)=1.299.

Owen
--

Jim,

Perhaps there's some misunderstanding about location of the meter and
what it is measuring. Let's try to clear it up.

Would you please do us both a favour by answering the following simple
question?

There is a 50 ohm line feeding a 100 ohm antenna.

There is an SWR meter located at the line-antenna junction.

The meter has a reading. Does the reading apply to SWR of the
antenna, or does it apply to the SWR along the feedline?

Antenna or Feedline?
----
Reg.

"Reg Edwards" wrote:
There is a 50 ohm line feeding a 100 ohm antenna.
There is an SWR meter located at the line-antenna junction.
The meter has a reading. Does the reading apply to SWR of the
antenna, or does it apply to the SWR along the feedline?
______________

It applies to the match of the RF network that follows the SWR meter to the
impedance for which the SWR meter was calibrated.

And if in your example the SWR meter has been calibrated for 50 ohms, and is
moved to the input end of that line+antenna RF network, it will also have a
reading -- which will be the same as when it was inserted at the
antenna-line junction, less the round-trip RF attenuation of the
transmission line (assuming that the transmission line is 50 +/- j0 ohms
throughout its length).

In fact it is a common practice to optimise the transmission line/antenna
match of commercial FM and TV broadcast antenna systems by use of a variable
transformer inserted at the antenna input, whose adjustment is made by
reference to the far-end reflection seen at the sending end of the
transmission line, using a high-directivity reflectometer, or SWR meter.

The same physics applies to ham antenna systems and methods/means of
measurement.

RF

Visit http://rfry.org for FM transmission system papers.

Richard,

If's and But's are not required.

The antenna is just an arbitrary load.

Does the meter reading indicate SWR on the feedline (which is what is
usually required), or does it not?

This is not a "catch question". It is not a troll.

"Antenna or Feedline?" please.

KISS
----
Reg.

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

"Richard Fry" wrote in message
...
"Reg Edwards" wrote:
There is a 50 ohm line feeding a 100 ohm antenna.
There is an SWR meter located at the line-antenna junction.
The meter has a reading. Does the reading apply to SWR of the
antenna, or does it apply to the SWR along the feedline?
______________

It applies to the match of the RF network that follows the SWR meter
to the
impedance for which the SWR meter was calibrated.

And if in your example the SWR meter has been calibrated for 50
ohms, and is
moved to the input end of that line+antenna RF network, it will also
have a
reading -- which will be the same as when it was inserted at the
antenna-line junction, less the round-trip RF attenuation of the
transmission line (assuming that the transmission line is 50 +/- j0
ohms
throughout its length).

In fact it is a common practice to optimise the transmission
line/antenna
match of commercial FM and TV broadcast antenna systems by use of a
variable
transformer inserted at the antenna input, whose adjustment is made
by
reference to the far-end reflection seen at the sending end of the
transmission line, using a high-directivity reflectometer, or SWR
meter.

The same physics applies to ham antenna systems and methods/means of
measurement.

RF

Visit http://rfry.org for FM transmission system papers.

Reg Edwards wrote:
Jim,

Perhaps there's some misunderstanding about location of the meter and
what it is measuring. Let's try to clear it up.

Would you please do us both a favour by answering the following simple
question?

There is a 50 ohm line feeding a 100 ohm antenna.

There is an SWR meter located at the line-antenna junction.

What does this mean?

The meter has a reading. Does the reading apply to SWR of the
antenna, or does it apply to the SWR along the feedline?

The reading is the SWR at that point.

Antenna or Feedline?
----
Reg.

An SWR meter reads the SWR of the thing connected to its output port
with respect to the reference impedance the meter was designed for.

The SWR meter reads the SWR *AT THE POINT OF CONNECTION* of the
connected system.

Not the middle of the system, not the other end (if it has one)
of the system, but the input point.

If you measure a SWR (50 Ohms reference) of 2:1 for a black box,
what is in the box?

A. A 25 Ohm resistor.
B. A 100 Ohm resistor.
C. A cable spool of coax with some impedance at the end of it.
D. Could be any of the above.

In general there is no guarantee that the SWR at any point of a
transmission line will be equal to the SWR at any other point on a
transmission line other than for special cases.

What seems to have you terribly confused is that all the transmission
lines, tuners, antennas, connectors and whatevers become a *SYSTEM*
and the SWR at the input connector to the *SYSTEM* is not guaranteed
to be the same as the SWR at some arbitrary point inside the system.

When you measure the SWR of a line with a load on the end, you are
measuring the SWR of the entire system relative to your reference,
not the load.

--
Jim Pennino

Remove .spam.sux to reply.

Jim, I'm sorry you are unable to answer the simple question "Feedline
or Antenna?".
----
Reg.

Owen Duffy wrote:
On Tue, 27 Sep 2005 02:54:31 +0000 (UTC),
wrote:

Owen Duffy wrote:
On Tue, 27 Sep 2005 02:25:11 +0000 (UTC),
wrote:


SWR is nothing more than a dimensionless impedance ratio.

The fundamental definition of SWR flows from the behaviour and
properties of RF transmission lines.

And power=EI. And it also equals I^2*R and E^2/R.

SWR can be expressed in terms of power ratios, current ratios, and
impedance ratios.

When a transmission line is terminated in an impedance other than its
characteristic impedance, there will be both a forward wave and a
reflected wave of such magnitude to resolve the conditions that must
apply at the termination.

Irrelevant.

The forward wave and the reflected wave sum at all points along the
line having regard for their magnitudes and relative phase to produce
a "standing wave". The Standing Wave Ratio (SWR or VSWR) is defined to
mean the ratio of the maximum to the minimum of the magnitude of the
standing wave voltage pattern along the line.

Is is also defined as a current ratio and an impedance ration.

The SWR on a lossless line can be calculated knowing the complex
characteristic impedance of the line and the complex load impedance.

What no waves, just impedences!! Now you are contidicting yourself.

The SWR on the line does not depend in any way on some unrelated
independent reference resistance as you suggest in your formula.

Read it again.

The R is the R of the thing at the end of the line.

The X is the X of the thing at the end of the line.

The X is the impedance of the line.

You seem to be suggesting that your redefined SWR is a really good
(obscure) way to talk about an impedance (independently of a
transmission line) in terms of some standardised reference value, and
you can throw away the fundamental meaning of SWR to support your
SWR(50) concept. In your terms (independently of a transmission line),
for instance, a Z of 60+j10 would be SWR(50)=1.299, and so would an
infinite number of other Zs have SWR(50)=1.299... how is that of
value. To know Z is 60+j10 is to know more than to know SWR(50)=1.299.

The equations given are general and can be derived from first priciples.

The Z in the equations is the Z of your reference, i.e. 50 for a 50
Ohm system.

SWR is *ALWAYS* relative to some reference impedance.

Owen
--

--
Jim Pennino

Remove .spam.sux to reply.