I have not insisted, not even mentioned, that SWR can exist only on
50-ohm lines.

All I have said, somewhere, is that the usual SWR meter gives the
correct answers only on 50-ohm lines.
----
Reg.

Reg Edwards wrote:
I have not insisted, not even mentioned, that SWR can exist only on
50-ohm lines.

All I have said, somewhere, is that the usual SWR meter gives the
correct answers only on 50-ohm lines.
----
Reg.

If I connect a 100 Ohm antenna through a 100 Ohm transmission line to
a SWR meter designed for 50 Ohms and then to a transmitter which expects
a 50 Ohm load, does the meter read correctly with respect to the desired
transmitter loading?

Of course it does.

--
Jim Pennino

Remove .spam.sux to reply.

On Mon, 26 Sep 2005 20:38:01 +0000 (UTC),
wrote:

Reg Edwards wrote:
I have not insisted, not even mentioned, that SWR can exist only on
50-ohm lines.

All I have said, somewhere, is that the usual SWR meter gives the
correct answers only on 50-ohm lines.
----
Reg.

If I connect a 100 Ohm antenna through a 100 Ohm transmission line to
a SWR meter designed for 50 Ohms and then to a transmitter which expects
a 50 Ohm load, does the meter read correctly with respect to the desired
transmitter loading?

Of course it does.

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

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.

Yet we would expect the "SWR meter designed for 50 Ohms" to which it
is connected (on the load side) to read VSWR=2:1, so is it measuring
the SWR on the load side of the meter as you earlier stated?

Perhaps typical SWR meters are actually sampling current and voltage
on a very short section (almost a point sample) of transmission line
that is part of the instrument, and they are indicating what the
observed SWR would be on an extended line of that type, provided that
attenuation was insignificant, and that the extended line was
sufficiently long to allow full development of the standing wave
pattern.

The inference of what is happening on adjacent line is ours, not the
instruments, as demonstrated by your example above.

Owen
--

Owen Duffy wrote:
On Mon, 26 Sep 2005 20:38:01 +0000 (UTC),
wrote:

Reg Edwards wrote:
I have not insisted, not even mentioned, that SWR can exist only on
50-ohm lines.

All I have said, somewhere, is that the usual SWR meter gives the
correct answers only on 50-ohm lines.
----
Reg.

If I connect a 100 Ohm antenna through a 100 Ohm transmission line to
a SWR meter designed for 50 Ohms and then to a transmitter which expects
a 50 Ohm load, does the meter read correctly with respect to the desired
transmitter loading?

Of course it does.

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.

Yet we would expect the "SWR meter designed for 50 Ohms" to which it
is connected (on the load side) to read VSWR=2:1, so is it measuring
the SWR on the load side of the meter as you earlier stated?

Yep.

Perhaps typical SWR meters are actually sampling current and voltage
on a very short section (almost a point sample) of transmission line
that is part of the instrument, and they are indicating what the
observed SWR would be on an extended line of that type, provided that
attenuation was insignificant, and that the extended line was
sufficiently long to allow full development of the standing wave
pattern.

The SWR that most people care about is that of the antenna.

Usually you have a 50 Ohm transmitter connected to 50 Ohm line, and
then to an antenna you hope is 50 Ohms.

To accurately measure the antenna SWR without any error being introduced
by line losses, you have to put the SWR meter at the end of the line
adjacent to the antenna.

This is usually impractical and we normally put the SWR meter near the
transmitter.

In this case the meter measures the SWR of the entire system, i.e. the
line going from the meter to the antenna and the antenna.

The net practical effect of the line loss for real line and real antennas
is that the observed SWR will indicate a lower value than if the meter
were directly connected to the antenna.

The inference of what is happening on adjacent line is ours, not the
instruments, as demonstrated by your example above.

Owen
--

--
Jim Pennino

Remove .spam.sux to reply.

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.


Yet we would expect the "SWR meter designed for 50 Ohms" to which it
is connected (on the load side) to read VSWR=2:1, so is it measuring
the SWR on the load side of the meter as you earlier stated?

Yep.

No, it isn't.

The SWR meter in your example reads 2:1 when the SWR on the 100 ohm
line is 1:1. Your example demonstrates that a typical SWR meter does
not measure, or necessarily indicate the SWR of the (actual)
transmission line on the load side of itself.

Owen
--

"Owen Duffy" wrote -
Your example demonstrates that a typical SWR meter does
not measure, or necessarily indicate the SWR of the (actual)
transmission line on the load side of itself.

----------------------------------------------------------------------
----------

You must have been reading what I've been writing for the last 6 or 7
years.
----
Reg.

On Tue, 27 Sep 2005 00:24:51 +0000 (UTC), "Reg Edwards"
wrote:


You must have been reading what I've been writing for the last 6 or 7
years.

Reg, Yes, I have probably read and learned a good deal from stuff you
have written, but I have skipped over a lot of what you have
written... probably most noticeably when you and others trade kicks to
the groin to see who is the last man standing.

Back on topic:

A point that you hinted at, but might have been overlooked by some is
that it can be relatively unimportant that the SWR bridge's sampling
line has the same characteristic impedance as the impedance at which
its detector has been nulled.

For example, a typical SWR meter designed originally for 75 ohms, with
a 0.1m long ideal 75 ohm sampling section, but with the detector
adjusted to read nil reflected power with a 50+j0 load on the
"antenna" terminals of the meter, will in most cases operate just as
well as a 50 ohm SWR meter on 7MHz, as the detector will truly show
when it has a 50 ohm load, the indicated VSWR for other loads will
substantially correct (ie within typical accuracy for the type of
instrument), and the insertion VSWR (~1.02:1) because of the 0.1m of
75 ohm line will be insignificant in practice.

In many amateur reflectometer designs (and in some commercial
implementations), very little attention has been given to the
characteristic impedance of the sampling section, and in some cases to
the insertion VSWR (that results).

I recall testing a relatively expensive SWR meter rated from 1.8 to
150MHz, and noting that whilst it indicated a VSWR1.1 at 144MHz on a
good dummy load a Bird 43 ahead of it indicated an insertion VSWR
1.5:1. So whilst it was good at indicating a 50+j0 ohm load on its
"antenna" terminals, it was not very capable of delivering that load
to its "transmitter" terminals.

Owen
--

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.

If you use a 100 Ohm SWR meter you get 1:1.

The SWR is a function of the TOTAL SYSTEM impedance connected to the
load side.

Replace the 100 antenna with a 100 Ohm resistor and the reading doesn't
change.

Eliminate the line and connect the 100 Ohm resistor directly to the
meter and the reading doesn't change.


Yet we would expect the "SWR meter designed for 50 Ohms" to which it
is connected (on the load side) to read VSWR=2:1, so is it measuring
the SWR on the load side of the meter as you earlier stated?

Yep.

No, it isn't.

The SWR meter in your example reads 2:1 when the SWR on the 100 ohm
line is 1:1. Your example demonstrates that a typical SWR meter does
not measure, or necessarily indicate the SWR of the (actual)
transmission line on the load side of itself.

The SWR of the SYSTEM, line and antenna, is NOT 1:1 for a 50 Ohm
reference.

Owen
--

--
Jim Pennino

Remove .spam.sux to reply.

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.

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.

Owen
--