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 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
--

Inaccurate crap equipment has nothing to do with the arguement.

--
Jim Pennino

Remove .spam.sux to reply.

Owen,

SWR meters with a sampling line.

The only experience I've had has been I once made one for HF. It was
of the type where a second wire is drawn alongside the inner conductor
of a short length of coaxial line of impedance in the same street as
the system it is to work with.

Operating frequencies covered the whole of the HF band. That is a very
wide band. Which indicates that line length plays no part in
measuring accuracy once calibrated.

To explain how the thing works it is necessary to return to what it
really is. It is a resistance bridge. All so-called SWR meters,
whatever the circuit or form of construction, are resistance bridges.

The bridge has 3 internal ratio arms. The 4th arm is the variable
transmitter load. If all 4 arms are of same resistance we have a very
sensitive arrangement suitable for QRP transmitters. However, 3/4 of
the TX power is dissipated in the 3 internal bridge arms.

For higher power transmitters it is necessary to use high ratios for
the ratio arms. In the case of meters which use a little ferrite ring
as a current transformer, a resistor of the order of 30 to 100 ohms
can be shunted across the current transformer secondary winding while
the primary winding has an input resistance of the order of 0.1 ohms
which forms the value of the ratio arm in series with the external
load. This 0.1-ohm arm is capable of carrying the load current of
several amps with only a small power loss.

The other two ratio arms can be a pair of high value resistors in the
same ratio as occurs via the current transformer. If the input
resistance of the current transformer is 0.1 ohms then the bridge
ratio is 50 / 0.1 = 500:1 where 50 ohms is the usual value of the
load resistance when the bridge is balanced and SWR = 1:1

The two high impedance arms can be capacitors in the same ratio of
500:1 which have zero power dissipation but have a minor effect on
accuracy. They introduce a small phase angle into the load as seen by
the transmitter through the meter. The error increases with
increasing frequency.

It will be seen that the take-off point is effectively the same for
both current and voltage.

Returning to the so-called sampling line.

There is a bridge configuration which is not quite so obvious. But
instead of a current transformer the current is picked off by means of
a short length of wire in parallel with the coaxial inner conductor by
virtue of their mutual inductance. The line is too short for
propagation effects to play a significant part.

Voltage is picked off at the same point by virtue of the capacitance
between the wire and coaxial inner conductor. The phase relationship
between volts and amps can be reversed just by reversing the direction
of propagation through the meter.

The bridge ratio is set partially by the ratio of the impedances Zo of
the additional wire and inner coax conductor. The length of coaxial
line affects only the bridge sensitivity and power dissipated in the
meter. As you must be aware, sensitivity falls of fast with decreasing
frequency and 160 meters was my favourite band. So the home-brewed
meter was soon discarded and I returned to ferrite rings.

I was left with the impression it was very easy to make and that
almost anything would work.

Hope you can understand the foregoing.
----
Reg.

Reg Edwards wrote:
There is a bridge configuration which is not quite so obvious. But
instead of a current transformer the current is picked off by means of
a short length of wire in parallel with the coaxial inner conductor by
virtue of their mutual inductance. The line is too short for
propagation effects to play a significant part.

The pickup lines in my Heathkit HM-15 are terminated on one
end with a 50 ohm resistor. One pickup line thus attenuates
the reflected traveling wave and allows the forward traveling
wave to be rectified. The other pickup line attenuates the
forward traveling wave and allows the reflected traveling wave
to be rectified. Knowing the peak values of both of these two
traveling waves allows a calibrated meter to indicate SWR.
--
73, Cecil http://www.qsl.net/w5dxp

Cec, you have YOUR explanation and I have MY explanation.

Which is the most simple?

There is a bridge.

When the variable arm, the load, is 50 ohms the bridge is balanced and
the meter indicates SWR = 1:1

When the variable arm is either 0 ohms or infinite ohms, the meter
indicates SWR = infinity :1

What can be more simple than that? How it works can be visualised.

But the meter is ambiguous. It cannot distinguish between loads of 0
ohms and infinite ohms. Additional information is required.

This serious ambiguity also applies to your weird contraption. ;o)
----
Regards, Reg.

Cec, I notice that you and others have begun to use my description of
"indicate" rather than "measure".
----
Reg.

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.