Dr. Slick wrote:
BTW, how do you know the accuracy of your homebrew SWR meter?

I have a bunch of 50W 600 ohm non-inductive resistors that I use
for calibration purposes. And I really don't know the accuracy.
An SWR of 20:1 looks the same as an SWR of 25:1 on the scale.
I have an upper and lower acceptable limit to the SWRs with the
matching method I use. The SWR meter tells me if the SWR is
outside of that acceptable range.
--
73, Cecil http://www.qsl.net/w5dxp



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Dr. Slick wrote:

As you might know, the input S11 or SWR will change when you go
from an antenna analyzer or network analyzer to measuring with the
actual full power PA and meter hooked up. This may be partly due to
the fact that the meter is usually not a perfect 50 ohm thru, and
partly due to the fact that the analyzers outputs are closer to 50
ohms than the PA.

If S11 or the SWR actually does change, you've either got a nonlinear
transmission line or a nonlinear load. That is, the impedance changes as
the signal level changes. If you *measure* a different SWR or S11, it
means that either the SWR or S11 is actually changing for the reasons I
just stated, or the meter is nonlinear in the sense that its reading
changes with power level (possibly due to RF ingress, but it could be a
host of other things), or you're measuring with two different meters
that don't agree.

It's not because of the different source impedances.

Sure, you can normalize a Smith chart to anything you'd like. That
doesn't make the SWR change with source impedance.

. . .

Roy Lewallen, W7EL

W5DXP wrote:
Dr. Slick wrote:
BTW, how do you know the accuracy of your homebrew SWR meter?

I have a bunch of 50W 600 ohm non-inductive resistors that I use
for calibration purposes. And I really don't know the accuracy.
An SWR of 20:1 looks the same as an SWR of 25:1 on the scale.
I have an upper and lower acceptable limit to the SWRs with the
matching method I use. The SWR meter tells me if the SWR is
outside of that acceptable range.

It's interesting to see an example of an SWR meter for a Z0 that isn't
50 ohms, because it helps to confirm that they all work in basically the
same way.

If [V] is a sample of the line voltage, and [i] is a sample of the line
current, then the forward reading is the sum of two RF voltages, [V] +
[i]R, where R is the resistor that converts the [i] sample into a
voltage.

The reverse reading is the difference, [V] - [i]R. The "calibration to
Z0" procedure consists of terminating the line in the design value of
Z0, and then adjusting R so that the reverse reading [V] - [i]R is zero.

The RF voltages are either summed or subtracted, and then the resultant
is detected by the diode.

Just one small point, though... it is not necessary that R = Z0. The
value required depends on the sampling factors kV and kI that relate the
voltage and current in the line to the sampled values [V] and [i]. In
full, the instrument is calibrated to Z0 when:

kV*V - kI*I*R = 0

In a typical bridge, two out of the three constants kV, kI and R are
fixed, and the third is adjustable. In a Bruene bridge, kI is fixed by
the number of turns on the current-sampling toroid, R is fixed, and you
calibrate the bridge by adjusting the kV factor in the voltage divider.
However, it would be equally good to build-in fixed values of kV and kI,
and balance the bridge by making R a small trimpot. So R really does not
have to equal Z0... and in most published circuits, it doesn't.

This can also be shown in a different way, by thinking of it as a
Wheatstone bridge, with Z0 as one arm. The requirement for balance is
only that Z0/R2 = R3/R4. It is not necessary for any of the other
resistors R2, R3 or R4 to equal Z0 in order to achieve balance.

AFAIK, the only situation where the "terminating" resistor truly needs
to equal Z0 is in parallel-line couplers for microwaves, when the
sampling line approaches a quarter-wavelength long and its own
characteristic impedance is Z0 too.


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

Dr. Slick wrote:
As you might know, the input S11 or SWR will change when you go
from an antenna analyzer or network analyzer to measuring with the
actual full power PA and meter hooked up. This may be partly due to
the fact that the meter is usually not a perfect 50 ohm thru, and
partly due to the fact that the analyzers outputs are closer to 50 ohms
than the PA.

Sorry, that is exactly wrong. S11, SWR and the impedance itself, do
*not* change when you connect a different instrument to the same load.
All the changes you have described are due entirely to instrument
errors.

That's how the instrument errors are determined... by knowing for a fact
that, whatever all the different instruments may say, the impedance
they're trying to measure is the one thing that has *not* changed.

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

"Ian White, G3SEK" wrote in message ...

Sorry, that is exactly wrong. S11, SWR and the impedance itself, do
*not* change when you connect a different instrument to the same load.
All the changes you have described are due entirely to instrument
errors.

That's how the instrument errors are determined... by knowing for a fact
that, whatever all the different instruments may say, the impedance
they're trying to measure is the one thing that has *not* changed.


You are right, but i never stated that the impedance we are
feeding ever changes, only the measured SWR.

Hey, we live in the real world, with real instrument errors. This
is the case i am talking about when i started the thread. I usually
DON'T measure the same SWR from antenna analyzer versus PA and meter
hooked up, and this may be due to the fact that the PA has a different
source impedance than the analyzer. I'm not claiming that the
impedance we are feeding has changed. And if you read my original
post, you will notice that the SWR didn't change when the coax length
was changed, mainly the incident power.

How would you explain what Cecil wrote? How are some people
improving SWR by changing coax length, when in theory they shouldn't
be able to do this?

Do you think the series reactance a system offers a PA may
actually improve it's incident power?

Slick

Dr. Slick wrote:
And i don't think you can expect the measured SWR with the meter
and PA to be exactly the same as what you get with a small-signal
analyzer. It's usually a bit different.

The actual conceptual SWR and the difficulty in measuring the SWR are two
different things. Low power level SWRs are difficult to measure because
of the diode voltage drops. Better to use a class-B amp for such where the
zero-crossing points are aligned.
--
73, Cecil http://www.qsl.net/w5dxp



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Dr. Slick wrote:
"Ian White, G3SEK" wrote in message
...

Sorry, that is exactly wrong. S11, SWR and the impedance itself, do
*not* change when you connect a different instrument to the same load.
All the changes you have described are due entirely to instrument
errors.

That's how the instrument errors are determined... by knowing for a fact
that, whatever all the different instruments may say, the impedance
they're trying to measure is the one thing that has *not* changed.


You are right, but i never stated that the impedance we are
feeding ever changes, only the measured SWR.

Oh dear... just when I thought it was safe to go back into the waves...

[On the other points, I'll reply to your second, corrected, posting.]


How would you explain what Cecil wrote?

Who else but Cecil would dare attempt that? :-)

How are some people
improving SWR by changing coax length, when in theory they shouldn't
be able to do this?

There are two possible reasons. One is instrument error - SWR meters are
not perfect.

The other possible reason is that the *outer* surface of the coax has
currents on it, so it has become part of the antenna. In that case,
changing the length of coax is not only changing the length of feedline
(the inner surfaces of the coax), but also is changing the antenna
itself. The voltage and current distributions on all the wires will
shift around, resulting in a different V, I and relative phase at the
top of the coax - in other words, a different feedpoint impedance. Then
the SWR (as measured on the *inside* of the feedline) genuinely will
change.

This SWR change is usually quite difficult to predict, because you
didn't mean there to be any current on the outer surface of the coax in
the first place. The only practical way to see if there could be a
problem is to use a clamp-on RF current meter to see how much surface
current is present.

If the SWR changes with feedline length *and* you have significant
surface current, then you know one probable reason... but in all cases,
these can also be instrument error in the SWR meter.

Do you think the series reactance a system offers a PA may
actually improve it's incident power?

To answer your exact question: I don't think there is a valid general
answer. It depends on the specific PA design, and also on what you mean
by "improve".

What I do know is that changes in the load impedance presented to a PA
will change several of its operating conditions, all at the same time.
Some of those changes will be "improvements" - but others definitely
won't be.

For example, reducing the load impedance will usually make the output
device operate in a more linear way... but the efficiency drops and the
greater heat dissipation and current are likely to shorten the lifetime
of the device. Is that an improvement?


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

Ian White, G3SEK wrote:
The RF voltages are either summed or subtracted, and then the resultant
is detected by the diode.

I know you know this, Ian, but let's make sure that everyone understands
that the summation (or subtraction) is a phasor (vector) summation. It is
the phasing between the total voltage and total current that allows the
forward wave to be separated from the reflected wave, and vice versa. The
directional coupler designer assumes that the ratio of Vfor/Ifor = Z0
and that the ratio of Vref/Iref = Z0. If that assumption is incorrect,
the SWR meter will still assume that the assumption is correct.

Just one small point, though... it is not necessary that R = Z0.

That's true and is just a habit on my part. I set R=Z0 and then adjust the
voltage accordingly for calibration purposes. For awhile, I was using a
450 ohm load for ladder-line with a measured Z0 of 388 ohms. It still
worked pretty well.

AFAIK, the only situation where the "terminating" resistor truly needs
to equal Z0 is in parallel-line couplers for microwaves, when the
sampling line approaches a quarter-wavelength long and its own
characteristic impedance is Z0 too.

In most of the slotted line pickups that I have seen, the internal load
resistor is equal to the Z0 of the slotted line. I don't know if that
is necessary or not.
--
73, Cecil http://www.qsl.net/w5dxp



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Dr. Slick wrote:

Roy Lewallen wrote:
Sure, you can normalize a Smith chart to anything you'd like. That
doesn't make the SWR change with source impedance.

I disagree on this point. You are caught up in the 50 Ohm world,
which i admit is easy to do. The SWR is based on the ratio of the
forward to the reflected power. If you had an analyzer that was
calibrated to 20 Ohms (the same as normalizing the Smith for 20 Ohms
in the center) you would certainly have reflected power and high SWR
going into a 50 Ohm load.

Not if the feedline has a Z0 of 50 ohms. The problem would be in believing
what you believe about what the 20 ohm SWR meter is trying to tell you.

It is trying to tell you that it is being misused but you are inferring
that it is trying to tell you the actual SWR. It is not.

And a 20 Ohm load would have a 1:1 SWR.

Not if the feedline has a Z0 of 50 ohms. If a DC voltmeter gives an
erroneous reading for RF voltage, do you blame the voltmeter or the
user of the voltmeter?
--
73, Cecil http://www.qsl.net/w5dxp



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Dr. Slick wrote:
"Ian White, G3SEK" wrote in message
...
Dr. Slick wrote:
As you might know, the input S11 or SWR will change when you go
from an antenna analyzer or network analyzer to measuring with the
actual full power PA and meter hooked up. This may be partly due to
the fact that the meter is usually not a perfect 50 ohm thru, and
partly due to the fact that the analyzers outputs are closer to 50 ohms
than the PA.

Sorry, that is exactly wrong. S11, SWR and the impedance itself, do
*not* change when you connect a different instrument to the same load.
All the changes you have described are due entirely to instrument
errors.

That's how the instrument errors are determined... by knowing for a fact
that, whatever all the different instruments may say, the impedance
they're trying to measure is the one thing that has *not* changed.


On second thought, i believe we are all wrong to equate S11 with
SWR!

Input S11 of a system will certainly never change. But the SWR
is absolutely dependant on the source impedance.

No! SWR, S11, return loss, rho, Y-parameters, Z-parameters, etc, etc are
all different derived functions of the same two variables: an arbitrary
complex impedance, and the system reference impedance Z0 (a constant
which may or may not be defined as complex).

Only those two variables are involved, so all of these functions are
locked together. If one variable changes, all the derived functions
change too. Either all change, or none change; nothing else is logically
possible.

As Roy says, the equations relating any one of these parameters to any
other are all well known. NONE of them ever involves source impedance.


If you had a network
analyzer calibrated for 20 Ohms, you would certainly have reflected
power and high VSWR going into 50 Ohms, and a 1:1 SWR going into 20
Ohms.

This would be the same as re-normalizing the Smith Chart for 20
Ohms in the center. You certainly can do this in MIMP.

I don't blame anyone for believing it's a 50-Ohm-only world!

No argument about any of that... but it's a totally separate point that
has no relevance whatever to your earlier statements about source
impedance.



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