This has been explained many times, to no avail.

So instead of one of us explaining it yet again, I suggest that you do
the following experiment. It requires only a transmitter, one or two
dummy loads, an SWR meter, and no more than five minutes of your time.

1. Connect the transmitter to either a dummy load or an antenna through
the SWR meter and measure the SWR.

2. Connect the transmitter in parallel with a dummy load by using a tee
connector. Connect this parallel combination to the input of the SWR
meter, and the output of the SWR meter to the same load as before (dummy
load or antenna).

Do you see any change in the SWR?

If you don't, then something is wrong with your theory -- since the
source impedance is clearly different for the two measurements --, and
you should take the effort of resolving it with your recent observations.

Roy Lewallen, W7EL

Richard Fry wrote:
"Ian White, G3SEK"wrote:

Richard Fry wrote:

"Ian White, G3SEK wrote

The meter measures nothing that involves the source, except
the level of RF that it supplies. It does not respond in any way
whatever to the source impedance.

Not that I said it did in my part of the thread, but nevertheless the

above

statement is not strictly true. In the case where the source Z of the tx

PA

does not match its load Z (which is typical), power reflected from the

load

mismatch will at least partly be re-reflected from the PA -- which then
contributes to the power sensed by a "wattmeter" in the output path.

Sorry, that statement cannot be correct. It would mean that the
impedance you measure at the near end of a transmission line (terminated
by some arbitrary load at the far end) would depend on the internal
impedance of the device that's doing the measuring - and that is not
true, either in transmission-line theory or in the real world. It is a
function only of the line and the load. etc

____________

How, then, do you explain the "ghost image" that can occur* in analog(ue) TV
transmission systems arising from reflections at/near the antenna end of the
station's transmission line?

*with sufficient round-trip propagation time in the transmission line

RF

Ian White, G3SEK wrote:

Reg Edwards wrote:

For those who have forgotten how or have never measured SWR.


Hang on, Reg - didn't you spend your career working on VLF cables that
went under the ocean?


How did you keep the water out of the slotted cable? And how far did you
have to swim between Vmax and Vmin?

Roy Lewallen, W7EL

Let me suggest an additional exercise for Richard and anyone else that
believes that source impedance affects the SWR.

Those of us who believe otherwise can easily calculate the SWR which
will exist on a line, and the SWR that will be read by an SWR meter at
any point in a system, by knowing simply the line length and impedance
and the load impedance. We don't require knowledge of the source
impedance. The equations we use can be found in numerous places, and
these have been used for over a century to design working systems.

You must use other equations to predict SWR -- equations which include
source impedance. It would be very interesting to see those equations.
Your equations and ours will predict different results from the simple
test I proposed. So if you'll show us the equation you use to calculate
SWR which includes source impedance, it'll be easy to see whether it's
correct or not.

Roy Lewallen, W7EL

Richard Clark wrote:
I see you have yet to respond to this very matter attended to quite at
length by Chipman.

I have recently realized that those terms in Chipman's equations are
interference terms. EM wave interference is not understood very well
by RF people although it is understood very well by optics people.

For instance, the superposing of two coherent voltages in a Z0
environment is well known.

Vtot = V1 + V2 (assume V1 and V2 are in phase)

Squaring both sides and dividing by Z0 yields the power.

Vtot^2/Z0 = (V1+V2)^2/Z0

Vtot^2/Z0 = V1^2/Z0 + V2^2/Z0 + (2*V1*V2)/Z0

Note that the first term to the right of the equals sign is the
power associated with the V1 wave and the second term is the
power associated with the V2 wave. The third term is the
interference term. If V1 and V2 are in phase, the third term
will be constructive interference.

If the phase angle between V1 and V2 is less than 90 degrees,
the interference is constructive, i.e. cos(theta) is positive.

If V1 and F2 were 180 degrees out of phase, the interference
would be destructive.

If the phase angle between V1 and V2 is between 90 degrees and
180 degrees, the interference is destructive, i.e. cos(theta)
is negative.

Interference is the reason for those extra terms in Chipman's
equations. It always happens when the sum of two voltages
are squared to get the power.

Reference: _Optics_, by Hecht.
--
73, Cecil http://www.qsl.net/w5dxp


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"Roy Lewallen" wrote
Let me suggest an additional exercise for Richard and anyone else that
believes that source impedance affects the SWR. (etc)
____________________

Just one sec, please. I didn't say that the true SWR connected to the tx
output connector was affected. I said that the RF power measured at the
sample point(s) in the transmitter can be affected by the source and load
impedances of the tx, for the reasons stated.

The true load SWR does not change under these conditions, but it cannot then
be determined by such a meter. Attempting to do so will yield some value,
but it will be wrong.

RF

"Walter Maxwell" wrote in message
...
On Fri, 3 Sep 2004 17:16:48 -0300, "Another Voice"
wrote:

**** Post for FREE via your newsreader at post.usenet.com ****

"Richard Harrison"
Just how short can a transmission line be
and still enforce its Zo?

The whole thing is perfectly clear if one imagines applying a step
function
(rising edge) to any short, even VERY short, length of transmission line.
The current in the short line will be equal to V/Zo - at least until the
reflections (if any) start arriving back at the input. If the line
happen
to be terminated with Zo, then no reflections and I=V/Zo is the steady
state.

The only issue of shortness is that a very short line means very short
time
until the reflections arrive.

The step function makes things a lot easier to understand than RF. It
'enforces' the distinction between the transient period and steady state.

IMO, the length of the line is irrelevant when using a device such as the
Bruene
bridge or a Bird 43. Each of those instruments are designed or adjusted to
indicate the forward or reflected power, based on three things: 1) ratio
of the
foward and reflected voltages, the voltage reflection coefficient 2) the
scale
numbered from 0 to 1, where 0 indicates the reflection is zero, and 1
equals
total reflection, but the significant point is that a 3:1 mismatch gives a
reflection coefficient of 0.5, which then means that the half-scale
reading of
0.5 indicates the 3:1 mismatch, or a 3:1 SWR, and 3) the device is so
designed
or adjusted so that the voltage ratios indicate the correct value because
it's
inherent characteristic impedance, Zo, is 50 ohms.

Thus, no transmission line is necessary. For example, the device can be
connected directly to the antenna terminals, or any other device you
desire to
determine the mismatch, and power it directly from the signal source--no
transmission line is needed on either port for the device to indicate the
degree
of mismatch.

Walt, W2DU

Walt,

I hope people are listening to what you are saying. I built up a Bruene
meter in SWCAD using 0% tolerance components and other ideal parts. Works
exactly like Bird claims their meter does, except that the error only
depends on the PC floating point arithmetic. Transmission line or not makes
no difference. BTW, it is kind of neat to see the directional coupler
properties, by driving the two sides with different signals, and then being
able to separate them.

Tam/WB2TT

Sorry, I must have misinterpreted your earlier posting.

But we seem to now have a "true SWR" as opposed to some other kind of
SWR. And "true SWR connected to the tx output" doesn't have any meaning
at all to me. I also have no idea of what "sample points within the
transmitter" might be. So let me explain what I (and virtually all
published literature) mean by SWR.

If we connect a transmitter to an SWR meter, and then to a long piece of
lossless cable with the same Z0 as the SWR meter, and finally to a load,
the SWR meter reading will be the same as the VSWR on the cable, i.e.,
the ratio of maximum to minimum voltages on the line. This ratio of
voltages is, by definition, the VSWR -- which equals the ISWR, and is
often referred to simply as SWR.

If we measure or calculate the impedance seen looking into the line,
then disconnect the line from the SWR meter and replace it and the load
with lumped elements of the same impedance, the SWR meter reading won't
change(*).

Now, I can calculate the what the SWR meter reading will be under this
condition also. In both cases, the source impedance won't affect the SWR
meter reading, the positions or relative magnitudes of the maximum and
minimum voltages on the line, or the voltage or current within the SWR
meter line section. (This last condition assumes that the net power
delivered by the source stays the same; otherwise, the ratio of voltage
to current, and their phase angles, stay constant, regardless of the
power delivered.)

I have no idea how all this relates to your "true SWR". But do you agree
with what I've said above? If not, I'll describe a couple of simple
experiments which will test it against any alternative view you might
propose.

(*) We can also replace them with a load at the end of a line of
different Z0. As long as we choose the load Z and the line length to
make the impedance seen at the line input the same as before, the SWR
meter will read the same as before -- even though it no longer equals
the actual VSWR on the transmission line. The SWR meter is really
indicating the impedance seen looking into the line, not in this case
the actual line VSWR. (That's the essence of Reg's objection to the SWR
meter designation. Of course, if I connect my ammeter across a resistor,
it's not measuring the current through the resistor, either.)

Roy Lewallen, W7EL


Richard Fry wrote:
"Roy Lewallen" wrote

Let me suggest an additional exercise for Richard and anyone else that
believes that source impedance affects the SWR. (etc)

____________________

Just one sec, please. I didn't say that the true SWR connected to the tx
output connector was affected. I said that the RF power measured at the
sample point(s) in the transmitter can be affected by the source and load
impedances of the tx, for the reasons stated.

The true load SWR does not change under these conditions, but it cannot then
be determined by such a meter. Attempting to do so will yield some value,
but it will be wrong.

RF

Ian White, G3SEK wrote:

Reg Edwards wrote:

For those who have forgotten how or have never measured SWR.


Hang on, Reg - didn't you spend your career working on VLF cables that
went under the ocean?


How did you keep the water out of the slotted cable? And how far did you
have to swim between Vmax and Vmin?

Roy Lewallen, W7EL
===============================

Water can be kept out of slotted cables by the ship's radio operator who
never has anything else to do. We used to toss him overboard with a ladle
and pump. On occasions we used the ship's doctor when he wasn't propping up
the bar boozing duty-free scotch.

Didn't have to swim anywhere. The propagation velocity is so low at ELF in
sea water it is necessary only to sit on a stool in a diving suit and wait
for the max's and min's to pass by.
---
Reg.

"Roy Lewallen" wrote

But we seem to now have a "true SWR" as opposed to
some other kind of SWR. And "true SWR connected to
the tx output" doesn't have any meaning at all to me.

My "true SWR" term is used is an attempt to differentiate between the SWR of
the antenna system, and the inaccuracies associated with trying to measure
it with devices that cannot isolate the incident power in the system from
internal reflections of that power. For the conditions and reasoning
outlined in my earlier posts in this thread, and even though the system SWR
is a constant -- the normal SWR meter used in/with an operating transmitter
working into a mismatched load won't have the ability to give strictly
accurate measurement of that SWR. That is all I'm saying.

I also have no idea of what "sample points within the
transmitter" might be.

In broadcast gear, these are the directional couplers whose pickup probes
are inserted transversely into the coaxial line between the harmonic filter
output and the tx output connector. I haven't been a licensed ham for over
40 years (when I went into the broadcast field), but I expect some ham txs
might have the same setup. Otherwise it could be a Model 43 or the like
inserted between the output connector of the ham tx and the transmission
line to the antenna.

I hope this is understandable now.

RF

Richard Fry wrote:

Ian White wrote:
The so-called SWR meter is a steady-state instrument, so it always makes
sense to use that quicker, easier way of thinking. Since you're the one
who chooses to think of this particular situation in terms of multiple
reflections, any difficulties you encounter are entirely yours.

This reads to me as though you know they are there, but choose to
ignore them...?

Oh no, quite the opposite - but since these difficulties are entirely of
your own making, you get to do the work :-)

If you ever see a conflict between two different theories that explain
the same observed facts, then there's an error somewhere.

We agree on the subject of conflict resolution, but apparently not on
the location of the error.

Thank you for the more detailed explanation below... which, sure enough,
revealed where the error is.

If the multiple-reflection theory is extrapolated to infinite time, so
that it calculates results for the steady state, it *must* give identical
results to the steady-state theory. But whenever the steady-state
theory can be used, it will always get you there much more quickly.

This is true only to the extent that all the power ever generated by
the transmitter eventually either is radiated by the antenna or is
dissipated by losses somewhere.

That is exactly true in the steady state.

For simplicity, let's assume a tx with a source impedance of zero ohms
feeds a lossless transmission line of uniform impedance throughout its
length to a mismatch at the far end. The mismatch reflects a
percentage of the incident power back down the line to the tx, and
continues to do so as long as the transmitter generates power. The tx
will re-reflect the reflected power back to the far end -- in this case
all of the reflected power it ever sees, in fact. To this easily-seen,
real-world reality you agreed above ("Yes, that is a true observation, ...").

The re-reflections combine with the power generated by the tx at that
instant to create a vector sum at the sample point used by the meter.

There's the error: you can't "combine... power" in that way. You can
only create vector sums of voltage; and separately, vector sums of
current.

To make the multiple-reflection theory work correctly, you have to do
two separate vector sums at output port of the transmitter. First you
add all the voltage vectors: the 1st (original) forward, the 1st
reflected, the 2nd forward (re-reflected), 2nd reflected... and so on,
summed to infinity to give the correct result for the steady state.
Then you do the exactly same for all the forward and reflected current
vectors.

In order to account for reflection from the transmitter, you have to
assume some value of source impedance. Any value will do, for reasons
we'll see in a moment.

Now you can calculate two things: the vector ratio, which is the complex
impedance that the transmitter sees as a load; and the scalar product,
which is the power the transmitter can deliver into that load.

If you vary the source impedance of the transmitter, it will change all
the summed voltage vectors and all the summed current vectors - but each
voltage term in the sum will be changed by exactly the same factor as
its corresponding current term. Certainly the product (the output
power) will change, but the ratio (the load impedance) will not.

So, when correctly worked out, the load impedance is *not* a function of
the transmitter output impedance or the output power. Likewise, the
indication of the SWR meter is not a function of either the transmitter
output impedance or the output power - this last one being a well-known
fact.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek