VSWR doesn't matter?
 

Jerry Martes wrote:
"David G. Nagel" wrote in message
...
Tim Wescott wrote:
billcalley wrote:

We are all told that VSWR doesn't matter when using low loss
transmission lines, since the RF energy will travel from the
transmitter up to the mismatched antenna, where a certain amount of
this RF energy will reflect back towards the transmitter; after which
the RF will then reflect back up to the antenna -- where the energy is
eventually radiated after bouncing back and forth between the
transmitter and antenna. I understand the concept, but what I don't
quite understand is why the reflected RF energy isn't simply absorbed
by the 50 ohm output of the transmitter after the first reflection?
For the RF to bounce back and forth, wouldn't the transmitter's
impedance have to be very, very high (or low) when the reflected RF
energy hit its output stages? I know I'm missing something vital
here...

That's assuming you use an antenna tuner. The tuner will transform the
transmitter's output impedance* just as it transforms the line. Were the
transmitter output impedance actually at 50 ohms, on the other side of
the tuner it would have the same VSWR as the line when everything was
tuned up.

Having said that, the VSWR _does_ matter somewhat when using low loss
lines, both because the line loss is low but not zero, and the tuner loss
will tend to go up as you correct for higher and higher VSWR.

* I am _not_ going to start the Big Transmitter Output Impedance Debate.
sed denizens -- just don't comment on what a transmitter's "actual"
output impedance may be, lest you start a flame war.

If you want a quick lesson in high vswr find a ham with an old tube
transmitter and see if he will hook it up to a mismatched load. The cherry
red plates are the reflected energy being absorbed. Transistors will just
turn to smoke under the same conditions.

Dave WD9BDZ

Hi david

Wouldnt it be OK to have a high VSWR along the transmission line if the
"tank ckt" can be adjusted to match the load to the transmitter output
impedance? That is, the VSWR along the transmission could concievely be
high, yet, with proper "tank ckt" adjustment that impedance seen by the
output circuit (plate) wouldnt result in a "cherry red plate".
What I am asking is ? is the transmission line VSWR directly related to
"plate reddening"?
I'm more asking than *telling*.

Jerry


Jerry;

The point I was trying to make is that the reflected current is
disapated as heat in the finals if the transmitter isn't matched to the
load.
In a tube radio the tank circuit is the equivilent of an antenna
match/tuner and converts the 2000 or so ohms at the plate to the 50 ohms
of the transmission line and the unknown ohms of the mis matched antenna.


Dave WD9BDZ

VSWR doesn't matter?
 

On Mar 12, 4:56 am, Richard Clark wrote:
On 11 Mar 2007 20:39:46 -0700, "Bob" wrote:

The active part of the transmitter output isn't 50 ohm.
That would cause half the power to be lost as heat in
the output stage.

Hi Bob,

Well, aside from the initial misunderstanding of how power gets to the
load (much less back, and then to the load again); I will put to you
a question that has NEVER been answered by those who know what the
transmitter output Z ISN'T:
"What Z is it?"

73's
Richard Clark, KB7QHC

As Tim Williams alludes, it depends on the transmitter design.
It will often be complex rarther than resistive. Since the active
device changes impedance during a single cycle of the RF
signal it may not even be adequately described by a single
value in ohms for a paticular frequency if you wish to
analyse the case of forward and reflected power.

Consider a class C or class E output stage with an
output transistor that is low impedance during
most of the positive half of a cycle of signal and mostly
somewhere near open circuit for the negative half
of the cycle. It seems to me that the effect of reflected
power is going to be different depending its phase
relative to the forward power.
I think this also applys to a lesser extent to a class
A PA with a nice hi-Q tank circuit.

As usually whan this topic comes up, It don't feel
like we have arrived at a usefull and convincing model
of what happens, possibly because simple
descriptions don't cover everything.

Bob

VSWR doesn't matter?
 

On 12 Mar 2007 23:00:59 -0700, "Bob" wrote:

As Tim Williams alludes, it depends on the transmitter design.

Hi Bob,

No quantifiable answer I see. It's not unexpected, everyone who knows
what it isn't has never been able to say what it is. It seems like
the stock answer you give the cop who asks if you know the speed
limit.
"No. But I wasn't speeding!"

The dependency here started with a conventional Ham transmitter, one
so ordinary as to be a commodity. The design is not so exotic as to
elude a very simple value - except for those who know it isn't 50
Ohms.

73's
Richard Clark, KB7QHC

VSWR doesn't matter?
 

"Cecil Moore" wrote in message
...
Of course, that was a tongue-in-cheek posting.
But if you could design a Thevenin equivalent
source with a 0.1 ohm source impedance, wouldn't
the efficiency calculate out to be pretty high?

Class D rules. (Using MOSFETs, the Thevenin equivalent is quite easy to
spot, too!)

Tim

--
Deep Fryer: A very philosophical monk.
Website @ http://webpages.charter.net/dawill/tmoranwms

VSWR doesn't matter?
 

"Roy Lewallen" wrote
The problem is that the idea of "reflected energy" turning the plates hot
is so easy to understand, that people aren't willing to abandon it simply
because it isn't true.
_____________

But reflected energy/power does exist.

For an easy example, such reflections are evident in the picture seen on an
analog TV receiver when the match between the transmit antenna and the
transmission connected to it is bad enough.

In analog TV transmit systems with a typical 500+ foot length transmission
line from the tx to the antenna, a 5% reflection from a far-end mismatch can
be quite visible, showing as a "ghost" image that is offset from the main
image as related to the round-trip propagation time of the transmission
line.

RF

VSWR doesn't matter?
 

Jimmie D wrote:
"Dan Bloomquist" wrote in message
...

billcalley wrote:


We are all told that VSWR doesn't matter when using low loss
transmission lines, since the RF energy will travel from the
transmitter up to the mismatched antenna, where a certain amount of
this RF energy will reflect back towards the transmitter; after which
the RF will then reflect back up to the antenna -- where the energy is
eventually radiated after bouncing back and forth between the
transmitter and antenna.

As pointed out, VSWR does matter. A lot of bouncing means you heat the
transmission line with the power instead of radiating the power. 'Doesn't
matter', really means it can be tolerated if need be.


I understand the concept, but what I don't
quite understand is why the reflected RF energy isn't simply absorbed
by the 50 ohm output of the transmitter after the first reflection?
For the RF to bounce back and forth, wouldn't the transmitter's
impedance have to be very, very high (or low) when the reflected RF
energy hit its output stages? I know I'm missing something vital
here...

Here is what you are missing. In the case of the output, (real/resistive
component of the transmitter), seeing the reflected wave, it is _not_
reflecting that power back up the transmission line as you think it is. It
would go back to that real impedance and heat the transmitter. Here is
what is done with a miss match in the real world.

trans-output - match - line - antenna

The 'match' is where the magic happens. All the energy coming down the
line that got reflected from the antenna 'sees' the 'trans-output -
match' as a perfect reflector and gets bounced back[*]. On the other side
of the match is the trans-output. There the trans-output sees a perfect
impedance, (technically, the conjugate of the trans-output), so that all
the power travels through the match toward the antenna.

The magic is that when the match is tuned, both of the above conditions
are satisfied.

*The reflected wave sees a purely reactive reflector not just because of
the network but also because of the output power of the transmitter.
Without transmitter power the impedance as seen from the load will
dramatically change.

Best, Dan.



Saying that SWR doesnt matter is a rather broad statement(like saying never
or always) but I have know of antenna systems having an SWR of 30:1 and his
was normal. The feedline was balanced line made of 1 inch copper. Of course
an SWR lie this on coax could be fatal to coax and equipment. A more common
example of this is the 1/4 wl matching section on a J-pole antenna. It
matches 50 ohms to a few Kohms so an SWR of 60: 1 or so would not be unusal
here.S oas long as the feedline can handle the current and voltage peaks
without much los it doesnt matter much as long as the source impedance is
matched to the impedance at the input to the transmission line.Im sure there
is a practical limit though.

Hi Jimmie,
Keep in mind I'm answering in the context of the op's post. And the
theoretical SWR on a stub is infinite. The point of the stub at the
antenna is to keep the SWR on the transmission line in a reasonable
range, to make a match if you will. To put high SWR on the feedline
instead of matching at the antenna isn't a great idea in my book.

OTOH.
I finally did some sidebanding a couple of months ago. (First time on
HF.) I got my hands on an old swan 500c. After changing the 6je6's and
supply caps, I had to find out what it was like to get on the air. I ran
outside and hung a wire between the lab and the shop. 40-50 feet. Put a
couple of alligator clips on the end of a chunk of rg-58 and into the
window. I started looking for the antenna through the trans-match with
an antenna bridge. The tuning was very sharp, lots of Q. I don't know if
I could have found it without the bridge :) I was willing to tolerate
the miss match to get on the air.

Well, it worked out. I made some great QSLs across the mid west and into
northern CA. I live in Vernon AZ. I'm pleased this turned out to be as
great a radio location as I thought. It shouldn't be long before I get a
beam on a tower. By then I'll look to match at the antenna and keep the
SWR off the feed line as much as possible.

Best, Dan.

VSWR doesn't matter?
 

Roy Lewallen wrote:

Yes! All that matters to the transmitter is the impedance it sees. It
doesn't know or care that you've mathematically separated the delivered
power into "forward" and "reverse" components. It doesn't know or care
what the SWR is on the transmission line connected to it, or even if a
transmission line is connected at all.

Well, without a line, you don't have a real component to tune into.
Drawing arcs on a smith chart from an open line with capacitors and
coils will only get you to another purely reactive point.

Best, Dan.

VSWR doesn't matter?
 

In rec.radio.amateur.antenna Roy Lewallen wrote:
David G. Nagel wrote:

If you want a quick lesson in high vswr find a ham with an old tube
transmitter and see if he will hook it up to a mismatched load. The
cherry red plates are the reflected energy being absorbed. Transistors
will just turn to smoke under the same conditions.

Unfortunately, you'd be learning the wrong lesson.

The cherry color is due to the transmitter being loaded with an
impedance it's not designed for, causing the final to run at low
efficiency. You can disconnect the antenna and replace it with a lumped
RC or RL impedance of the same value and get exactly the same result.
Alternatively, you can attach any combination of load and transmission
line which give the same impedance, resulting in a wide variation of
"reflected energy", and get exactly the same result. All that counts is
the impedance seen by the transmitter, not the VSWR on the line or the
"reflected power".

The problem is that the idea of "reflected energy" turning the plates
hot is so easy to understand, that people aren't willing to abandon it
simply because it isn't true.

See http://eznec.com/misc/Food_for_thought.pdf for more.

Roy Lewallen, W7EL

The fact that any transmission line and antenna combination can be
replaced with an RLC lumped load at the transmitter output and the
transmitter can't tell the difference is something that a lot of
hams seem to have a problem understanding.

What I've never understood is why so many hams have a problem with
the concept of equivalent circuits only when antennas and transmission
lines are involved.


--
Jim Pennino

Remove .spam.sux to reply.

VSWR doesn't matter?
 

"Dan Bloomquist" wrote in message
...
Jimmie D wrote:
"Dan Bloomquist" wrote in message
...



Hi Jimmie,
Keep in mind I'm answering in the context of the op's post. And the
theoretical SWR on a stub is infinite. The point of the stub at the
antenna is to keep the SWR on the transmission line in a reasonable range,
to make a match if you will. To put high SWR on the feedline instead of
matching at the antenna isn't a great idea in my book.

Sure yoiu can, that stub is a transmission line. It would matter if it s a
1/4 wl long or 21 1/4 wl long. If it is designed to handle the current and
voltage peaks it can transmit power with low loss when a high VSWR is
present. Its just that most people dont make there feedlines out of inch
copper tubing. Even with 450 ohm ladder line 10:1 VSWR is very acceptable.

VSWR doesn't matter?
 

Richard Fry wrote:
"Roy Lewallen" wrote
The problem is that the idea of "reflected energy" turning the plates
hot is so easy to understand, that people aren't willing to abandon it
simply because it isn't true.
_____________

But reflected energy/power does exist.

For an easy example, such reflections are evident in the picture seen on
an analog TV receiver when the match between the transmit antenna and
the transmission connected to it is bad enough.

In analog TV transmit systems with a typical 500+ foot length
transmission line from the tx to the antenna, a 5% reflection from a
far-end mismatch can be quite visible, showing as a "ghost" image that
is offset from the main image as related to the round-trip propagation
time of the transmission line.

RF

Richard,

You are undoubtedly correct, but you have also demonstrated what is
really the lifeblood of many arguments in RRAA. You have introduced both
transient behavior and multi-frequency behavior.

Clearly these are important in the real world. However, the vast
majority of models and calculations used as support for RRAA postings
are steady-state and monochromatic. Anyone who stayed awake through
calculus and differential equations might recall that the equations for
steady-state and transient behavior are often quite different.

No one denies the existence of reflections. Some people get confused by
the mathematics of power and voltage. But a big argument is about the
round-trip travel of energy in the steady-state. Some people seem to
believe that energy continues to flow back and forth from one end of a
(mismatched) transmission line to the other under steady-state
conditions, even simultaneously traveling in both directions. (Passing
like ships in the night?) Since energy is a scalar quantity, and any
given joule is not distinguishable from another, it is not clear how the
proponents keep track of the bookkeeping, but they muddle through somehow.

73,
Gene
W4SZ

VSWR doesn't matter?
 

Richard Fry wrote:
"Roy Lewallen" wrote
The problem is that the idea of "reflected energy" turning the
plates hot is so easy to understand, that people aren't willing to
abandon it simply because it isn't true.
_____________

But reflected energy/power does exist.

For an easy example, such reflections are evident in the picture seen
on an analog TV receiver when the match between the transmit antenna
and the transmission connected to it is bad enough.

In analog TV transmit systems with a typical 500+ foot length
transmission line from the tx to the antenna, a 5% reflection from a
far-end mismatch can be quite visible, showing as a "ghost" image
that is offset from the main image as related to the round-trip
propagation time of the transmission line.

Richard,

The round trip time on the transmission line is 1uS+, and the period of
the highest modulating frequency is 0.2uS, so transient performance of
the line is very important.

Run the numbers on typical ham SSB telphony where the rtt is more like
0.2uS+ and the period of the highest modulating frequency is 300uS, is
it any wonder transient performance isn't critical.

So, if a solution in the steady state solution doesn't degrade the
modulation, why complicate matters with pretend partial time domain
solutions. It is half baked thinking in both worlds that drives the
thinking that reflected power *must* be dissipated in the anode.

There is no doubt that under load end mismatch, there is a reflected wave
on the transmission line, and there is no doubt that under some
conditions, the anode dissipates more power, and they may be correlated,
but the simplistic explanation above that is commonly touted is BS.

Owen

VSWR doesn't matter?
 

In rec.radio.amateur.antenna Richard Clark wrote:
On Tue, 13 Mar 2007 16:05:02 GMT, wrote:

The fact that any transmission line and antenna combination can be
replaced with an RLC lumped load at the transmitter output and the
transmitter can't tell the difference is something that a lot of
hams seem to have a problem understanding.

Hi Jim,

Would it be fair to say there are a number of Hams (no need to go into
proportionality, could be equal number) who have difficulties of
understanding with going from lumped, equivalent circuits to antennas
and transmission lines?

The two perspectives are not exclusionary nor mutually incompatible,
only the arguers are.

I'd have to say that as soon as a circuit contains a radiator or a
transmission line the arm waving begins.

--
Jim Pennino

Remove .spam.sux to reply.

VSWR doesn't matter?
 

On Mar 12, 11:50 pm, Roy Lewallen wrote:
The problem is that the idea of "reflected energy" turning the plates
hot is so easy to understand, that people aren't willing to abandon it
simply because it isn't true.

It also isn't true that there is no energy in the reflected wave, that
such beliefs are gobbledegook, and that RF standing wave energy
just sloshes around in a transmission line at less than light speed.
To really understand what is going on, one has to understand
superposition and interference between RF energy waves. You
are on record as not caring to understand reflected energy. Please
don't condemn those of us who are trying to understand.
--
73, Cecil, w5dxp.com

VSWR doesn't matter?
 

On Mar 12, 11:53 pm, Roy Lewallen wrote:
Yes! All that matters to the transmitter is the impedance it sees. It
doesn't know or care that you've mathematically separated the delivered
power into "forward" and "reverse" components. It doesn't know or care
what the SWR is on the transmission line connected to it, or even if a
transmission line is connected at all.

Think about this - if the transmission line is exactly one-wavelength
long
and lossless, the transmitter sees exactly the same impedance as the
load. At the load, we know reflections occur, but they are same-cycle
reflections so during steady-state with no modulation, exactly the
same
conditions exist at the transmitter as exist at the load if the
transmitter
has the same impedance as the transmission line. So even if we
cannot measure the reflections back into the transmitter, they are
no doubt, there - that is, unless one denies the existence of
reflections
in which case, one needs to explain how standing waves are possible
without reflections in a single-source system.
--
73, Cecil, w5dxp.com

VSWR doesn't matter?
 

On 13 Mar 2007 14:19:12 -0700, "Cecil Moore" wrote:

On Mar 12, 11:50 pm, Roy Lewallen wrote:
The problem is that the idea of "reflected energy" turning the plates
hot is so easy to understand, that people aren't willing to abandon it
simply because it isn't true.

It also isn't true that there is no energy in the reflected wave, that
such beliefs are gobbledegook, and that RF standing wave energy
just sloshes around in a transmission line at less than light speed.
To really understand what is going on, one has to understand
superposition and interference between RF energy waves. You
are on record as not caring to understand reflected energy. Please
don't condemn those of us who are trying to understand.

Did you guys on this thread know that it's been proven statistically that five
out of four people have trouble with fractions?

Walt,W2DU

VSWR doesn't matter?
 

On Tue, 13 Mar 2007 16:05:02 GMT, wrote:

The fact that any transmission line and antenna combination can be
replaced with an RLC lumped load at the transmitter output and the
transmitter can't tell the difference is something that a lot of
hams seem to have a problem understanding.

Hi Jim,

Would it be fair to say there are a number of Hams (no need to go into
proportionality, could be equal number) who have difficulties of
understanding with going from lumped, equivalent circuits to antennas
and transmission lines?

The two perspectives are not exclusionary nor mutually incompatible,
only the arguers are.

73's
Richard Clark, KB7QHC

VSWR doesn't matter?
 

On Tue, 13 Mar 2007 21:15:03 GMT, wrote:

The two perspectives are not exclusionary nor mutually incompatible,
only the arguers are.

I'd have to say that as soon as a circuit contains a radiator or a
transmission line the arm waving begins.

Amen

VSWR doesn't matter?
 

Walter Maxwell wrote:
On 13 Mar 2007 14:19:12 -0700, "Cecil Moore" wrote:

On Mar 12, 11:50 pm, Roy Lewallen wrote:
The problem is that the idea of "reflected energy" turning the plates
hot is so easy to understand, that people aren't willing to abandon it
simply because it isn't true.
It also isn't true that there is no energy in the reflected wave, that
such beliefs are gobbledegook, and that RF standing wave energy
just sloshes around in a transmission line at less than light speed.
To really understand what is going on, one has to understand
superposition and interference between RF energy waves. You
are on record as not caring to understand reflected energy. Please
don't condemn those of us who are trying to understand.

Did you guys on this thread know that it's been proven statistically that five
out of four people have trouble with fractions?

Walt,W2DU


Walt,

I didn't know that, but I'm dain bramaged.

73,
Gene
W4SZ

VSWR doesn't matter?
 

Walter Maxwell wrote in
:

On 13 Mar 2007 14:19:12 -0700, "Cecil Moore"
wrote:

Did you guys on this thread know that it's been proven statistically
that five out of four people have trouble with fractions?

That's vulgar!

VSWR doesn't matter?
 

"Owen Duffy" wrote
Richard,
The round trip time on the transmission line is 1uS+, and the period of
the highest modulating frequency is 0.2uS, so transient performance of
the line is very important.
____________

Sorry, sir, but quite a few decades of experience in the analog TV broadcast
industry show otherwise (not to mention an accurate theoretical analysis of
this condition).

For example, a reflection within an analog TV broadcast signal that is
delayed by one microsecond from the main image equates to something like a
10% horizontal displacement of that reflected, or "ghost" image from the
main image (525/60Hz TV standard).

A ghost television image amounting to 5% of the main image, and offset by
10% of the width of even a fairly small display screen is not difficult to
see (or to be objected to) by an "average" observer at an "average" viewing
distance from that display screen.

Reflected r-f power may be less of a concern to amateur radio operators than
it is to commercial operators, but that doesn't mean that reflected power is
non-existent, or even unimportant.

RF http://rfry.org

VSWR doesn't matter?
 

"Richard Fry" wrote in :

"Owen Duffy" wrote
Richard,
The round trip time on the transmission line is 1uS+, and the period
of the highest modulating frequency is 0.2uS, so transient
performance of the line is very important.
____________

Sorry, sir, but quite a few decades of experience in the analog TV
broadcast industry show otherwise (not to mention an accurate
theoretical analysis of this condition).

Richard, I don't think you read my words. You are disagreeing with my
proposition that transient performance of the line is very important (in
the TV example you cited).

Owen

VSWR doesn't matter?
 

"Bob" wrote in message
ups.com...
On Mar 12, 1:08 am, "billcalley" wrote:
We are all told that VSWR doesn't matter when using low loss
transmission lines,

No, we are not all told that.
The active part of the transmitter output isn't 50 ohm.
That would cause half the power to be lost as heat in
the output stage. It's only 50ohm once it becomes a moving
wave in the transmission line.

Bob9


In that case...

Half power is only lost when terminated to a 50-ohm load;
i.e. no standing waves. What happens when there's a
mismatch and reflected energy :)



I'll go stand in a corner...

VSWR doesn't matter?
 

"Uncle Peter" wrote in message
...

"Bob" wrote in message
ups.com...
On Mar 12, 1:08 am, "billcalley" wrote:
We are all told that VSWR doesn't matter when using low loss
transmission lines,

No, we are not all told that.
The active part of the transmitter output isn't 50 ohm.
That would cause half the power to be lost as heat in
the output stage. It's only 50ohm once it becomes a moving
wave in the transmission line.

Bob9


In that case...

Half power is only lost when terminated to a 50-ohm load;
i.e. no standing waves. What happens when there's a
mismatch and reflected energy :)



I'll go stand in a corner...


A mismatch where, between the feedline and the antenna, feedline and source,
source and impedance seen at the input to the feedline.

Jimmie

VSWR doesn't matter?
 

On Tue, 13 Mar 2007 17:39:04 -0500, "Richard Fry" wrote:

"Owen Duffy" wrote
Richard,
The round trip time on the transmission line is 1uS+, and the period of
the highest modulating frequency is 0.2uS, so transient performance of
the line is very important.
____________

Sorry, sir, but quite a few decades of experience in the analog TV broadcast
industry show otherwise (not to mention an accurate theoretical analysis of
this condition).

For example, a reflection within an analog TV broadcast signal that is
delayed by one microsecond from the main image equates to something like a
10% horizontal displacement of that reflected, or "ghost" image from the
main image (525/60Hz TV standard).

A ghost television image amounting to 5% of the main image, and offset by
10% of the width of even a fairly small display screen is not difficult to
see (or to be objected to) by an "average" observer at an "average" viewing
distance from that display screen.

Reflected r-f power may be less of a concern to amateur radio operators than
it is to commercial operators, but that doesn't mean that reflected power is
non-existent, or even unimportant.

RF http://rfry.org

I know that Roy was heavily involved with TDR at Tektronix years ago. I began
working at the RCA Laboratories' antenna lab in 1958. I don't know what
Tektronix was doing relative to TDR at that time, but one of my colleagues at
the lab was Donald Peterson. Don was then working on TDR, and to our knowledge
then, his work on the subject was new. His experiments showed that using TDR we
could spot problems in a TV TX transmission line that was causing ghosts. Using
Don's technique, he traveled to many TV stations around the country that had
ghost problems, and with TDR he was able to determine the precise location of a
discontinuity in the transmission line that produced a reflection that caused
the ghost.

That was over 40 years ago, but I seem to remember that any discontinuity that
resulted in a VSWR greater than 1.005:1 produced a ghost that could not be
tolerated in the transmitted picture.

I'm sure this is the magnitude of reflections Richard F. is referring to.

Walt, W2DU

VSWR doesn't matter?
 

On Tue, 13 Mar 2007 22:28:02 GMT, Owen Duffy wrote:

Walter Maxwell wrote in
:

On 13 Mar 2007 14:19:12 -0700, "Cecil Moore"
wrote:

Did you guys on this thread know that it's been proven statistically
that five out of four people have trouble with fractions?

That's vulgar!

The stat ement or the fractions?

Walt, W2DU

VSWR doesn't matter?
 

On Tue, 13 Mar 2007 22:22:38 GMT, Gene Fuller wrote:

Walter Maxwell wrote:
On 13 Mar 2007 14:19:12 -0700, "Cecil Moore" wrote:

On Mar 12, 11:50 pm, Roy Lewallen wrote:
The problem is that the idea of "reflected energy" turning the plates
hot is so easy to understand, that people aren't willing to abandon it
simply because it isn't true.
It also isn't true that there is no energy in the reflected wave, that
such beliefs are gobbledegook, and that RF standing wave energy
just sloshes around in a transmission line at less than light speed.
To really understand what is going on, one has to understand
superposition and interference between RF energy waves. You
are on record as not caring to understand reflected energy. Please
don't condemn those of us who are trying to understand.

Did you guys on this thread know that it's been proven statistically that five
out of four people have trouble with fractions?

Walt,W2DU


Walt,

I didn't know that, but I'm dain bramaged.

73,
Gene
W4SZ

Gene, are you aware that religious dyslecsics pray to their dog?

Walt

VSWR doesn't matter?
 

Walter Maxwell wrote in
:

On Tue, 13 Mar 2007 22:28:02 GMT, Owen Duffy wrote:

Walter Maxwell wrote in
m:

On 13 Mar 2007 14:19:12 -0700, "Cecil Moore"
wrote:

Did you guys on this thread know that it's been proven statistically
that five out of four people have trouble with fractions?

That's vulgar!

The stat ement or the fractions?

5/4

VSWR doesn't matter?
 

"Walter Maxwell" wrote:
That was over 40 years ago, but I seem to remember that any
discontinuity that resulted in a VSWR greater than 1.005:1
produced a ghost that could not be tolerated in the transmitted picture.
I'm sure this is the magnitude of reflections Richard F. is referring to.
_____________

Analog TV transmission is not quite that sensitive to VSWR, fortunately.
Matti Siukola of the RCA Broadcast TV antenna group in Gibbsboro, NJ did
some experimental work showing that a 1% reflection (1.02 VSWR) or less is
unnoticeable to a critical observer, a 3% reflection (1.06 VSWR) is
noticeable but tolerable, and a 5% reflection (about 1.1 VSWR) and above is
objectionable. These values applied to the r-f spectrum from visual carrier
(Fcv) to Fcv +2.5 MHz or so, and for transmission line lengths of 500 feet
and more from the tx to the antenna.

These parameters were measured using an r-f pulse at the visual carrier
frequency having the transition times and r-f bandwidth corresponding to the
maximum bandwidth limits of the TV channel, only.

The more conventional broadband TDRs used a very short pulse with energy
from DC to far beyond the limits of the TV channel. It could resolve small
discontinuities along the transmission line, but many of them had no affect
on the quality of the transmitted television image, as they were not present
in the r-f spectrum of the TV signal. And the pulse return of a wideband
TDR is extremely high from the TV transmit antenna itself, which is a DC
short across the far end of the line.

RF (RCA Broadcast Field Engineer, 1965-1980)

VSWR doesn't matter?
 

Yes, there's no simple correlation between VSWR at a particular
frequency and the reflection coefficient seen by a step or pulse type
TDR. As Richard pointed out, these TDRs have energy extending from DC
(the step type) or some relatively low frequency (pulse type) to
extremely high frequencies. The units I was involved in designing had a
3 dB frequency response and step content of up to 60 GHz. The phase has
to be quite constant over this entire bandwidth, also, for good step
fidelity. This very wide bandwidth is necessary to produce a fast step
and step response (on the order of 10 - 15 ps for the units I worked
with) in order to resolve anomalies which are physically very close
together. It is possible to translate a TDR return into a spectrum of
complex reflection coefficients (that is, a plot of reflection
coefficient or SWR vs frequency), but this requires a Fourier transform.
However, the energy content at any particular frequency is very small,
so many repetitions have to be integrated to provide a usable
signal/noise ratio. Likewise, a network analyzer can be swept over a
very wide frequency range and S11 converted to a TDR waveform by use of
an inverse Fourier transform.

Because of the major difference in spectral content and methodology, a
lot of care has to be taken in translating what you observe with a TDR
system to what happens in a steady-state single frequency situation. For
just one example, with a TDR you can easily tell the difference between
a transmission line and load, and a lumped RC or RL circuit. You can
also easily see the difference if you use a signal generator and make
measurements at several different frequencies. Or if you watch the
transient behavior as you turn the generator on and off (as in the
frequency-limited TDR Richard described). But in a single frequency
steady state system, you can't tell any difference whatsoever, provided
that you choose the RC or RL to have the same terminal impedance as the
original transmission line/load combination. Whatever effects are seen
with all the "forward" and "reverse" power and energy bouncing around
the line are seen exactly the same with no line at all and just an RC or
RL as a load. So any explanation of the effects (such as the red plates
of the mismatched transmitter posed earlier) has to be made without
resorting to the bouncing energy. Why that seems so difficult for so
many to do is a puzzle.

Roy Lewallen, W7EL

Richard Fry wrote:
"Walter Maxwell" wrote:
That was over 40 years ago, but I seem to remember that any
discontinuity that resulted in a VSWR greater than 1.005:1
produced a ghost that could not be tolerated in the transmitted picture.
I'm sure this is the magnitude of reflections Richard F. is referring to.
_____________

Analog TV transmission is not quite that sensitive to VSWR, fortunately.
Matti Siukola of the RCA Broadcast TV antenna group in Gibbsboro, NJ did
some experimental work showing that a 1% reflection (1.02 VSWR) or less
is unnoticeable to a critical observer, a 3% reflection (1.06 VSWR) is
noticeable but tolerable, and a 5% reflection (about 1.1 VSWR) and above
is objectionable. These values applied to the r-f spectrum from visual
carrier (Fcv) to Fcv +2.5 MHz or so, and for transmission line lengths
of 500 feet and more from the tx to the antenna.

These parameters were measured using an r-f pulse at the visual carrier
frequency having the transition times and r-f bandwidth corresponding to
the maximum bandwidth limits of the TV channel, only.

The more conventional broadband TDRs used a very short pulse with energy
from DC to far beyond the limits of the TV channel. It could resolve
small discontinuities along the transmission line, but many of them had
no affect on the quality of the transmitted television image, as they
were not present in the r-f spectrum of the TV signal. And the pulse
return of a wideband TDR is extremely high from the TV transmit antenna
itself, which is a DC short across the far end of the line.

RF (RCA Broadcast Field Engineer, 1965-1980)

VSWR doesn't matter?
 

"Richard Fry" wrote in message
...
In analog TV transmit systems with a typical 500+ foot length transmission
line from the tx to the antenna, a 5% reflection from a far-end mismatch
can
be quite visible, showing as a "ghost" image that is offset from the main
image as related to the round-trip propagation time of the transmission
line.

I've noticed that, at least in this area, Fox 39 (WQRF) has a ghost of a few
microseconds (I forget what exactly, I've calculated it before). Something
like 500 feet, IIRC.

Tim

--
Deep Fryer: A very philosophical monk.
Website @ http://webpages.charter.net/dawill/tmoranwms

VSWR doesn't matter?
 

On Mar 13, 2:48 pm, Gene Fuller wrote:
No one denies the existence of reflections.

But some people deny that there is any energy in the reflections -
see below. :-)


But a big argument is about the
round-trip travel of energy in the steady-state. Some people seem to
believe that energy continues to flow back and forth from one end of a
(mismatched) transmission line to the other under steady-state
conditions, even simultaneously traveling in both directions. (Passing
like ships in the night?) Since energy is a scalar quantity, and any
given joule is not distinguishable from another, it is not clear how the
proponents keep track of the bookkeeping, but they muddle through somehow.

Again Gene, to be able to prove your point, you need to present an
example of a standing wave that exists without a forward traveling
energy wave and a reverse traveling energy wave. You keep implying
that is possible, but have presented no proof.

Water is also a scalar. If you had one gallon per minute flowing into
a barrel and two gallons per minute flowing out of the barrel, would
you argue that there is no water flowing into the barrel and only one
gallon of water flowing out of the barrel? Or would you say the *net*
water flow is one barrel per minute out of the barrel?
--
73, Cecil, w5dxp.com

VSWR doesn't matter?
 

On Mar 13, 9:27 pm, Walter Maxwell wrote:
That was over 40 years ago, but I seem to remember that any discontinuity that
resulted in a VSWR greater than 1.005:1 produced a ghost that could not be
tolerated in the transmitted picture.

And of course, the same thing can happen in a simple system consisting
of
a TV source, a transmission line, and a mismatched load. TV ghosting
can
prove that the reflected traveling waves make multiple round trips to
the load
and back to the source, yet many continue to deny that proof. The best
(copout) argument that I have heard against it is that, "That is not
steady-state",
and therefore doesn't count. :-)
--
73, Cecil, w5dxp.com

VSWR doesn't matter?
 

On Mar 14, 6:53 am, Roy Lewallen wrote:
So any explanation of the effects (such as the red plates
of the mismatched transmitter posed earlier) has to be made without
resorting to the bouncing energy.

That's simply not true. When the load is connected directly to the
source, incident power is often still rejected, it just doesn't
have very far to "bounce". And since it is internal to the source, the
"bouncing" is difficult if not impossible to quantitize.

If you hang a purly reactive load on a source output, it rejects all
the the incident power just like it does at the end of a one-
wavelength long transmission line. If we leave the source output
terminals open, i.e. an infinite impedance, all of the source power is
rejected at the source output terminal, i.e. there is a standing wave
on the internal wire (often coax) connected to the source connector.

In the same way that a source doesn't know whether it is connected to
a transmission line or a lumped circuit, a purely reactive load
doesn't know whether it is connected to a source or to a transmission
line. Either way, it does an immediate rejection of incident power.
Whether the load is connected to a transmission line or directly to a
source, the reflection at the load is a same-cycle reflection. Since
it happens at the load with a transmission line, why are you surprised
that it happens at the load with a source?
--
73, Cecil, w5dxp.com

VSWR doesn't matter?
 

On Wed, 14 Mar 2007 03:51:01 GMT, Owen Duffy wrote:

Walter Maxwell wrote in
:

On Tue, 13 Mar 2007 22:28:02 GMT, Owen Duffy wrote:

Walter Maxwell wrote in
:

On 13 Mar 2007 14:19:12 -0700, "Cecil Moore"
wrote:

Did you guys on this thread know that it's been proven statistically
that five out of four people have trouble with fractions?

That's vulgar!

The stat ement or the fractions?

5/4

Yer right, that's what I thought you mean't.

Walt

VSWR doesn't matter?
 

Cecil Moore wrote:
On Mar 13, 2:48 pm, Gene Fuller wrote:
No one denies the existence of reflections.

But some people deny that there is any energy in the reflections -
see below. :-)


But a big argument is about the
round-trip travel of energy in the steady-state. Some people seem to
believe that energy continues to flow back and forth from one end of a
(mismatched) transmission line to the other under steady-state
conditions, even simultaneously traveling in both directions. (Passing
like ships in the night?) Since energy is a scalar quantity, and any
given joule is not distinguishable from another, it is not clear how the
proponents keep track of the bookkeeping, but they muddle through somehow.

Again Gene, to be able to prove your point, you need to present an
example of a standing wave that exists without a forward traveling
energy wave and a reverse traveling energy wave. You keep implying
that is possible, but have presented no proof.

Au contraire, mon frere. You continue to claim that a standing wave MUST
be made up of two traveling waves, but without proof.

My contention is that this distinction is merely a matter of
mathematical preference. When standing waves occur, there is absolutely
no physical difference between the standing wave and its traveling wave
constituents. If you find some physically significant difference due to
considering traveling wave constituents rather than the standing wave,
then you have made a mistake in your calculations.

Water is also a scalar. If you had one gallon per minute flowing into
a barrel and two gallons per minute flowing out of the barrel, would
you argue that there is no water flowing into the barrel and only one
gallon of water flowing out of the barrel? Or would you say the *net*
water flow is one barrel per minute out of the barrel?

This is totally irrelevant to the issue at hand. Try to keep on task.

73,
Gene
W4SZ

VSWR doesn't matter?
 

billcalley wrote:

snip

I've been reading the posts on this. One poster said this has been going
on for twenty years! (For the other groups, this thread has life on
rec.radio.amateur.antenna) It doesn't need to be so.

First, there should be no doubt that reflected power on a transmission
line is real. Sure, you can replace the line with a lump but that
doesn't clear up the question for others.

For the next two examples, see page 179:
http://cp.literature.agilent.com/lit...4753-97015.pdf
All examples assume the same impedance for source and line.

First example, step into an open line with a Thevenin source. The energy
is divided between the source and the line. Half the energy is moving
down the line and when it returns changes the impedance the source sees
to an open circuit. The energy does not flow back into the source, so,
where did it go? It is stored in the capacitance of the line.

Second example, step into a shorted line. When the energy returns the
source now sees a short. The energy does not flow back into the source,
so, where did it go? It is stored in the inductance of the line.

So here are two examples where the energy sent down the line do not
return to the source.

Third example. Send a pulse down the line. The Thevenin voltage source
will go to short, as it should, when the pulse falls. The pulse is
reflected from either an open or a short at the end of the line. All the
energy is dissipated in the source impedance when this pulse returns.
That is where the energy goes. And it is obviously the _same_ energy
created at the source.

Sure, non of the cases above represent steady state AC. But they do show
that energy may or may not be returned to the real component of the source.

With the above in mind, it can be shown, (in some part II), that a real
accounting of energy from source to load and back is possible.
Equivalent circuits are just that, the trading of line for lump. But,
and this is really important, the only reason the effective impedance at
the input of a 50 ohm line is not 50 ohms is because of reflected energy.

Best, Dan.

VSWR doesn't matter?
 

"Richard Fry" wrote in :

In analog TV transmit systems with a typical 500+ foot length
transmission line from the tx to the antenna, a 5% reflection from a
far-end mismatch can be quite visible, showing as a "ghost" image that
is offset from the main image as related to the round-trip propagation
time of the transmission line.

Richard, read this carefully, I do NOT disagree with what you have said
in the paragraph above.

For a ghost to be visible to viewers getting signal off air, there is
more at play that a 5% reflection from a far end mismatch, the reflected
wave is heading towards the transmitter, and needed to encounter another
at least another reflection point with sufficiently high reflection
coefficient that a sufficiently large, time delayed copy of the original
modulated wave reaches the load (even if in turn part is reflected
again).

This second point of reflection could be a transmission line
discontinuity, but it is most likely that it was that the tx end of the
transmission line was not matched, that is that the tranmitter did not
terminate the line with a near perfect match.

It leaves the questions "are transmitters matched to Zo in real life, is
matching an unavoidable consequence of optimising power output?".

For avoidance of doubt, the discussion in this post is about the
transient behaviour of the tranmission line and waves, it is relevant to
long transmission lines with analogue TV modulation, but that doesn't
mean it is necessarily important to other applications.

Owen

VSWR doesn't matter?
 

"Owen Duffy" wrote
This second point of reflection could be a transmission line
discontinuity, but it is most likely that it was that the tx end of the
transmission line was not matched, that is that the tranmitter did not
terminate the line with a near perfect match.

It leaves the questions "are transmitters matched to Zo in real life, is
matching an unavoidable consequence of optimising power output?"
___________

Field experience in my tenure with RCA Broadcast when measuring and
minimizing the "ghosting" propensities of commercial analog broadcast TV
transmission systems showed/shows that a measured 5% or greater voltage
reflection from the transmit antenna/input elbow complex, when sufficiently
displaced in time from the main image, will result in an objectionable ghost
image seen on an off-air TV set tuned to that station.

That would not be true if such a nominal 5% far-end reflection from the
antenna system essentially was absorbed by the TV transmitter (whether or
not that turned the tube PA plates red, or caused the failure or other
compromise of a solid-state PA).

Maximizing the output power and efficiency of a broadcast r-f amplifier
dictates that its effective output Z must be greatly different than the load
Z it is expected to drive. In the case of broadcast transmitters, that
source impedance is low (a few ohms), compared to the typical 50 or 75 ohm
Zo of the load it is driving.

And this it the reason that much of the voltage reflected from an
antenna/far-end mismatch returns from the tx back to the antenna to be
radiated, and so to produce the TV ghost image seen under those conditions.

An equivalent effect is a reality in FM broadcasting, where a poor Z match
of the antenna system across the FM channel bandwidth produces synchronous
AM, and adds to stereo and SCA crosstalk.

RF

VSWR doesn't matter?
 

On Mar 14, 12:24 pm, Gene Fuller wrote:
Au contraire, mon frere. You continue to claim that a standing wave MUST
be made up of two traveling waves, but without proof.

On the contrary, I have presented at least three references as proof.
If I remember correctly, it was Ramo, Whinnery, Hecht, and Balanis.
You, OTOH, have presented none.

My contention is that this distinction is merely a matter of
mathematical preference. When standing waves occur, there is absolutely
no physical difference between the standing wave and its traveling wave
constituents.

Obviously false as proven by the different equations for the two types
of waves. We laid that one to rest long ago. In fact, it was you who
pointed out that standing wave phase is completely different from
traveling wave phase and cannot be used to measure phase shift through
a coil. If I remember correctly, it was the difference between
cos(x*wt) and cos(x)*cos(wt), i.e. *very* different.

Water is also a scalar. If you had one gallon per minute flowing into
a barrel and two gallons per minute flowing out of the barrel, would
you argue that there is no water flowing into the barrel and only one
gallon of water flowing out of the barrel? Or would you say the *net*
water flow is one barrel per minute out of the barrel?

This is totally irrelevant to the issue at hand. Try to keep on task.

No, it is virtually identical to your argument. Saying it is "totally
irrevelent" doesn't change anything. You are arguing that net energy
transfer is primary and the underlying energy components are
irrelevant if nonexistant.
--
73, Cecil, w5dxp.com

VSWR doesn't matter?
 

On Mar 14, 5:36 pm, "Richard Fry" wrote:
And this it the reason that much of the voltage reflected from an
antenna/far-end mismatch returns from the tx back to the antenna to be
radiated, and so to produce the TV ghost image seen under those conditions.

For whatever reason, ghosting of this sort proves that the reflected
energy
makes a round trip from the load to the source and back. The ghosting
delay is exactly what a speed-of-light EM traveling wave would
experience.
In fact, multiple ghosting is caused by multiple reflections.
--
73, Cecil, w5dxp.com