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