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

"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)

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)

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

"Cecil Moore" wrote
...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. etc
_______________

Does the lack of a technical response to Cecil's post (so far) mean
that his analysis and conclusions are understood and accepted?

Hopefully so.

RF

Richard Fry wrote:
"Cecil Moore" wrote
...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. etc

Does the lack of a technical response to Cecil's post (so far) mean
that his analysis and conclusions are understood and accepted?

The "eliminate the transmission line" sword cuts both
ways. If the source cannot tell the difference between
driving a one wavelength transmission line and driving
a lumped circuit load directly, it follows that the load
cannot tell if it is being driven by a one-wavelength
transmission line or being driven directly by a source.
The incident signal looks the same in either case and the
load rejects (reflects) the same amount of forward power
either way. Except for the energy stored in the one-
wavelength transmission line, conditions are the same
in either case.
--
73, Cecil http://www.w5dxp.com

"Cecil Moore" wrote in message
...
Richard Fry wrote:
"Cecil Moore" wrote
...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. etc

Does the lack of a technical response to Cecil's post (so far) mean
that his analysis and conclusions are understood and accepted?

The "eliminate the transmission line" sword cuts both
ways. If the source cannot tell the difference between
driving a one wavelength transmission line and driving
a lumped circuit load directly, it follows that the load
cannot tell if it is being driven by a one-wavelength
transmission line or being driven directly by a source.
The incident signal looks the same in either case and the
load rejects (reflects) the same amount of forward power
either way. Except for the energy stored in the one-
wavelength transmission line, conditions are the same
in either case.
--
73, Cecil http://www.w5dxp.com

in steady state... where your favorite s equations hold. this is true. it
is not true in the general case where you account for startup transients.

Richard Fry wrote:

Does the lack of a technical response to Cecil's post (so far) mean
that his analysis and conclusions are understood and accepted?

Hopefully so.

In my case, it's because I plonked him long ago.

Roy Lewallen, W7EL

Roy Lewallen wrote:
Richard Fry wrote:
Does the lack of a technical response to Cecil's post (so far) mean
that his analysis and conclusions are understood and accepted?

Hopefully so.

In my case, it's because I plonked him long ago.

For pointing out that an antenna is a distributed
network, not a lumped circuit.
--
73, Cecil http://www.w5dxp.com

I teach my students that prior to analysis of an electrical, electronic, or
EM network/system one must ask and answer a critical question. The question
is: Is the network/system linear, close enough to linear for engineering
purposes, or not linear?

If linear, or essentially linear, one brings into play linear analysis.
Thevenin equivalents, which are only equivalent as far as what they do to
the outside world, are a part of linear analysis.

Most RF power amplifiers that deliver more than one or two watts are
non-linear circuits. Typically, the active device conducts for only a
fraction of each cycle. How else could one get DC power to RF power
efficiencies of over 50 %? Great care must be taken in modeling such
circuits.

A simple example: Consider a transformer fed bridge rectifier (very
non-linear) that is connected to an (old fashion) series L, shunt C filter.
In steady state, if L is large enough, one may model the rectifier as a
series of series connected voltage sources with harmonically related
frequencies (and a DC source). It is left as an exercise for the student to
decide on the sizes, frequencies, and phases of the sources. (Because of
the LPF properties of the LC network, one does not need many harmonics.)
Then one may apply superposition (the essence of a linear process) to
estimate the ripple on the load. However, the model just described is
invalid if L is too small or if L is non-linear. The model is insufficient
to predict the losses in the rectifier. This example is not likely to be
found in current electronic texts, but we all know for whom they are
written.

Techniques exist for dealing with many non-linear networks. They must be
used with great care. If one holds one's nose, one might find an
"equivalent" for a transmitter that suffices for describing what happens
outside of the transmitter, but not inside of the transmitter. Please do
not make conclusions about the "equivalent" itself. Please discriminate
between linear and non-linear networks.

Thus ends the lecture. 73 Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
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