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)