VSWR doesn't matter?
 

Dan Bloomquist wrote:

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.

Wrong, wrong, wrong. Energy is not created or destroyed, but it can be
converted back and forth to mass. See mass defect and hydrogen fusion.


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.

Less wrong, line length makes no significant difference (unless it is really
lossy).


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.


Yes they do. Same model, same reflected power, same heating effect on the
last amplifier stage.

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.


Wrong, wrong, wrong again. Transmission line impedance is strictly a matter
of physical dimensions, and surrounding materials permitivity and
permeablity.

Best, Dan.

--
JosephKK
Gegen dummheit kampfen die Gotter Selbst, vergebens.Â*Â*
--Schiller

VSWR doesn't matter?
 

Richard Fry wrote:
For example, a reflection from a mismatch between a 1,000 foot long,
air-dielectric transmission line and the TV transmit antenna connected
to it
produces a ghost with ~ 2 µs delay from the main image. The active scan
width of an NTSC TV line is about 53 µs, so 2/53 = ~4% of the width of the
screen, or maybe 5% counting overscan. This ghost is easy to see in a
typical TV set/viewing setup.

We performed that exact experiment at Texas A&M
in the 50's. The ghosts were right where the
reflected waves predicted they would be. I
wonder how the modulation in a reflected wave
moves up and down the line without the reflected
wave also moving up and down the line? :-)
--
73, Cecil, w5dxp.com

VSWR doesn't matter? (Standing - travelling waves)
 

"Cecil Moore" wrote

Plus they need to be linked to reality. Standing waves existing
without the component forward and reverse traveling waves is
divorced from reality. Neither you nor anyone else has been able
to provide even one real-world example of such.

Forward traveling wave + reflected traveling wave = standing wave

What happens to the standing wave when you take away the reflected
wave?

Forward traveling wave + nothing = forward traveling wave

i.e. there is no standing wave. So please tell us again how
you can build a standing wave from a single traveling wave.

... so you can forget about bringing up all of your TV ghost arguments.

That rug of yours under which you try to sweep all the
reflected energy is going to explode one of these days. :-)
--
73, Cecil, w5dxp.com

Which brings us back to the great loading coil dispute.

In the resonant quarter wave monopole (say 80m), loaded with solenoid coil,
about 2/3 up the radiator, we experience significant (about 40%) current
drop at the top end of the coil. This is also demonstrated by bottom of the
coil getting warmer or hot proportionately, indicating that we have standing
wave circuit and some real current in the system, fortifying Cecil's
argument. RF is flowing along the radiator, "seeing" high impedance tip at
the end, being reflected, flowing back and being superimposed with the
forward wave. Reality that W8JI and other "defenders" had hard time to
swallow.

In the case of traveling wave antenna, like Beverage, terminated with
resistance, we can see the uniform current along the wire. Coil or slinky
inserted in such system will show the same current along the coil (minus
ohmic losses).

There is real life proof about what Cecil is saying above.

Relating to the standing wave circuit, I had question in my mind: how
important is to control the resistance and consider it in standing wave
antenna system. Example is that the current above the loading coil is
appreciably smaller than at the base, hinting that you perhaps do not need
low resistance (copper tubing vs. SS whip).

But...
if the standing wave is made of forward and reverse traveling waves, should
not we be trying to keep the resistance low in the system? Or is it
insignificant?
My pet peeve tells me that it would gain significance in the multi element
loaded arrays. Do the modeling programs capture that? They show slight
increase of current at the bottom (few turns) of the coil (when loading
inductance is properly modeled). Would that be due to the loss from that
point on, when forward and reflected wave is "meeting" the losses to
resistance and radiation and then with lesser amplitude superimposing with
forward wave?

Yuri, K3BU.us

VSWR doesn't matter? (Standing - travelling waves)
 

Yuri Blanarovich wrote:
But...
if the standing wave is made of forward and reverse traveling waves, should
not we be trying to keep the resistance low in the system? Or is it
insignificant?

The I^2*R losses in the conductors depend upon the net
current which is the phasor sum of the forward current
and reflected current. Since the net current is lower
above the coil in a mobile center-loaded antenna, the
resistance of the stinger is not as important as the
resistance of the bottom shaft which carries maximum
current. Whether the I^2*R losses in a stinger are
significant or not is a subjective call. If one is
looking for that last 0.1 dB that will win a mobile
antenna shootout, replacing the stainless steel
stinger with one-inch copper tubing might do the
trick.

Incidentally, in a thin-wire 1/2WL dipole, the reflected
current arriving back at the feedpoint is approximately
90% of the forward current, i.e. only about 10% of the
current is lost to radiation in its round trip to the
end of the antenna and back. The same is true of forward
and reflected voltage. The feedpoint impedance of a
1/2WL dipole results from the superposition of and
interference between the forward and reflected waves
on the standing-wave antenna at the feedpoint.

If we made a Z0=600 ohm 1/4WL open stub out of resistance
wire such that the feedpoint impedance is 73 ohms, we
would have a pretty good simulation of an antenna where
energy is converted to heat instead of being radiated.
The voltages and currents on the stub would correlate
closely with the voltages and currents on a 1/2WL dipole.
--
73, Cecil, w5dxp.com

VSWR doesn't matter?
 

On Mar 16, 10:22 am, Cecil Moore wrote:
Richard Fry wrote:
For example, a reflection from a mismatch between a 1,000 foot long,
air-dielectric transmission line and the TV transmit antenna connected
to it
produces a ghost with ~ 2 µs delay from the main image. The active scan
width of an NTSC TV line is about 53 µs, so 2/53 = ~4% of the width of the
screen, or maybe 5% counting overscan. This ghost is easy to see in a
typical TV set/viewing setup.

We performed that exact experiment at Texas A&M
in the 50's. The ghosts were right where the
reflected waves predicted they would be. I
wonder how the modulation in a reflected wave
moves up and down the line without the reflected
wave also moving up and down the line? :-)
--
73, Cecil, w5dxp.com

Awww come on Cecil - think it through...
First the standing wave is only standing by stroboscopic effect, i.e.
the instantaneous alignment (constructive and destructive) of the
forward and reflected wave fronts at a given point along the line...
The currents continue to stream past the standing point (in both
directions) carrying both the carrier and it's instantaneous
modulation products with it...
Also, the ghosts should be multiple... In the example cited of 2 uS
delay, there should be a ghost every 2 uS across the screen, albeit
each subsequent ghost a fixed number of dB weaker that the one before
due to the % radiated (plus line losses) for each round trip...

denny / k8do

VSWR doesn't matter?
 

Denny wrote:
Cecil Moore wrote:
I
wonder how the modulation in a reflected wave
moves up and down the line without the reflected
wave also moving up and down the line? :-)

Awww come on Cecil - think it through...
First the standing wave is only standing by stroboscopic effect, i.e.
the instantaneous alignment (constructive and destructive) of the
forward and reflected wave fronts at a given point along the line...
The currents continue to stream past the standing point (in both
directions) carrying both the carrier and it's instantaneous
modulation products with it...

OK, I'll change my wondering - I wonder how any rational
person can believe that the modulation in a reflected wave
moves up and down the line without the reflected wave also
moving up and down the line? :-)

Also, the ghosts should be multiple... In the example cited of 2 uS
delay, there should be a ghost every 2 uS across the screen, albeit
each subsequent ghost a fixed number of dB weaker that the one before
due to the % radiated (plus line losses) for each round trip...

Yes, that's exactly the way it appeared to be. Multiple
ghosting getting fainter across the screen.
--
73, Cecil http://www.w5dxp.com

VSWR doesn't matter?
 

"Denny" wrote
Also, the ghosts should be multiple... In the example cited of 2 uS
delay, there should be a ghost every 2 uS across the screen, albeit
each subsequent ghost a fixed number of dB weaker that the one
before due to the % radiated (plus line losses) for each round trip...
_____________

The ghost image needs to be at least 1% of the peak amplitude possible for
the primary video signal before it becomes noticeable So in order for the
2nd and successive ghosts in my example to be visible to a TV viewer, the
reflection producing the first ghost would be so objectionable that a TV
station would not tolerate it, and fix the problem.

RF

VSWR doesn't matter?
 

Denny, K8DO wrote:
"The currents continue to stream past the standing point (in both
directions) carrying both the carrier and its instantaneous modulation
products with it."

Yes. Point is the a-c amplitudes of both forward and reflected currents
are varing at the r-f frequency but their phase relationship is fixed so
that the two currents may be represented by vectors (phasors) at the
point.

Amplitude modulation is a mixing process which produces sum and
difference frequencies that are not synchronized with either original
frequency. Stable frequency generators produce fixed locations for
maxima and minima*along the transmission line, however.

Cecil`s tests were designed to demonstrate atanding waves and phasor
addition. They likely worked as intended.

Best regards, Richard Harrison, KB5WZI

VSWR doesn't matter?
 

Richard Fry wrote:
The ghost image needs to be at least 1% of the peak amplitude possible
for the primary video signal before it becomes noticeable So in order
for the 2nd and successive ghosts in my example to be visible to a TV
viewer, the reflection producing the first ghost would be so
objectionable that a TV station would not tolerate it, and fix the problem.

As I remember, the Texas A&M experiment used a reflection
coefficient of 0.707, i.e. 50% of the signal energy was
reflected at the TV receiver. There also had to be considerable
reflections at the source but that fact was never discussed.
--
73, Cecil http://www.w5dxp.com

VSWR doesn't matter?
 

Richard Harrison wrote:
Cecil`s tests were designed to demonstrate atanding waves and phasor
addition. They likely worked as intended.

One fact to note is that the virtual impedance changes
all up and down a transmission line yet no additional
reflections occur while the Z0 is constant. Reflections
occur only at *actual* impedance discontinuities, e.g. at
a junction of two different Z0s.
--
73, Cecil http://www.w5dxp.com