Henry wrote,

I know that any power not dissipated by an antenna is reflected back to the
transmitter. Then the transmitter "reflects" this reflection back to
antenna, ad nauseum until its all gone. I also know that a short or an open
is required to reflect power and I'm searching for which it is, an open or a
short. I'm inclined to think it's a virtual open but I'm at a loss to
understand that and I wonder if someone has a good explanation or analogy
and some math wouldn't hurt.
tnx
Hank WD5JFR



This post is guaranteed to get Cecil revivified. Here's a hint: quit thinking
solely in
terms of power, that's for fellows who want to explain how it all works without
going into
any of the complicated details. Get an undergraduate physics text that
discusses
waves, and read it, or, better yet, take a class. Take all explanations you
read in
amateur publications with a grain of salt. After you've done this, you still
won't be
able to argue with Cecil, because that requires an extensive knowledge of the
moronic,
unfair, and downright pathalogical debating techniques of which Cecil is a
master.
But, it will be harder for you to fall for some of the crackpot ideas you're
liable to
read on this newsgroup, and it will give you something to think about when
sipping
your after-dinner port.

73,
Tom Donaly, KA6RUH

Tam/WB2TT wrote:
I am not quite sure what you are saying. But, I ran a SPICE simulation of
the following:
1V 1MHz source with resistor R0 feeding a 50 Ohm 250 ns transmission line
shorted at the far end. Independent of R0, in steady state the voltage at
the input end of the transmission line will be 1V. The effect of R0 is to
limit how long it takes to reach steady state. For R0 = 50 Ohms, it is one
cycle; for R0 = 500 Ohms, it is about 8 cycles, as eyeballed off the
waveform display.

Does SPICE report the steady-state forward and reflected waves
or just the superposition of those two waves? We all know what
they look like when superposed. The question is whether the
identity of the forward and reflected waves disappear after
they are superposed. To the best of my knowledge, the very
existence of standing waves requires the existence of a forward-
traveling wave and a rearward-traveling wave.

I have asked for examples of standing waves void of rearward-
traveling waves and none has been forthcoming.
--
73, Cecil http://www.qsl.net/w5dxp



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Tdonaly wrote:
This post is guaranteed to get Cecil revivified. Here's a hint: quit thinking
solely in terms of power, that's for fellows who want to explain how it all
works without going into any of the complicated details.

Like Hewlett Packard in their AN 95-1 publication? "Another advantage
of the s-parameters springs from the simple relationship between the
variables a1, a2, b1, and b2, and *various power waves*. ... The previous
four equations show that s-parameters are simply related to power gain and
mismatch loss, quantities which are often of more interest than the
corresponding voltage functions."
--
73, Cecil http://www.qsl.net/w5dxp



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I just want to know the reflection physics in the Tx, no antenna tuner, just
a mismatched antenna. I recall no pysics book that tell me how the
reflection sees the transmitter.
73
Hank WD5JFR
There are no stupid questions, just stupid people asking!
"Tdonaly" wrote in message
...
Henry wrote,

I know that any power not dissipated by an antenna is reflected back to
the
transmitter. Then the transmitter "reflects" this reflection back to
antenna, ad nauseum until its all gone. I also know that a short or an
open
is required to reflect power and I'm searching for which it is, an open
or a
short. I'm inclined to think it's a virtual open but I'm at a loss to
understand that and I wonder if someone has a good explanation or analogy
and some math wouldn't hurt.
tnx
Hank WD5JFR



This post is guaranteed to get Cecil revivified. Here's a hint: quit
thinking
solely in
terms of power, that's for fellows who want to explain how it all works
without
going into
any of the complicated details. Get an undergraduate physics text that
discusses
waves, and read it, or, better yet, take a class. Take all explanations
you
read in
amateur publications with a grain of salt. After you've done this, you
still
won't be
able to argue with Cecil, because that requires an extensive knowledge of
the
moronic,
unfair, and downright pathalogical debating techniques of which Cecil is a
master.
But, it will be harder for you to fall for some of the crackpot ideas
you're
liable to
read on this newsgroup, and it will give you something to think about when
sipping
your after-dinner port.

73,
Tom Donaly, KA6RUH

Henry Kolesnik wrote:
I just want to know the reflection physics in the Tx, no antenna tuner, just
a mismatched antenna. I recall no pysics book that tell me how the
reflection sees the transmitter.

The reflected waves obey the laws of physics. The kicker is that we
don't know (and apparently cannot directly measure) the source impedance.
What the reflections can do is modify the designed-for load line through
superposition of the forward and reflected waves. Modification of the
designed-for load line is not desirable and, if unprotected, can cause
over-voltage, over-current, or phase problems.
--
73, Cecil http://www.qsl.net/w5dxp



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OK, if we shine a flashlight at a mirror the light bounces back and what
ever is caught by the reflector will be reflected. Take away the reflector
and the reflection just keeps going. If someone can tell me what the hot
filament does perhaps I can understand what happens in the finals, or
whatever.
tnx
Hank WD5JFR
"Cecil Moore" wrote in message
...
Henry Kolesnik wrote:
I just want to know the reflection physics in the Tx, no antenna tuner,
just
a mismatched antenna. I recall no pysics book that tell me how the
reflection sees the transmitter.

The reflected waves obey the laws of physics. The kicker is that we
don't know (and apparently cannot directly measure) the source impedance.
What the reflections can do is modify the designed-for load line through
superposition of the forward and reflected waves. Modification of the
designed-for load line is not desirable and, if unprotected, can cause
over-voltage, over-current, or phase problems.
--
73, Cecil http://www.qsl.net/w5dxp



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Henry Kolesnik wrote:
OK, if we shine a flashlight at a mirror the light bounces back and what
ever is caught by the reflector will be reflected. Take away the reflector
and the reflection just keeps going. If someone can tell me what the hot
filament does perhaps I can understand what happens in the finals, or
whatever.

The hot filament provides a source of electrons. How many
electrons are emitted depends on the instantaneous voltages
on the other elements, the plate and grids. Reflected waves
have an effect on those instantaneous voltages but IMO,
there's not much sense in pursuing the "filament" line of
reasoning.
--
73, Cecil http://www.qsl.net/w5dxp



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It's true in the case of transmission lines that there are standing
waves and reflections but, unfortunately, this concept has somehow
come to dominate and confuse the concept of matching a tranmitter to
an antenna -- a generator to a load. Like in many areas of science,
mathematicians and scientists often find convenient ways to
mathematically describe and predict physical phenomenon that hinders,
even misleads, the understanding of how it actually works.

If you leave out the complex part of impedences for the moment and
think of 100 volt generator that has a 50 ohm internal impedance
driving a 50 ohm load, current is 1 amp and the power dissipated by
the load is 50 watts. There is also 50 watts dissipated by the
generator's internal impedance, for a total of 100 watts dissipated by
the entire system. Therefore, the "available" power for this generator
is 50 watts.

Maximum available I^2*R power only occurs when the load impedance is
equal to the generator's characteristic impedance, 50 ohms (do the
math). Any load impedance higher or lower, ALWAYS produces less
"available" power.

Herein lies one of the big problems with the "reflection" definition,
conceptually. The generator (transmitter) is not a constant-power
device. When a manufacturer says that it's XYZ transmitter produces
100 watts, it only produces (has available) 100 watts (after internal
dissipation) into a 50 ohm load. Any other load *always* produces less
available power, due to simple I^2*R laws. It has nothing to do with
reflections or standing waves, although, mathmatically, reflection
formulas accurately describe it.

A couple of examples using a 100 volt constant voltage generator and
an internal impedance (RG) of 50 ohms:

1) Load (RL) = 50 ohms. Current = 100 / (50 + 50) = 1 amp
Power dissipated in RL (PL) = (1)^2*50 = 50 watts
Power dissipated in RG (PG) = (1)^2*50 = 50 watts
SWR = RL/RG = PL/PG = 1:1

2) Load (RL) = 100 ohms. Current = 100 / (50 + 100) = .667 amp
Power dissipated in RL (PL) = (.667)^2*100 = 44.5 watts
Power dissipated in RG (PG) = (.667)^2*50 = 22.25 watts
SWR = RL/RG = PL/PG = 2:1

3) Load (RL) = 25 ohms. Current = 100 / (50 + 25) = 1.34 amp
Power dissipated in RL (PL) = (1.34)^2*25 = 44.9 watts
Power dissipated in RG (PG) = (1.34)^2*50 = 89.8 watts
SWR = RG/RL = PG/PL = 2:1

Notice that the total power dissipated in all three examples is
different. The transmitter is NOT a constant-power source, but it's
also not a unlimited power source and has operational limits.

Therefore, what is commonly called "reflected power" is power that
never leaves the transmitter and is dissipated as heat by the
transmitter's internal 50 ohm impedance (if the transmitter's design
doesn't prematurely shut down first).

Al


"Henry Kolesnik" wrote in message ...
I know that any power not dissipated by an antenna is reflected back to the
transmitter. Then the transmitter "reflects" this reflection back to
antenna, ad nauseum until its all gone. I also know that a short or an open
is required to reflect power and I'm searching for which it is, an open or a
short. I'm inclined to think it's a virtual open but I'm at a loss to
understand that and I wonder if someone has a good explanation or analogy
and some math wouldn't hurt.
tnx
Hank WD5JFR

alhearn wrote:
Herein lies one of the big problems with the "reflection" definition,
conceptually.

That's why I often resort to a signal generator with a circulator/load
to illustrate my point. That signal generator *is* a constant power
source.

Therefore, what is commonly called "reflected power" is power that
never leaves the transmitter and is dissipated as heat by the
transmitter's internal 50 ohm impedance (if the transmitter's design
doesn't prematurely shut down first).

You can mount an argument that if the source doesn't see its
source impedance, then there is a reflection at that internal
mismatch. But that's not what is commonly called reflected power.

When we talk about reflected power on this newsgroup, we are usually
referring to the forward power rejected by a mismatch between the
transmission line Z0 and the antenna impedance (associated with mismatch
loss). In a typical ham radio antenna system, the "lost" reflected power
is forced to engage in destructive interference at the tuner and thus
joins the forward power wave.
--
73, Cecil http://www.qsl.net/w5dxp



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I thought filaments produced photons/light waves as well. You took me to
light now you want to leave!
Come on I just want a good basic understand on what it is the the Tx
reflects the power , how it does it and a little simple math. My dad used
to say if you can't explain something you think you know to someone else it
might because you don't understand it yourself or lack command of the
language. In this case for me it's both.
73
Hank WD5JFR
"Cecil Moore" wrote in message
...
Henry Kolesnik wrote:
OK, if we shine a flashlight at a mirror the light bounces back and what
ever is caught by the reflector will be reflected. Take away the
reflector
and the reflection just keeps going. If someone can tell me what the
hot
filament does perhaps I can understand what happens in the finals, or
whatever.

The hot filament provides a source of electrons. How many
electrons are emitted depends on the instantaneous voltages
on the other elements, the plate and grids. Reflected waves
have an effect on those instantaneous voltages but IMO,
there's not much sense in pursuing the "filament" line of
reasoning.
--
73, Cecil http://www.qsl.net/w5dxp



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