If there is incident energy upon a correctly matched transmitter's output,
it need not absorb 100% of the energy.

If the transmitter is an ideal, linear, Thevenin/Norton source, it must, as
is consistent with linearity, and the power transfer theorem, and
transmission line theory, and all that.

But, it is quite common for transmitters to reflect incident power. This
characteristic is captured by the scattering parameters, namely s22, the
output reflectance. (Well, that would be Gamma_22, but they're equivalent
parameters in the end.)

This corresponds, in turn, with the dynamic output impedance, which you may
recall from audio amplifiers, need not equal the "best load" impedance. For
instance, an amplifier for 8 ohm loads might have a dynamic impedance of
0.03 ohms, a very good voltage source in comparison -- acting like a short
circuit to incident energy (and therefore reflecting energy, out of phase,
back towards the loudspeaker, thus giving it a high "damping factor").

The same is true of RF amplifiers, except rather than constant voltage
characteristics due to the use of voltage negative feedback, the
characteristic is usually constant current (high impedance), due to the lack
of negative feedback (or the use of current feedback) and the high intrinsic
impedance of the devices (i.e., the collector / drain / plate resistance).

As the energy "piles up on" and reflects off the transmitter, internal
voltages and currents may get into dangerous ranges, causing excessive
dissipation or breakdown; this is partly why transmitters shouldn't be
operated with high SWR (the other part being, efficiency and power capacity
suck).

The only amplifiers that have a relatively matched intrinsic output
imepdance are vacuum tube triode amplifiers. Though even these tend to have
a poor match, either being operated in class AB with R_L Rp, or class B/C
with R_L Rp (and lots of grid current to push plate current up there).
Because, again, nonlinear devices don't need to obey the linearity theorems,
and can have impedances different from the "best load" value.

Tim

--
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com

"rickman" wrote in message
...
On 7/4/2015 9:43 PM, Jeff Liebermann wrote:
On Sat, 04 Jul 2015 19:33:30 -0400, Jerry Stuckle
wrote:

On 7/4/2015 7:22 PM, Jeff Liebermann wrote:
On Sat, 04 Jul 2015 19:04:01 -0400, Jerry Stuckle
wrote:
Think of it this way, without the math. On the transmitter side of
the
network, the match is 1:1, with nothing reflected back to the
transmitter.

So you have a signal coming back from the antenna. You have a perfect
matching network, which means nothing is lost in the network. The
feedline is perfect, so there is no loss in it. The only place for
the
signal to go is back to the antenna.

Wikipedia says that if the source is matched to the line, any
reflections that come back are absorbed, not reflected back to the
antenna:

https://en.wikipedia.org/wiki/Impedance_matching
"If the source impedance matches the line, reflections
from the load end will be absorbed at the source end.
If the transmission line is not matched at both ends
reflections from the load will be re-reflected at the
source and re-re-reflected at the load end ad infinitum,
losing energy on each transit of the transmission line."

And you believe everything Wikipedia says? ROFLMAO.
But that also explains your ignorance.

Let's see if I understand you correctly. You claim that with a power
amplifier (source) output impedance that is perfectly matched to the
coax cable, but not necessarily the load (antenna), any reflected
power from the load (antenna) is bounced back to the load (antenna) by
the perfectly matched source (power amp). Is that what you're saying?

Yet, when I have a perfectly matched load (antenna), all the power it
is fed is radiated and nothing is reflected. You can't have it both
ways because the reflected power from the load (antenna), becomes the
incident power going towards the source (power amp). Matched and
mismatched loads do NOT act differently depending on the direction of
travel. If you claim were true, then transmitting into a matched
antenna or dummy load would reflect all the power back towards the
transmitter.

I think this is one of those situations where a casual explanation won't
work. You can use a "casual" explanation when the various qualifications
for a simplification apply. But to do that, the qualifiers have to be
fully understood and no one here is showing what the qualifiers are much
less that they are met. So until we get a real explanation I will stick
with what I recall. In the end, to settle this we may have to use the
math.

I'm sure someone in s.e.d could explain this properly. Some of them may
be purely argumentative, but some really know their stuff. I believe the
description of a conjugate match is the mathematical inverse of the
complex impedance of the antenna "viewed" through the feed line, but I
have to admit I don't really know what that implies or if it is even an
accurate description.

--

Rick