Cecil Moore wrote:

Assume a one second long lossless unterminated transmission line.
Pour 1000 watts into it for one second. During the next second, we
disconnect the line from the source and you grab the two wires, one
in each hand. Then tell us whether reflected power exists or not.


So if we had an infinitely long antenna, would the power put into it
last forever?

- Mike KB3EIA -

It's true that even a simple DC battery circuit can be described in
terms of incident and reflected power, SWR, and reflection
coefficients, but that's very unrealistic when trying to conceptually
understand what's going on.

I suppose a point that I'm making is that understanding how to match a
source with a load doesn't have much to do with reflections and
standing waves although, again, it can certainly be described
(unrealistically) that way.

Take the case of a transmitter and an antenna connected together with
a two-port black box between them, and that black box happened to
contain a transmission line. That unseen transmission line has
standing waves and reflections (assuming a mismatched condition),
losses, etc. all contained within the box. However, the transmitter
only sees a steady state complex impedance when looking into the box
(at a single frequency). Obviously there's some conjugate matching
that needs to take place for maximum power transfer, but there are no
reflections or standing waves involved outside the box -- unless
transmission line stubs are used for matching.

In short, transmission lines have reflections and standing waves, and
as a "black box" they affect how the load is seen by the source. But
extending the power reflection concept outside of that black box only
confuses things, even though it works mathmatically.

Al

Cecil Moore wrote in message ...
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.

Mike Coslo wrote:

Cecil Moore wrote:
Therefore, radar cannot work since it relies upon reflected
joules/second. Mirrors also cannot work since there is an ExB
amount of power in those reflections.

I made note of this in a thread I just started. Is this a good
analogy? Certainly the signal goes out, hits something and then comes
back. Wouldn't this scramble the signal by some definable amount in an
antenna?

Of course, but the point is that there exists energy as power, ExH,
in the reflected wave. If there is no energy in a reflected wave,
radar wouldn't work. Anyone who says there is no energy in a
reflected wave is just pulling your leg.

Assume a one second long lossless unterminated transmission line.
Pour 1000 watts into it for one second. During the next second, we
disconnect the line from the source and you grab the two wires, one
in each hand. Then tell us whether reflected power exists or not.

At any rate, the answer should be pretty easy to verify by using a
very long but practical sized antenna. Anyone done that?

Anyone who has ever used a TDR on a mismatched transmission line.
--
73, Cecil http://www.qsl.net/w5dxp



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Mike Coslo wrote:

Cecil Moore wrote:

Assume a one second long lossless unterminated transmission line.
Pour 1000 watts into it for one second. During the next second, we
disconnect the line from the source and you grab the two wires, one
in each hand. Then tell us whether reflected power exists or not.

So if we had an infinitely long antenna, would the power put into it
last forever?

It would have a feedpoint impedance of around 600 ohms and I
don't know how long it would have to be to eliminate reflections.
A terminated rhombic comes close to the characteristics of an
infinitely long antenna.
--
73, Cecil http://www.qsl.net/w5dxp



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alhearn wrote:
Take the case of a transmitter and an antenna connected together with
a two-port black box between them, and that black box happened to
contain a transmission line. That unseen transmission line has
standing waves and reflections (assuming a mismatched condition),
losses, etc. all contained within the box. However, the transmitter
only sees a steady state complex impedance when looking into the box
(at a single frequency).

That's a steady-state shortcut which assumes pure sine waves that
don't exist in reality. Please don't confuse steady-state shortcuts
with reality. Noise and modulation cause the "steady-state complex
impedance" not to be steady-state at all. Many will say it's close
enough, but one cannot understand reflections by assuming an un-
varying steady-state.

In a TV system with ghosting due to reflections, the unvarying steady-
state condition doesn't exist. In fact, when you assume steady-state
conditions, you eliminate ghosting, at least in your own mind. In
reality, steady-state doesn't really exist because of random noise
and unpredictable modulation.
--
73, Cecil http://www.qsl.net/w5dxp



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"Cecil Moore" wrote in message
...
.................................
Of course, but the point is that there exists energy as power, ExH,
in the reflected wave. If there is no energy in a reflected wave,
radar wouldn't work. Anyone who says there is no energy in a
reflected wave is just pulling your leg.

But its not free. If you tap it, it won't be there to re reflect. So, I
think in the steady state you have changed the load impedance. BTW, (not a
joke or sarcasm) Have you heard of Zero Point Energy? They calculate there
is something like 10**26 Joules in a cubic meter of empty space.

Tam/WB2TT

"Richard Fry" wrote in message
.........................
................................
Concept below

However this is not an accurate model of a transmitter.

For an example, take an old Heathkit DX-100 generating a measured 180
watts
of CW RF into a matched 50 ohm load. To do this, it does NOT also
dissipate
180 watts of RF into some "virtual" internal RF load in the DX-100. In
fact, the PAs and power supply in the DX-100 could not produce a total RF
output power of 360 watts without exceeding their ratings.

The dissipation in the PA is essentially related only the DC to RF
conversion efficiency of the PA, which in this case probably is about 75%,
max (Class C). So a PA input power of about 240 watts DC is required to
produce 180 watts of RF output power. The other 60 watts of plate input
power is converted to heat by the PA tube anodes.

The entire RF output generated by the PA stage is applied virtually 100%
to
the output connector. How much of that is absorbed by the load connected
there is a function of load SWR and system losses.

- RF
There is a Motorola ap note that agrees with what Richard is saying. To
paraphrase it, if the the DX100 had an output impedance of 50 Ohms, then the
overall efficiency would be 37.5%.
.....................................

Aviation Week & Space Technology. March 1, 2004. P50.

Tam

Tam/WB2TT wrote:

"Cecil Moore" wrote:
Of course, but the point is that there exists energy as power, ExH,
in the reflected wave. If there is no energy in a reflected wave,
radar wouldn't work. Anyone who says there is no energy in a
reflected wave is just pulling your leg.

But its not free. If you tap it, it won't be there to re reflect. So, I
think in the steady state you have changed the load impedance.

Moral: don't use it up, just use a negligible portion for measurement
purposes. A Bird wattmeter consumes very little actual power.

BTW, (not a
joke or sarcasm) Have you heard of Zero Point Energy? They calculate there
is something like 10**26 Joules in a cubic meter of empty space.

Yes, some people think that is the energy being measured, off and on,
in the cold fusion experiments. The question is: If one taps into
the energy being used to support the structure of space itself,
what happens to that space structure? IMO, space is the result of
the annihilation of matter and anti-matter in the early universe
and certainly still contains that annihilation energy.
--
73, Cecil http://www.qsl.net/w5dxp



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Richard Clark wrote:

Hi Ian,

Perhaps in this immediate thread. However, I have demonstrated both
sides coming to the same conclusions several many times, and one
example as recently as within this last week.

This issue is not about being right, it is about ego foremost else why
all the debate? Hank has asked a fairly straightforward question
with rather simple terms he could accept as a compelling case. To
this point (some 22 entries) that has been largely abandoned with each
scribbler answering their own imagining of how to discover the
philosopher's stone.

73's
Richard Clark, KB7QHC

I haven't been here terribly long, maybe 6 weeks, but I have noticed
that your comment applies to around half of the threads longer than 4 or
so comments.

A better ratio than many newsgroups, and people seem to have a sense of
humor, which a lot of other newsgroup's participants decidedly do not.
So, all in all, it could be worse.

tom
K0TAR