From: ming@enel.ucalgary.ca (Yang Ming)
Subject: Re: SWR Tuning and PLL
Date: Fri, 08 Jan 93 19:42:27 GMT
Organization: ECE Department, U. of Calgary, Calgary, Alberta, Canada

In article <1993Jan7.231323.6634@ke4zv.uucp> gary@ke4zv.UUCP (Gary Coffman) writes:
>>
>>OK. You ARE talking about linear circuits. You are right that E
>>changes due to the reflected wave, but you forget that I changes
>>as well, and E/I is constant is linear circuits.
>
>No. If E/I is constant, you get an unvarying DC static line. That's
>not true in a power amplifier, only in a small signal amplifier.

OK. You agree that in a small signal amplifier E/I does not change. This
is the point I have been trying to clarify in the last few posts : 
linear or non-linear. 

>Also, the voltage and current in a reflected wave are sign reversed
>from the forward wave. The return current is out of phase with the
>forward current while the return voltage is in phase with the forward
>current, plus line delay phase shifts of course. Maxwell covers this
>in chapter four better than I can. 

This doesn't neccesarily mean that E/I has to be zero or what ever
causes your total reflection.

>
>
>The same analogy still holds, however, with reduced amplitude reflections.
>In this case you just pair the reflected wave with a *part* of the forward
>wave being generated by the source. If you didn't do that, there wouldn't
>be any output from the amplifier, thus no reflected wave, etc. As long as
>the reflected wave is smaller than the forward wave being generated by
>the source, and it has to be, then the net power flow is forward in all
>cases.

Are you trying to say that the forward wave kills or cancels the reflected
wave, so that reflected one wont be able to enter the source?
If so, it is misconcepted.

>
>>>Let me give a crude analogy to illustrate what happens when a
>>>reflected wave enters an operating amplifier's output port, think
>>>of it as trying to piss into the nozzle of a operating firehose. 
>>>It all gets reflected back to you. The conjugate mirror effect is 
>>>what causes total re-reflection of the wave.
>>>
>>
>>This is N/A. EM wave on the transmission line is not the same thing.
>
>But EM waves trying to get into an operating *source* are the same
>thing. You have to realize that the behavior at the output terminal
>of an active device is not transmission line behavior. Let's try
>another analogy to try to get the point across. Assume a 12 volt
>battery feeding a 6 ohm load. There are two amps forward current
>flowing. Now insert a 1.5 volt battery in series with the 12 volt
>battery, but of reversed polarity. This is your reverse wave. No
>power from the 1.5 volt battery is dissipated by the 12 volt battery.
>The voltage can't overcome the forward voltage to get back in the
>bigger battery. It is cancelled out by 1.5 volts of the forward
>voltage leaving 10.5 volts for the load. The bigger battery now
>sees a higher output impedance and supplies less current to the 
>load. Like an amplifier, the current it *could* have supplied
>before doesn't *back up* in the amplifer, it simply isn't drawn
>from the power supply.
>

This analogy is not a good one either. First, you gave a DC example;
second, you put in another source, not a reflective impedance;
third, your three components are all in series but source-antenna
is kind of paralell like.

An operating source can be entered. My analogy would be this:
If you have a mirror or whatever shinny, you could reflect
the light from your headlight back into the light bulb.
If in an ideal case, you reflect all the light emitted back to
the focal point, you can burn down the filament. THis is power
absorbtion. If your antenna is not properly tuned and reflects all
the power, your amplifier could also be burnt down.


>>
>>Doesn't matter even it is output, if it is linear. You know S-parameters
>>that are used to describe a transistor? They are got from maesurement
>>in linear region and used in amp design. They don't consider S22
>>non-linear.
>
>It does matter. Two port and four port networks have different behavior.

No. It doesn't. We are not talking about how many ports. We are
talking about linear circuits. These S-parameters
can be used to describe any N-port circuits. There are many books
on how to use them to design amplifiers and oscillators, etc. Again,
they are linear parameters. They work fine until reaching nonlinear region.
But circuits start with small signal in linear region, and 
reach non-linear region at large signal. 

>
>>
>>Ah. Finally I got the keyword that you ARE talking about nou-linearity.
>>Fine, transistors are better described with nonlinear parameters,
>>no doubt about it. But befor nonlinearity takes over, at small signals,
>>they can be nicely described with linear terms. The point I have been
>>trying to make is that reflected wave CAN be absorbed by the amplifier,
>>partly or tatolly, depending on the whole network, while your point is
>>that it is totally reflected. If you agree that there is a linear part
>>in amplifier parameters, then you can't ignore that some reflected
>>wave is absorbed by the amplifier.
>
>Transmitter output stages are almost *by definition* large signal
>devices. Small signal parameters are essentially worthless in
>describing their behavior. Solving by parts is the only reasonable
>way of dealing with the problem. As long as the reflected wave is
>opposed by the forward wave *at the active device terminal* no
>power can flow back into the active device.

The word *opposed* you just used, together with your two analogies,
again convinces me that you think that the forward wave PUSHES
the reflected wave back to the antenna, or concels it. This is
definitely wrong in linear circuit theory. The truth is, at
a crosssection along the transmission line, the net power flow
is the difference of these two waves, but the power is still 
being absorbed by the two loads the two ends.

Before each question is answered, it has to be clarified it's 
about linear circuits or nonlinear ones.
I don't know what kind of nonlinear theory was used by Maxwell. But
there are some common sense regarding the nonlinear methods.
An important part is that nonlinear theory grows up from the
linear theory to better describe the device. Therefore there
is a linear part in nonlinear theory. As a matter of fact,
this linear part should be the main part, in all practical
amplifiers, in order to have neglectable nonlinear effect on
the signal.

Gary, I think there are two hard to answer questions in your posts:

1) If reflected wave can be absorbed by the amplifier at small
signal (linear region), how come as you turn up the power,
the amplifier turns out to be reflecting *all* the reflected wave?
What makes the reverse? You believe linear and non-linear work
together?

2) If all the reflected wave from the antenna is re-reflected by
the source, where does it go? Since this is a already-generated wave,
and lines are usually very low loss, this part of wave will go back and
forth between source and the antenna, each trip gets absorbed (radiated)
a bit by the atenna at the load end, and totally reflected at the 
source end. This way you should be able to eventually send all generated
wave to atenna. Do you still need match them? Isn't that too good to
be true?



Ming.
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