[Prev][Next][Index][Thread]
Re: Bang the rocks together harder lumpophiles
>Original Poster: "Robert Jones" <alwynj48-at-earthlink-dot-net>
>>A lossless transmission line, or a lossless coil, is a purely reactive
>>element, and so, in sinusoidal steady state conditions (CW), all
>>voltages
>>are at +/-90 degrees with all currents. In the mechanical analogs,
>>this would be the relation between positions and speeds.
>
>I am sorry to be blunt but this is not correct the impedance of a
>losseless transmission line is resistive, hence for a PROPAGATING
>wave the voltage is ALWAYS in phase with the current.
This is wrong. The line is open at the other end. It would look resistive in
STEADY STATE only if terminated on its characteristic resistance. It's true
that
a long line behaves as a resistive element, but only for SHORT signals, that
last for less time that the required for travel along the entire line and back
to the input side. And even so, with the other end open, there is a reflection,
and current is drawn from the input after the signal returns.
Think at what happens at DC, for example. There is no return path for the
input current, and the open-ended line can't be considered a resistor. It's
just a capacitor. It behaves as a capacitor until the first resonance. It
alternates between inductive and capacitive phase input characteristics at
each resonance.
>Your mechanical analogue appears odd, V x I = power, position x speed=?
>V x dV/dt=? I would expect force x velocity = power
The equations and the behavior are similar, not the units. You can consider
the tension in the cord (a force proportional to the displacement from rest)
and the speed to get power.
>I dont think I understood the above paragraphs. If the line is not
>resonating
>the in to out phase could be anything it depends on the length of line. At
>resonance what is being measured is the phase of a standing wave with
>respect
>to the input. A standing wave is just that it does not move. Its the sum of
>a forward and backward travelling. It has constant phase it only
>changes in amplitude. Its phase is pinned to the reflection at the end of
>the line
>that created it. If you could remove the standing wave you would be able to
>observe the progressive phase change of the input signal.
With the output open, in sinusoidal steady state, in a lossless line, no
average
power
can enter or leave the line, or even move along the line. At any point where
you cut the line you will observe 90 degrees phase relationship between
voltage and current, at any frequency. Otherwise there would be energy
moving along the line.
>At resonance the input current is in phase with the input voltage thats one
>definition of resonance. ( assuming resistive drive). If the were no losses
> the input voltage would be zero ie it would look like a short.
Correct, but with a resistive drive you are effectively measuring the relation
between the input current (the input voltage is just a small voltage drop in
the resistive losses of the line) and the output voltage. 90 degrees
relationship.
>I suspect you are talking about the relative phase of the current and
>voltage in a standing wave which is 90deg.
>I think there has been a lot of confusion about standing wave and
>propagating
>waves. This is what produced the "no phase shift so its not a transmission
>line" view.
>In effect the two types of waves are spatial and temporal.
>One moves in space and one moves in time.
>The difficulty is that if you put a scope probe on either one you get the
>same sinewave hence confusion. At first it fooled me too.
>
>It may be possible to have a internally consistent theory of how the coil
>works
>using standing waves with particular properties. I prefer the main stream or
>traditional theory of forward and backward travelling waves.
>I missed the posting of your model I will look for it.
I don't like to think about travelling waves. For short signals the concept
is
useful, but things become very confuse as reflected signals appear back at
the input.
Antonio Carlos M. de Queiroz
http://www-dot-compuland-dot-com.br