Gary,
An impedance mismatch is the same thing as a reflector. How do you
think mirrors work? It's a matter of perspective.
"Reality" of power flow is debatable. Let me describe it from my
viewpoint as someone brought up as a physicist.
The basic issue is what are the voltages and currents in your system
at each point? "Power" only has tangible meaning when it is delivered
somewhere in a useful form (e.g., heat). Otherwise, it's a
mathematical construct - a way of doing the accounting to make sure
energy is conserved. On your transmission line, there is voltage and
current at each point, which are described by differential equations
and boundary conditions. (Ultimately, Maxwell's equations.) If you
have a match, V and I are uniform along the line. If not, you have a
standing wave. (No issue of "power" yet.) The forward/reverse power
numbers you measure from samples of V and I on a transmission line
turns out to be useful in controlling transmission line losses (and
possible over-current and over-voltage conditions) and for maximum Tx
output. In some cases (TDR) it is easy to relate forward and reverse
power to a particular "piece" of energy - a wave packet, and an
impedance mismatch is recognizable as a mirror - a source of reflected
energy.
In the extreme, our arguments here resemble the wave/particle debates
in physics. Sometimes electrical signals behave more like particles
(radar, TDR), sometimes more like waves (CW network analysis). Neither
description is wrong.
An aside: I am annoyed by tuners and switches and other items that
are rated for "power handling" capabilty. E.g., a 1 kW balun. These
devices don't care about power, except they will have a thermal
dissipation limit. They care about voltage and/or current. At 50
ohms, the power level tells you V and I, but if you have anything
other than 1:1 SWR at 50 ohms, V and I can be very different. So you
see "1 kW" baluns burning up at 100 W power levels, etc.
This is a never-ending discussion, as someone pointed out at the
beginning of the thread. It's very interesting, but the connection to
"towers" is not very clear!
Cheers
Martin AA6E
On 5/13/05, Gary Schafer <garyschafer@comcast.net> wrote:
>
> Here is another way to look at it: With a circulator at the transmitter
> output and a load on the return port of the circulator, disconnect the
> antenna line from the circulator. Now all the power out of the
> transmitter goes into the load on the circulator and the transmitter
> still sees a flat 50 ohms.
>
> Is there really reflected power involved or does the power get dumped
> into the load because of an impedance mismatch at the antenna port on
> the circulator?
>
> We could look at reflected power not even existing on transmission lines
> at all. In fact we could look at it as once the transmission line is
> initially charged at transmitter turn on there are no more reflections.
> What we see are only impedance mismatches that our meters are showing
> us. As others have pointed out the swr bridge or watt meter is not
> really showing power but the indication is the result of an impedance
> mismatch from the value that the meter was calibrated for.
>
> 73
> Gary K4FMX
>
>
--
martin.ewing@gmail.com
http://blog.aa6e.net
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