Paul Christensen wrote:
> > If all the power was reflected back and absorbed by the ten-tec finals with
> > an
> > open termination on the line then it would be dissipating the full 100
> > watts in
> > the final?
>
> Quid Pro Quo: That's my assertion, what is yours? Where do you believe
> 100-watts of source power is being dissipated in an
> unterminated line? Even if the transceiver's maximum output power was not
> attainable under this condition, where then is *any*
> power being dissipated in a near-lossless, unterminated line?
>
> > How about when there is no line at all connected? What about if the line
> > was a
> > quarter wave length and open at the far end. The radio would see a dead
> > short.
>
> I did not test a shorted line quarter-wave line, but I believe the same tests
> would essentially prove identical results of a line
> with a shorted termination versus one with an open termination. In effect,
> the line SWR is close to infinity, notwithstanding line
> loss. Without answering a question with a question, what is your assessment
> of this?
>
Paul,
I don't mean to sound controversial on this, just trying to stimulate some
thinking on everyone's part. I don't profess to have all the
answers. It can be a complicated subject and sometimes things don't really work
exactly like it looks like they do.
We have to remember that when a transmission line is involved and there are
mismatches, there is impedance transformation going on. Any
time the load is not matched to the line the line no longer presents the same
impedance at the watt meter or the transmitter that it
did before.
What I am saying is that I don't think the bird meter will read true power
unless it sees a 50 ohm resistive load. So what is seen on
the meter is not necessarily what is going on in the transmission line.
If a coax line is unterminated the bird meter reads 100 watts forward and 100
watts reflected as you saw but is there really 100 watts
there? If there is zero current and some finite voltage then is there really
any power actually present? The transmission line is no
longer 50 ohms at the watt meter because of the impedance transformation.
(sorry about the additional questions but I don't have a
definitive answer)
What I was saying about an open quarter wave line is that an open circuit at
the end of a quarter wave line will be transformed into a
short at the other end due to impedance transformation of the line. If this is
hooked to the watt meter then the watt meter would be
looking into a short. Would it still read 100 watts? The transmitter probably
could not put out an infinite amount of current at zero
voltage to get a true 100 watts at that point.
If the reflected power were to get absorbed by the transmitter finals then an
open wire transmission line would never work but in a few
instances where it was matched to the antenna. In reality the reflected power
on that type of line is often very high if you were to
measure it. The reason that it does work is that most all of the reflected
power gets re- reflected back to the antenna and is absorbed
by the antenna.
73
Gary K4FMX
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