> > Provided that the Bird directional coupler matches the characteristic
impedance of the line, the metered results should be
> > reasonably accurate. Interpreting the displayed results is another
story.
>
> It doesn't matter what the characteristic impedance of the line is. What
matters is what the meter sees.
Not according to my Bird 43 operating manual. If you have one, see section
3-36 on p. 19. Also see the notes in section 3-37 on p. 20.
> There is an impedance matching network in a solid state transmitter too.
It just happens to be broad banded.
Right, and as I've already noted, the source impedance of the transmitter is
relatively constant as a function of frequency. There are many stages of
impedance transformation in the transmitter. We're discussing the generator
source impedance at RF output connector.
> > What instant in time does not matter. Power is averaged over time that
we read on the meter.
> Pick any amount of time you want. The relationship between voltage and
current is not going to change. We have a certain amount of
> current across a certain resistance, That is how we measure power. In this
case the resistance is zero. What is the power.
And I can short the battery terminals on a 12-volt car battery. The
terminal resistance is zero. In your example, no power would be dissipated
when in fact, the power is generated and dissipated in the source resistance
of the generator. I suspect an explosion would be evidence of that power.
> An open wire line can be and is often used for a single antenna to operate
on multiple bands. On one band the antenna may match the
> characteristic impedance of the line and there is no reflected power. On
the next band that same antenna may present a very drastic
> mismatch to the line. The line may be 450 ohms and the antenna may present
a 2000 ohm load. There is no matching network used at the
> antenna. There is reflected power from the antenna mismatched to the line.
If we cut the line the right length or we use the proper
> lengths of another impedance line we can get it to present 50 ohms or any
impedance we wish at the transmit end. There will be high
> reflected power on the line. But we do this with no tuner, only
transmission line. This will work fine and the final will not know any
> difference than if we had a matched line and antenna hooked to it.
Great. And what is the mechanism that allows what you state without the use
of a tuner? The answer lies in the classic QST article from By Goodman, "My
Feed Line Tunes My Antenna!" What you've done is exactly what I previously
stated. And that is "The reason that an open-wire balanced line works in
most of our applications is because a *conjugate* network is placed
somewhere on the transmission line (typically at the transmitter with
an auto-tuner or transmatch)."
In your example your tuning your antenna by establishing a conjugate match.
The change feedline length is that conjugate mechanism.
> Again, is there really any power generated when there is no load. Or is
there only voltage.
Yes, there is power being generated and it is being absorbed by the
generating source when operating into an unterminated transmission line.
If what you state is true (no load, therefore no power) and the fact that my
Bird 43 measured 100-watts forward and reflected into an unterminated line,
my transmitter shouldn't be drawing 20-amps at 13.8 volts. One more
time....where else can the power be dissipated?????
-Paul, W9AC
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