Paul Christensen wrote:
> > > 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.
But you left off the most important part of what I said here. Try this sometime
with a 1/2 wave length of line. Take a piece of 75 ohm cable and terminate it
with a 50 ohm load. Put your watt meter on it and you will see a perfect match.
No reflected power. Forget about what the bird manual says. The meter will see a
perfect flat 50 ohms. Also try it with other impedance lines as I mentioned
here. You will get the same results.
It doesn't matter what the characteristic impedance of the line is. What
matters is what the meter sees. I can take a 1/2 wavelength
line that has a characteristic impedance of 100 ohms or 400 ohms or 900
ohms. If I put a 50 ohm load on the end of that line and the
other end connected to the watt meter the meter will see 50 ohms with no
reflected power. Impedance transformation.
>
>
> > 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.
Yes and the reason that I mentioned it is that you earlier stated that the
reflected power coming from the antenna was re- reflected by the matching
network in a tube radio but in a solid state radio there was no matching
network. My point being that if you claim that the reflected power gets re-
reflected by a matching network in a tube radio why would it not also be re-
reflected by the matching network in the solid state radio.
>
>
> > > 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.
Well ok if you don't like a shorted line how about an open line with infinite
resistance. Now how much power is your battery going to dissipate if it sees an
infinite resistance?
Analogy is we have a 1/2 wave open line or a shorted 1/4 wave line hooked to the
transmitter. The transmitter sees an infinite resistance in either case.
>
>
> > 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.
Well you don't tune the antenna with the transmission line that is an old
misunderstood tale. The only way you tune the antenna is by changing some
parameter of the antenna. The transmission line is not part of the antenna.
Whatever you do to the transmission line will not change the antenna or the
impedance that the antenna presents to the feed line. And if the feed line
impedance does not match the antenna impedance there will be reflected power on
the transmission line.
What I am saying is that by the proper types of feed line lengths to the
transmitter you can make the transmitter see 50 ohms with no reflected power but
yet the feed line has reflected power on it. And we know that it will have
reflected power on it because it's impedance does not match the antenna.
>
>
> > 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?????
What happens if you disconnect the feed line completely from the transmitter.
What happens if you hook the bird watt meter directly to the output terminal of
your transmitter with just a double male connector (no cable length. Then don't
hook any line at all on the other side of the meter. What will it read?
What will you read if you put your voltmeter across your 12 volt car battery
with nothing else connected to it? You have a generator setting there that can
deliver power but if no load is applied is it delivering any power. Is it
generating power and all the power being reflected back into the battery.
73
Gary K4FMX
>
>
> -Paul, W9AC
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