[Amps] BirdŽ 43 Manual

Paul Christensen w9ac@arrl.net
Thu, 28 Mar 2002 07:27:19 -0500

> > Not according to my Bird 43 operating manual.  If you have one, see
> > 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
> with a 1/2 wave length of line. Take a piece of 75 ohm cable and terminate
> with a 50 ohm load. Put your watt meter on it and you will see a perfect
> 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
> here. You will get the same results.

Granted.  It is a principle of transmission line theory that the impedance
is identical on either side of a half-wavelength.  I believe the point in
the Bird manual is that we are not always dealing with 1/2-wave lines in our
measurements.  Most of our measurements in this hobby are conducted on line
of unknown length.

> 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.

Fine.  See above.

> 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
> reflected by a matching network in a tube radio why would it not also be
> reflected by the matching network in the solid state radio.

Because the source impedance remains relatively constant at 50-ohms with
solid-state, broadband transmitters.  Re-reflection requires a conjugate
(reactance cancellation) of the antenna + feedline.  The solid-state
transmitter has no way of achieving this without an external mechanism.
This can be satisfied with a transmatch, auto-tuner....and yes, changing the
feedline length to tune the antenna + line as a complete "system" as you
previously stated.  However, without that mechanism in place, any reflected
power caused by the load-to-feedline mismatch is absorbed in the

> Well ok if you don't like a shorted line how about an open line with
> resistance.

Heh...I'm not the one who doesn't like your shorted line hypothetical.  We
started with open lines and you moved to shorted lines.  Let's stick to one
point at a time.  Please explain why a zero-resistance short on an
automobile battery would produce no power?

> Well you don't tune the antenna with the transmission line that is an old
> misunderstood tale.

Not quite.  The antenna *system* which includes the line + antenna can be
tuned by changing the length of feedline.

> 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.

But you haven't expained why this works.  The answer is that by changing the
line length, the system reactance is cancelled and the antenna + feedline is
now at resonance, even when the line-to-antenna is mismatched.  This is the
essence of conjugate matching.  The transceiver sees only the resultant
resistive component.

> > 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
> > 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
> 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
> hook any line at all on the other side of the meter. What will it read?

The exact same reading:  100-watts forward and 100-watts reflected.  No
surprise there.  I tried it again this morning.  Mt transceiver is still
drawing 20-amps at 13.8 VDC with or without a feedline connected to the
radio.  Where is the power being dissipated?  By now, you know the answer.

-Paul, W9AC