Dear Group:

I add a vote for avoiding the arguable position about the "output Z" of
a transmitter (note, transmitter) and instead using words similar to those
used by Mr. Owen Duffy: a transmitter designed to operate into a load Z of
50+j0 ohms

Conclusions reached with a minimum of needed assumptions are preferred.
Roy's observations do not require a particular output Z and are true.

A signal generator with an emitter-follower output stage having a 50 ohm
resistor in series with the output port can have an output Z of very close
to 50 ohms over a broad frequency range.

73, Mac N8TT
--
J. McLaughlin; Michigan, USA
Home:
"Owen Duffy" wrote in message
...
Roy,

The mention of reactance means we are talking in the frequency domain,
and a steady state solution is being discussed.


Roy Lewallen wrote in
:

...
Power will indeed be transferred through such a match. Using your
antenna as an example, suppose that a transmitter with output Z of 50
ohms is connected to a tuner that transforms its output impedance to
0.6 + j1250 ohms. Connect the output of the tuner to a half wavelength

Does the use of "output impedance" here mean that he transmitter can be
validly represented by a Thevenin equivalent circuit, and that "output
impedance" is the Thevenin equivalent source impedance.

Without getting into that arguable postion and reinforcing the notion
that a transmitter rated for a nominal 50 ohm load has a source impedance
of 50+j0, you could say:

... Using your antenna as an example, suppose that a transmitter designed
to operate into a load Z of 50+j0 ohms is connected to a tuner that
transforms the antenna (0.5-j1250) to its preferred load impedance (50
+j0). Connect ...


600 ohm transmission line to the antenna. The transmitter will see 50
+ j0 ohms, the antenna will see an impedance of 0.6 + j1250 ohms, and
full power will be transferred. Power transfer has nothing to do with
the SWR on the line or the match between the line and antenna.

In practice, the line loss will increase some due to the very high
SWR, but the loss increase won't be much if the matched line loss is
low.

I chose a half wavelength for simplicity, but it's not necessary.
Other lengths of line will transform the antenna impedance to
different values. All that's necessary is to readjust the tuner
accordingly to match the different impedance.

And:

All that's necessary is to readjust the tuner accordingly to deliver the
transmitter its rated load impedance.


Amateurs have successfully been using this method to feed nonresonant
and multi-band antennas for decades.

Roy Lewallen, W7EL


As you will have noted, some band the term 'match' around with abandon,
and it means different things in different contexts, and to different
readers.

Take a transmitter designed for a 50+j0 load, connected by an electrical
half wave of 70 ohm coax to a 50 ohm load. Is it 'matched'? Well, from
the information, it is correctly loaded, it is designed for a 50+j0 load,
and it has a 50+j0 (approximately) load. We don't actually know the
source impedance, and even if we did, it this case, whether the
transmitter is 'matched' to the line, and whether the line is 'matched'
to the load is unimportant.

'Output impedance' is another term that is used differently, some use it
to mean the equivalent source impedance, some to mean the required load,
and some insist the foregoing are naturally the same, or will be if the
transmitter is 'matched' for maximum power output.

Owen