[Lowfer] XFX on the air -- NOT

[John Davis]

>Had the antenna system tuned to resonance ... guess the SWR could still be
>high?  What can I do about that?
>


Tuning the antenna to resonance only means that you've tuned out the
reactive component of its impedance, or maximized the antenna current.
(These two are not precisely the same thing, but close enough for most
purposes.)  Unfortunately, it doesn't say anything about the impedance
presented to the amplifier by the antenna.  You can cancel the reactance
easily enough, and yet the remaining resistive component may not be anywhere
near the value you want the amplifier to see.

This is not normally a problem in Part 15 gear, where the output network
provides filtering, impedance transformation, and antenna tuning, all in
one.  We can do that because (1) thanks to the rules limiting total length
of transmission line and antenna, its physically more practical to have all
these functions lumped in the tank circuit; and, (2) the power levels are
low enough that a modest mismatch usually doesn't cause fatal overheating,
voltage transients, etc.

However, when you go to ham and broadcast power levels, it's best to do
things the way the pros do.  That means defining a point (the transmitter
output terminals) at which you expect to see a known load impedance (usually
50 or 75 ohms resistive).  You build the matching network and harmonic
filter in the transmitter to work into that impedance.

(In Dave Bowman's circuit that you will be using, T2 allows the transistors
to see a very low impedance, needed for delivering adequate power from the
relatively low drain voltage, while stepping the impedance up to 50 ohms for
the output.  The filter after the transformer is two sections of relatively
tolerant, non-impedance-transforming lowpass designed to work with 50 ohms
in and out.)

Then you connect suitable transmission line to the transmitter and implement
suitable matching networks at the antenna to ensure that impedance is
actually present.  At the transmitter output, you also want metering of your
forward AND REFLECTED power.  This enables you to tune the antenna more
accurately, and can be incorporated into protective circuits to foldback
your drive level or cut drive entirely if excess reflected power is present.

It would be great to have an actual calibrated reflectometer that could give
you both forward and reflected power in watts; but if that's not practical,
I would at least use an impedance bridge to continuously monitor the VSWR.
I've seen a couple of articles on constructing bridges suitable for LF, but
can't remember where at the moment.  Perhaps someone else will recall.

John