In article ,
D. Huizinga wrote:

Hi there.

My 40 mtr doublet is fed with a balanced line of 300 Ohms. Nicely tunable
to 1:1 . But after twenty minutes or so the coil gets rather hot! It is
a quality
coil of 3 mm and shiny silvered or chromed on a ceramic body of 1 inch.

Not at all unusual.

An antenna tuner (or "transmatch" as some prefer) is a tuned circuit,
which can develop some very high circulating currents which flow
between the inductive components (e.g. the coil) and the capacitive
components (the internal capacitors, or the capacitive reactance of
the load). These high currents can result in a substantial amount of
loss in the tuner components (mostly in the coils).

How are your experiences with a 100 W tx ? I calculate just 0.3 Amps thru it.
The mind boggles on this..!

If I recall correctly, 100 watts is developed into a 50-ohm load via
70.7 volts at 1.4 amperes.

Those are the voltages and currents at the transmitter output. The
voltage and current relationships will be different at other points in
the circuit... at the antenna itself, and at various points inside the
tuner... because the impedances will be different.

In the case of extreme impedance transformations (e.g. transforming 50
ohms down to match a 5-ohm load, or up to a 1000-ohm load), an
antenna tuner can have to handle either very high currents (leading to
high I^2*R losses) or high voltages (leading to arcing).

The popular "T" antenna tuner configuration can be a bit tricky...
it's notorious for being able to "tune" some very difficult loads,
even down to a short circuit in some cases. In this latter situation,
the tuner is actually "tuning" its own internal losses (e.g. in the
coil). There are plenty of tales around about peoples' antenna tuners
"burning up" when this occurred - all of the transmitter power went
into heating up the coil, and melted the coil form!

Where does that heat come from ?

From the transmitter! You may actually have several amperes of RF
flowing through the coil. The coil's series resistance at RF is
higher than it is at DC, due to skin and proximity effects.

You might be able to reduce the losses and heating by changing the
length of the 300-wire line you are using, so that the antenna's
feedpoint impedance is transformed into an impedance at the tuner that
doesn't require extreme impedance transformation, and requires the use
of lower amounts of reactance during tuning.

If I recall correctly, the ARRL Handbook has some tables (and
software?) which can let you calculate a tuner's efficiency and
losses, based on the impedance of the load and some reasonable
assumptions about the Q of the inductor. Lower efficiency == more
loss == more heating.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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