Reg Edwards wrote:
If a tuner doesn't burst into flames with a 1 KW transmitter then it must be
resonably efficient. There must be a lot less than 1/2 S-unit loss in
signal strength.

Why all the concern about efficiency of tuners? Old wives' ?

No, because coils actually do melt and switch contacts do arc, even at
the 100W power level. This is because commercial tuners are notoriously
over-specified, and always use components that are at least one size too
small.

Rule of thumb: divide the rated power level by at least 3 - and you
still may not be entirely safe.

The T-match tuner can match a very wide range of impedances, but it is
also prone to operator error - specifically, using too much inductance.

For the lowest possible stress on the components, you must use the
*lowest* value of inductance that will allow the two capacitors to be
adjusted to give an impedance match.

You can also get a match using higher inductance, but the internal
voltages and circulating currents will be unnecessarily high. That's
where the inefficiency problems arise. With T-tuners, these problems are
especially acute when trying to match low impedances at low frequencies.

The ARRL Antenna Handbook has a lot more to say about this, and the
CD-ROM includes a program that lets you map out the efficiency as a
function of load impedance and frequency. Some of the numbers down in
that low-frequency, low-impedance corner are plain scary - most of the
power is going into the tuner itself, and it comes out as smoke.

This situation is not entirely the fault of manufacturers. It's we
customers who demand a tuner that claims to match every impedance, at
every frequency, with the highest possible power rating, and all for
less than the lowest possible cost. The T-tuner probably comes closest
to this ideal, which is why they choose that configuration... but it
isn't always the right choice.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)

http://www.ifwtech.co.uk/g3sek