In article ,
Fred W4JLE wrote:

Let's put this an easier way, how many of you have a low pass filter in
line. Most manufacturers claim a .25dB or less insertion loss. These filters
consist of a boat load of coils and caps.

A tuner normally has 2 varible caps and a single silver plated coil.

While any device indeed has loss, here we are picking fly crap out of
pepper.

I think you're making an inapplicable comparison.

The low-pass filters you're referring to have a relatively low loss
when used as directed, it's true... but the "use as directed"
condition usually mandates having the proper source and sink
impedances on either side. The loaded Q is usually modest.

A transmatch being used to match a difficult load can be in a
distinctly different situation. If the load is a harsh one (low R,
high magnitude of X), tuning up can create a situation with a high
network Q and high circulating currents in the tuner components.

The April '95 QST has an article which shows some actual calculations
(and if I recall correctly the Handbook has a bunch of tuner loss
tables).

One example in the QST article shows the results of using a T tuner to
tune an antenna which is just too darned short. The tuner sees a load
of 4.40 - j35 ohms. Although a match can be established with the
specified tuner components, the estimated power loss in the tuner is
almost 2/3 of the input (4.54 dB!). The transmission line eats enough
11.6 dB, in their example.

Granted, this is a rather pathological situation, but it does occur
from time to time in the real world. I've heard numerous reports of
people "burning up" their tuners by trying to match a low-resistance,
highly-reactive load, and frying the coil. Some of the MFJ tuners
seem to have a reputation for suffering from heat damage to the coils
(the plastic coil form melts).

--
Dave Platt AE6EO
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