In article ,
Ronald wrote:

- sometimes dummyloads are used in input stages of a valve or mosfet p.a.
because valves and mosfet only use the rf voltage NOT the power.
So the driver stage (xmttr) has to be dummyloaded and tapped.
Understand ?

Sure. And, if those are truly "dummy loads", they are *not* simply LC
circuits. They have a resistor of some sort incorporated into them,
which is responsible for providing the correct termination impedance
and which serves to dissipate the power from the driver stage.

In other cases, what you call "dummy loads" for driving a MOSFET are
*not* simply "dummy loads". Rather, they are impedance transformers,
to step up the RF voltage to the correct level needed to switch the
MOSFET or tube properly.

In many cases, an LCR input stage to a final amplifier may have both
of these functions... impedance transformation *and* load termination.

-Then the remark : " a LC circuit won't radiate efficiently " . So what?
Thats exactly what i need here.

In standard radio technology, a "dummy load" is a device which
dissipates most of the power fed to it.

An LC circuit isn't one. You *need* something with resistance in it,
if you want to terminate the driving signal properly (i.e. dissipate
its power).

- so every LC series circuit you see in schematics, ìf you see them, you call
shortcuts ??

How in the world did you come to that ridiculous conclusion? Please
don't put words into my mouth.

LC circuits have *many* useful functions in a radio schematic. Two of
the primary ones are impedance transformation (matching), and
filtering (bandpass or notch).

Acting as a dummy load (in the accepted sense of the term) is *not*
one of the good functions of an LC.

- If you use an antennatuner for a 'nittingneedle' you will get a 1:1, and we don't care the
bad radiation pff, and that antennatuner has a series LC circuit inside !!

That antenna tuner has an LCR in it. The "R" is the loss in the
coil... as I pointed out several postings ago, a coil does have a loss
resistance.

In the case you're talking about, "matching" a nitting-needle load
with a typical T-configured antenna tuner may result in an apparent
match. What's happening, is that you're creating a rather high-Q
resonance inside the tuner... and all of the power is being dissipated
in the coil's loss resistance. Very little of the power is actually
going into the load - it's not really "matched".

The coil heats up, and (if you've got a cheaply-made tuner) you melt
the coil form and burn up the tuner.

The problem with depending on this sort of circuit (e.g. using a coil,
and a couple of capacitors) to dissipate power (acting as a very crude
sort of "dummy load") is that it's unpredictable, dangerous, and inefficient.

It's unpredictable because the series resistance of the coil is
typically not specified by the manufacturer. In fact, the better the
coil (heavier wire, open-spaced windings) the lower the series
resistance and the losses, which means that you have to use a higher-Q
tuning (more critical adjustment) to establish the match.

It's dangerous, because the sort of small coils you'd normally find in
the driver-to-final network of an amplifier can't dissipate the amount
of power you're talking about (20 watts). They'll burn up.

It's inefficient, because for the price of one adjustable coil which
can be tuned in this way (with a couple of capacitors), will give the
correct termination impedance (typically 50 ohms), and not melt down
or catch fire or otherwise self-destruct, you can buy *dozens* of very
well made 50-ohm noninductive resistors, which will give a good flat
termination impedance over a *much* wider frequency range than a
high-Q resonant circuit. Such resistors will be cheaper, smaller,
and more reliable than a lossy-LC-circuit kluge, and won't require any
tuning at all to work properly.

So no shortcut at all, dummy.
So whats your point ?

My point is, you're attempting to choose the *wrong* technical
solution for your problem. You'll waste money, and end up with a
device which is bigger, tricker to tune, and less reliable than is
necessary.

I only asked about the practice of the consept.

And, the accepted engineering practice of this concept is "This is a
bad idea, we don't do it that way. We use resistors."

--
Dave Platt AE6EO
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