Hi Mike,
Rich rote:
> >>With half of the total P being dissipated in the
> >>generator, and half of the power being delivered
> >>to the load R, the efficiency is obviously 50%.
You asked:
> Is it being suggested, in essence, that if an amplifier delivers (as per
> Bird wattmeter) 1500W RF to the coax, that due to the power transfer rule,
> the amp's tubes must be generating at least 3KW RF? My HV and plate
> current suggest otherwise.
That's isn't exactly what he is saying, but it is close. Rich is saying
if a circuit is adjusted for maximum power transfer, efficiency is
only 50%.
Of course that isn't true, unless the source impedance is
dominated by a dissipative resistance. We pretty much (using
common sense) all know that isn't true, even if for the sake of
argument people like to claim it is.
Let me give an few examples:
1.) You adjust the pi-net in your PA so maximum power, for a fixed
drive level, is delivered to the antenna. You calculate efficiency and
find it is nearly maximum, and much more than 50%. You change
the load and find no matter what you do with the load, efficiency
and power drop.
That situation fits all the requirements of the maximum power
transfer theorem, and conjugate matching, yet efficiency is more
than 50%.
2.) You have a generator running at a constant shaft speed, and
limit available power by supplying a fixed field magnetic field level.
You do a load pull and find a load resistance that allows maximum
possible power in the load. Measuring generator heat, you find that
efficiency is well over 50%. Yet anything you do to the load from
that optimum value only reduces power, and reduces efficiency.
That is also a common situation.
The source impedance, while it always has some dissipation, does
not have to dissipate half the power when delivering maximum
power or when the load is optimized to have maximum power from
the source.
That mistake comes from not reading or applying the rules of the
model, and applying rules governing standard dissipative
resistances to a source limited by a non-dissipative "impedance" (if
you like).
In mechanical engineering they have terms that are different than in
electrical engineering, and the mechanical engineer has no
problem understanding the concept of power limited systems
where torque and distance is conjugately matched to provide
energy transfer at high efficiencies.
Unfortunately many EE's and some non-EE's who are otherwise
pretty smart think in terms of "it must be a resistor because it is in
ohms".
Find the hidden resistor that makes your 50 ohm cable 50 ohms
and causes peak efficiency to occur with a 50 ohm load where the
efficiency is only 50%, or the resistor that makes your antenna
50% efficient with power transferred to space when the antenna is
matched to the feedline for optimum power transfer.
We certainly need another name for that "resistance", and people
don't like non-dissipative resistance, so maybe we should call it
something else. How about a non-reactive impedance???
> Or am I missing something?
Nothing that I see.
73, Tom W8JI
w8ji@contesting.com
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