Hello,
it should be remembered that the values of loading capacitance, tuning
capacitance and the inductance of the tank coil not only depend on
impedances and frequency, but also on Q. The designer has a wide degree
of control over these values.
For example, if we need to match 4000 ohm to 50 ohm, on 160 meters, and
we have a loading capacitor of just 1000pF, and we don't want to add
capacitors in parallel, we can pick a Q of 10. This leads to 215pF
tuning capacitance (including tube and stray capacitance), a 35.8uH
coil, and 882pF of loading capacitance.
If that's still too high, just let's decrease the Q to 9.5. That gives
us 204pF, 37.2uH, and 645pF.
Still too high? Use Q=9. Then we get 194pF, 38.4uH, and just 272pF of
loading capacitance!
In practice this would be done by tapping the tank coil at a spot where
the Pi circuit tunes up with the loading capacitor at the center of its
range, into a dummy load. Regardless of how much maximum capacitance
that capacitor has! That will give us the best matching range possible
for real antennas with non-perfect SWR. Of course, the smaller the
loading cap is, the narrower this tuning range becomes.
The actual penalty of having a very high plate impedance is that we
cannot use a lower Q. With 4000 ohm, we cannot go below a Q of 8.89.
This makes tuning a bit touchy, and we need a VERY good tank coil to get
acceptable losses. The loss depends on the ratio between the natural
(unloaded) Q of the components, mainly the coil in practice, and the
loaded Q. If the coil has a natural Q of 300, which is typical, and we
are running it at a loaded Q of 10, it will loose one thirtieth of the
total RF power. At 1500 watts, that's 50 watts heating the coil, which
may be a problem.
If the plate impedance instead is just 2000 ohm, the minimal Q we can
use is 6.4, in practice we might use 8 or so. That way the loss is a bit
lower, and tuning is a bit broader.
Of course we cannot go too low with the loaded Q, because that will
increase harmonic output to an unacceptable level. Old handbooks say
that actually we shouldn't go lower than Q=10 or so.
On the highest bands, the problem is another one: All that tube
capacitance and stray capacitance forces us to use a Q higher than we
would like! For example, on 10 meters with a 4000 ohm plate, a Q of 10
would need a tuning capacitance of just 14pF. Good luck finding a tube,
and a construction method, that keeps the tube and stray capacitance
lower than this! Likely that capacitance will be higher, forcing us to
use a higher Q. If the total tube and stray capacitance, plus the
minimal capacitance of the tuning cap, is 40pF, we need to use a Q of 30
(or higher). At Q-30, the coil becomes only 0.83uH. With a natural coil
Q of 300, we would have 150 watts of loss in that little coil! That's
why many amps use coils wound from thick, broad silver plated copper
strip for the high bands. Absolutely highest possible natural Q of that
coil is a strict need! And of course tuning gets much more touchy than
on the lower bands.
For these reasons, most amplifiers don't use the same loaded Q on all
bands. They use higher Q on the higher bands, to accomodate the tube and
stray capacitances, and lower Q on the lower bands, to keep the tuning
and loading caps from growing too much, and to allow the use of smaller
loading coils. High Q coils for low frequencies are large.
Well, that was a quick burst of antique tube technology, from my side.
Since it's monday morning, I hope I may be forgiven. Now I switch back
into 2013.
Manfred
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Visit my hobby homepage!
http://ludens.cl
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