on 4/4/01 10:26 PM, Wt8r@aol.com at Wt8r@aol.com wrote:
> What is your estimate of the magnitude of the reduction of capacitance due to
> the addition of the "step-down" coil, if that is what you infer in the first
> & second paragraph above?\
Actually, adding the coil actually INCREASES the total output capacitance of
the anode circuitry (Ctube + Csocket +Chousing). However, it decreases the
impedance.
Since you are adding a series reactance to the circuit, the effect is
frequency dependent. So I can't give you a specific "magnitude" across the
band. That magnitude of change will also depend on the size of the
inductor.
If at 28 MHz we take an impedance of 5000 Ohms and 15 pF of parallel
capcitance, and add a 1.5 uH inductor, the resulting impedance as a result
of the transformation will be about 1200 Ohms with about 30 pF capacitance
in parallel. At 3.5 MHz, however, the change is minimal with a resulting
impedance of 4920 Ohms in parallel with 15.08 pF.
Series to parallel transformations are covered in nearly any first level RF
circuitry textbook.
>
> What is the purpose/function of the "step-down" transformer you used in your
> design?
>
I explained this in my post, Dave. Please re-read it. Yes, I like to hear
myself talk, but not that much. I don't need to repeat it again. It's also
been posted before by me and would be in the AMPS archives from just about 2
years ago.
> Would a step-down transformer enhance the efficiency of my Pi-network output
> which has an impedance of 2000 ohms (3000V@ 750mA)? If so, what value would
> you recommend?
Why would you need it? I've also never claimed that the coil enhanced
efficiency other than allowing a tank circuit to be built that has
realizable component values and a Q that is not ridiculously high. At 2000
Ohms, it is much simpler to build a pi-net that matches to 50 Ohms than from
5300 Ohms to 50.
73,
Jon
NA9D
-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)
Member: ARRL, AMSAT, DXCC, NRA
http://www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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