> The current circulating in a pi-net tank coil divides between
> the load capacitor and the coax because they're effectively
> in parallel.
That isn't correct. They are in series, but have different resistances
loads across each one.
A better way to look at this is two L networks back-to-back.
> an 875 pf load C for a Q of 13 when transforming from 5000
> to 50 ohms. 875 pf has an impedance of 51 ohms.
Those values are off somewhat. Let's assume the Q of the
capacitors is 5000, and the Q of the inductor is 250. Power is 1500
watts.
For an effective Q of 13, the reactance of the tuning capacitor is
407.6 ohms. The Q of that end of the tank alone is 12.3. The
current is 6.3 amperes. Loss in the tuning capacitor is 3.5 watts
The loading capacitor has a reactance of 69.7 ohms, Q at that end
of the tank is .7, and loading capacitor current is 4 amperes. That
capacitor dissipates .2watts.
The inductor has 428.6 ohms reactance and handles 6.6 amperes,
loss is 73.9 watts in the inductor.
Phase shift is about -122 degrees.
> between the output coax and the load capacitor. At 1500W,
> the coax will take 5.5A RMS and the load C will take the
> same amount. So, the circulating current will be about 11 Amps.
That's close but not exactly true. Every component has a different
current. Circulating current in the inductor is the highest at 6.6
amperes.
> As a rule of thumb for a Pi-net at HF, it looks like the
> circulating current runs about twice the coax current.
> Assuming you design for generally recommended Q's.
> When designing for higher frequencies, Q's tend to run
> much higher.
That rule of thumb does not work.
73, Tom W8JI
W8JI@contesting.com
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|