[Milsurplus] BC-230 questions and observation

[Kd6b@aol.com]

In a message dated 3/9/2003 3:17:23 PM EST, kargo_cult@msn.com writes:

<< Thanks Ken for your input on the BC-230 output circuit. 
 I guess i still do not totally understand the action of the loading
 when dealing with reactive load. If the load is purely resistive,
 like a 1/4 or 1/2 wave antenna, i can see that the coil tap
 acts simply as an impedance transformer from the low tens
 ohms, near the bottom of the coil, or in the few thousands ohms,
 nearer the top of the coil. 
 However, here's where i have a problem. The short aircraft 
 antenna ( or car whip, or any short antenna ) is modeled as
 a low resistance, 10 ohms or less, in series with a capacity of
 something like 50-100 pF. The "phantom antenna" boxes all
 have some circuit like this in them. So on a BC-230 type 
 circuit, where approximately does the tap go to load such
 an antenna? >>>>>
Your observations are quite correct. However, there are two ways of 
representing an equivalent circuit for an antenna, a series circuit, as you 
pointed out, or as a parallel equivalent circuit.
Assume a series R=10 Ohms and C=50 pF. At 3 MHz the cap reactance is about 
1060 Ohms so the "capacitor" Q is about 100. The parallel equivalent circuit 
in this case is a cap of still about 50 pF but the parallel resistance is the 
series resistance times Qsquared or in this case about 100,000 Ohms. That 
would put the tap high on the coil. I suspect we need a little better numbers 
for the antenna series resistance but you get the point.
One of my 230 coils has a turns ratio of at least 3:1 which would step up the 
plate load impedance by a factor of 9, assuming tight coupling. The plate 
load is approx 350V/50ma or 7K. Therefore the top of tank coil would be about 
60K  Ohms,..... so the numbers are in the right range.
I hope to get this all together for ER but there is work work work and not 
much time for ham radio.
73
Ken