L = (a2N2) ÷ (9a + 10b)
Where N = Total Turns
a = radius length in inches
b = winding length in inches
Turns may be space wound or close wound.
The INDUCTANCE of coils with air cores is:
Q = 2πfL ÷ R
Where f is frequency in Hertz and R is the Resistance of the coil.
The Q of a coil is dependent on a number of relative factors such as physical size, wire diameter, form factor, etc.
To obtain highest Q:
The coefficient of coupling (K) between two coils:
K = M ÷ √L1, L2
Where M is mutual inductance L1 is inductance of first coil, L2 is inductance of second coil.
M, L1, and L2 can be in any units as long as they are the same in henries, micro-henries, etc.
Under conditions of resonance, the load, antenna, dummy, etc. couple resistance into the tank circuit in the amount:
(2πfM)2 ÷ R
where f is frequency in Hertz and R is the Resistance of the LOAD. Hence the LOADED Q of your tank circuit can be determined by knowing M if you know K.
If the tank coil is overheating it may not need larger diameter wire- excessive circulating current in the tank may be the cause. If this is the case your final is POORLY designed and only a SMALL PORTION of your power is ending up at the antenna. Now, we could suggest using a very large conductor, but this would not SOLVE the problem. This large circulating current is passing through your switches and capacitor which is not good and eventual failure is probable.
Here are some points to keep in mind:
The EFFECTIVE Q is the ratio of the coil reactance (ωL) to the SUM of the resistance inherent in the coil and resistance coupled into the tank circuit by the load.
Capacitor spacing based on representative voltages of input capacitor of a pi network or tank capacitor in a parallel tuned tank.
|BAND MHz||POWER INPUT||WIRE SIZE||CAPACITOR SPACING|
|14.0||1000 W||8 or 1/4" tube||0.25"|
|28.0||1000 W||8 or 1/4" tube||0.25"|
|28.0||500 W||8 or 1/4" tube||0.05"|
The output capacitor of pi network need not be more than 0.015" for a 50 ohms load at 1000 W input. For lower powers, receiving capacitors may be used in most cases.