J
John Woodgate
- Jan 1, 1970
- 0
I read in sci.electronics.design that [email protected] wrote (in
20 turns does look too many for an 80-turn primary. If you have a
standard 93 ohm input impedance in the radio, the load on the primary is
93 x (80/20)^2 = 1488 ohms. If your 365 pF cap tunes the primary to 540
kHz (I don't remember the lower bound of the AM band in US, but it's
around that frequency), it has an impedance of:
1/(2pi x 540.10^3 x 365. 10^-12) = 807 ohms,
so your circuit Q is only 1488/807 = 1.84.
Try 5 turns, would round the ground end of your coil. You have grounded
the frame of the variable cap to the case of the radio, haven't you?
I might try adding or subtracting a few turns from the secondary
winding to see if I can improve it further.
20 turns does look too many for an 80-turn primary. If you have a
standard 93 ohm input impedance in the radio, the load on the primary is
93 x (80/20)^2 = 1488 ohms. If your 365 pF cap tunes the primary to 540
kHz (I don't remember the lower bound of the AM band in US, but it's
around that frequency), it has an impedance of:
1/(2pi x 540.10^3 x 365. 10^-12) = 807 ohms,
so your circuit Q is only 1488/807 = 1.84.
Try 5 turns, would round the ground end of your coil. You have grounded
the frame of the variable cap to the case of the radio, haven't you?