Since the very helpful reply I received from Jean-Matthieu about the Le Tenor radio I have been searching for information about the tuned circuit.
I have read that this type of circuit is a double tuned circuit and so have been reading a lot about this. I think I see how they work so am just going to outline my ideas below in case I am missing something:
Initial data:
input impedance (from antenna): 50 ohms
input impedance to NE602 (1500 ohms)
required tuned frequency: 7MHz
toroid used: T50-2
calculations:
start by picking a capacitor: 280pF (chosen as I have seen this value used in several circuits)
inductor needed to resonate a parallel LC circuit at 7MHz: 1.8uH
turns needed on a T50-2 for 1.8uH: 19
calculate primary turns required to match a 50 ohm antenna to this tuned circuit:
I have 50 ohms on the primary, 17 turns on the secondary and need 1500 ohms later on so try these values first so get: 3 turns on primary
Using the values from above for the second transformer I can now say I require 1500 ohms on the primary of the second toroid and need 1500 ohms for the NE602 so I can just do a 1:1 ratio for the turns on the second toroid, so 17 turns each side.
As far as I have seen (at an initial basic level) this seems to be a good start.
(using Jean-Matthieu's windings of 3 on the primary and 25 on the secondary on a T50-2 for both toroids with 50 ohms primary impedance, I calculate a secondary impedance of 3472 ohms. As I need this matched to 1500 on the NE602 I then calculate I need 16 turns on the secondary of the second toroid to make this match when saying I have 3472 ohms on the primary side)
I do have one more question that I have been searching for the answer to - when impedance matching from say 50 ohms into the tuned circuit is it better to go for a higher value for the reflected impedance part, above I have chosen to use 1500 ohms but Jean-Matthieu calculations suggest 3472 ohms?
