ECC86 radio, antenna feeding

[neazoi]

Hello,
I consider this little radio attached. I would like to know why the antenna is fed at the center of the coil instead of the top?

What are the pros and cons of this?

 

[E Kafeman]

Higher Q -> Better selectivity

 

[neazoi]

Higher Q -> Better selectivity

So the LC is not loaded by the triode too much and also it matches better the impedance of the antenna to the receiver?

 

[E Kafeman]

Yes, LC is the only part that select which station that is received. It would not be stable or selective if antenna, with frequency depending undefined and unstable impedance load would be a dominant part of the tuning circuit.
Triode load is low and almost constant.
It is possible that antenna is better matched but it depends on how antenna is designed.

 

[chuckey]

An old fashioned "dummy aerial" is basically 400 ohms in series with the signal generator output, so long wire aerials are roughly this impedance. Once you have this set completed, I would be tempted to wind 5 or 10 turns of wire on the earthy end of your coil, earth one end of this and connect the aerial to the other. This will raise the Q of the tuned circuit and it might work better.
Frank