I am a beginner in electronics and I'm currently the most interested in RF-related topics. I did build simple AM receiver usind good old TA7642 and, after adding small 9V amp, the radio broadcast can be heard across entire house. It works well.
But now I want to modify the circuit so it can use different antennas and coils, so I can change receiver frequency range, RF signal strength etc.
This is the antenna input cirucit part. That's basically default TA7642 setting:
Now tests with different coils and antennas:
Ferrite rod with 2 coils separated magnetically - WORKS.
Ferrite rod with 1 coil, but with additional output in the 1/3 coils length - WORKS. It's interestig that RF signal is taken directly from the coil, without separating it magnetically.
Just a coil (not ferrite), capacitor and external antenna - NOT WORKING. Even though that's the most basic design actually. 3rd case is definetly wrong, I don't receive anything desired but very distorted... FM stations. Capacitor rotating doesn't change anything, adding or removing external antenna doesn't help, too.
Why doesn't it work? How to fix this?
I have a few air coils without ferrite or three outputs. I also have external antennas: one for SW on the balcony, another one (long-wire) for LW. But I cannot use them as the only 1 & 2 designs do work.
Can you help me, please? Thanks!
BTW: If I attach external SW or long-wire antenna to circuits 1 or 2, I can receive tons of shortwave radio stations, but rotating capacitor doesn't allow me to tune in to any station. I understad it as follows: the external antenna gives so strong signal that it overrides what ferrite rod gets. Because of high capacitance of variable capacitor (20 - 500 pF) and/or high inductance of ferrite coil (probably 400 - 500 uH) such radio is totally not selective, so I cannot tune in to anything. Is this explanation true?