Small integrated loop antenna for FM radio?
I am designing a small PCB (3in x 4in) with lots of digital ICs in the middle. I need to incorporate a small loop antenna for an FM radio around the edge of the board (I actually need one for AM as well).
I have done some reading and feel that I understand the basic facts and what needs to be achieved. However, the calculated inductance of my proposed 2in x 3in, rectangular single-winding loop (embedded along the edge of the PCB) is incredibly small (many orders of magnitude less than a μH). This would therefore mean that the tuning capacitor value would be comparably small.
Is this correct, or am I missing something? If this is true, then the antenna would have a very low Q, which is not supposed to be the case for 'small' antennas.
How will the presence of the GND/PWR planes in the middle of the PCB (and therefore in the middle of the loop antenna) affect the antenna gain/efficiency?
What am I doing wrong here? Am I on the right track?
Any help is appreciated. Thank you!
-Steve
My books tell ′me, that reducing the inductance rweuires to increase the capacitance of a resonant circuit, if you want to
keep the resonance frequency.
I don't think, that magnetic loop is a promising principle to design a FM receiver antenna. Actually, a receiver antenna don't
need to be tuned and impedance matched in any case. Most small receivers don't.
Hello,
A small inductance means that you need a relative large capacitor, as fres = 1/(2*pi*sqrt(LC).
A ground plane within the loop reduces the inductance and forces the magnetic field lines to flow around the loop instead of through it. This reduces the efficiency significantly.
As you are interested in receive only, you do not need a good matched antenna. As long as the noise contribution from the antenna exceeds that of the receiver circuitry, you are OK.
I have bad feelings regarding "lots of digital ICs in the middle", both for the FM as well as the AM case. Maybe you should consider using the headphone leads as antenna (if present).