Separating an FM antenna from a topology which uses headphone ground as antenna

Question

I have a FM radio module with Silicon Labs Si4703 integrated tuner module for Arduino. The module assumes use of headphones and use of ground wire as an antenna (advice is wire length of 1.1-1.45m). In my use case that is unpractical, because the output will be coupled with an amplifier nearby and all components will be inside an aluminum box.

I would like to decouple the antenna from the ground. In my research I found a Silicon Labs AN231 document which explains the function of the device, but nowhere could be found the information about decoupling the antenna.

This diagram on page eight seems to be very similar to the module layout:

As I am not experienced with RF, my question is, is there a simple hack that would allow me to connect an independent antenna? For example, by cutting the line from the headphones ground S1 to FMIP, connect the S1 directly to the ground and connect a dedicated antenna to the input of the LC circuit made of Lmatch and C4. Will I get a reception and do I have a need to change the value of Lmatch to get it to work?

Answer 1

The inductor is only there to provide a low (audio) frequency path to ground for the headphones while presenting a high-impedance to high-frequency RF. The antenna input is essentially TP1. You don't need Lmatch if you are using a separate antenna connection.

I would connect S1 directly to ground (for the return leg of the audio output). And connect the antenna directly to TP1. I would move those D3 diodes over to TP1 to protect the antenna input from static damage to the chip.

Answer 2

Everything you need to know is in the application note on Antennas: http://www.silabs.com/support%20documents/technicaldocs/an383.pdf