Modify Si4703 FM Tuner evaluation board to add an external antenna

Question

I have this Si4703 evaluation board which I would like to modify to attach an external antenna. The board usually uses the plugged in headphone cable as the antenna.

Here are the relevant schematics:

One of the comments on the linked page claims this:

The antenna connection is Pin 2 of the SI4703. It is connected to the shield (ground) of the headphones via C5. This keeps the low frequency audio out of the chip. You would need to cut the trace between C5 and L1 and solder the antenna wire to the free end of C5. Keep C5 between the antenna and the chip.

Is this advice correct? Can I simply desolder L1 and solder the antenna to one of the free ends (the one that is not attached to GND) or should L1 be kept? Will any of this effect reception?

Here's the circuit with C5 and L1 marked:

^{_{P.S.: all images are from https://www.sparkfun.com/products/retired/10344, CC BY-NC-SA 3.0 (same as this page), modified to point out items relevant to the question.}}

Answer 1

Just came across another description of that same mod: http://www.electronic.lt/failai/news/2016/si4703_arduino/Si470x-Eval-v11_ant_mod.pdf

Electrically he did the same mod you describe. Not sure how he did it physically. You can:

• cut the trace on the board
• desolder C5, turn it by 180 degrees and solder the (formerly) right contact back to the right contact on the board (so the other is “dangling” freely)
• desolder just the left contact of C5 and insulate it against the board

Then solder the antenna cable to the free (left) connection of C5, and solder the shielding to one of the ground connection on the board (e.g. the GND pin).

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EDIT: Just found the manufacturer’s recommendations for adding various types of antennas: https://www.silabs.com/documents/public/application-notes/AN383.pdf

You might want to take a look at these. Unfortunately, antenna jacks are not covered, but I’d follow the procedures for a whip antenna.

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If you want to improve reception, I highly recommend placing the whole board in a shielded and grounded enclosure. In my case (of a Si470x-based USB dongle, which I ended up wrapping in aluminium foil), that improved things by roughly the same level as going from a pigtail antenna to a proper cable.

Answer 2

Instead, leave all the parts in place and disconnect the ground wire from the earphone plug and use that for your ant. Connect the wire you would would have used for the audio out common to the ground plane on the board.

Answer 3

Instead, get the PL102RT-S V1.5, which breaks out the ANT pin:

https://nl.aliexpress.com/item/Free-Shipping-SI4703-V1-5-FM-RDS-FM-radio-module/32219366138.html

or

https://nl.aliexpress.com/item/Free-Shipping-SI4703-V1-5-FM-RDS-FM-radio-module/32649196701.html

Or get the SparkFun basic breakout board (not on AliExpress):

https://www.sparkfun.com/products/11083

In all cases, you will need to add your own amplifier, which is not necessarily a bad thing ;-).

I tried the mod but stripped the pad in the process, so I'm going down this route now. I've read reports that it should work fine with RdSpi.

Answer 4

I made a short cable consisting of a 3.5mm jack and a 3.5 mm plug. Just connected the audio wires from jack to plug and inserted a coil in series in the ground wire. The jack is the part that is plugged into the SI4503 module. On that site a antenna wire can be soldered. The coil wil decouple (block) high frequencies like FM signals but behaves as a connection for audio signals. The 10uH coil was one I had just laying around. Didn't do the math on cut-off frequency but it works!