zoomx yes I got that digital tone sketch working before starting this project. That thread is really interesting!
Mike I got this working somewhat by changing antenna to digital pin 2 (from PWM pin 9), and removing 5V-connected 100K resistor (no detected signal when this was connected), while keeping the 100K-to-GND resistor (for some reason keeping this helped signal). Suddenly radio received my audio input, so success. I experimented with capacitor values & keeping 47uF seemed best.
The "reception" (a mix of screech and music from input) was effective within 12" (1 foot) radius in proximity to Uno board, and from an AM loop antenna wired up across the room, with no reception in-between the Uno & the antenna. This short-range noise was being received all over AM kHz spectrum (whereas the post zoomx linked previously is very tight on a single frequency). My experiment was not being transmitted / received at all on FM—I checked.
Any suggestions to clean up signal & focus it on desired frequency would be appreciated!
I previously linked to FCC rules on Unlicensed Operation of Low Power Radio for the benefit of those of us in the US; I'm posting Part 15 Devices here, so low-power hobbyists in the US are aware of the law:
Part 15 Devices
Unlicensed operation on the AM and FM radio broadcast bands is permitted for some extremely low powered devices covered under Part 15 of the FCC's rules. On FM frequencies, these devices are limited to an effective service range of approximately 200 feet (61 meters). See 47 CFR (Code of Federal Regulations) Section 15.239, and the July 24, 1991 Public Notice Opens a New Window. (still in effect). On the AM broadcast band, these devices are limited to an effective service range of approximately 200 feet (61 meters). See 47 CFR Sections 15.207, 15.209, 15.219, and 15.221. These devices must accept any interference caused by any other operation, which may further limit the effective service range.