Electronic – How is volume not affected by signal strength in AM radio

For data transmission / reception, one of the less expensive options today is a pre-built module around the nRF24L01+ Transceiver IC. These modules typically offer a built-in PCB-trace antenna, 250 Kbps to 2 MBPS bandwidth before error correction, and are tried and tested.

Most important, they save you time in debugging and antenna tuning. After thousands of people have used these modules, which are built on the manufacturer's reference designs after all, most of the kinks are pretty thoroughly ironed out. Also, being able to tap the experience of many others on the internet who have used such a module, counts for a lot when trying to resolve issues.

For instance, this listing on eBay is for a mere US$2.10 with free international shipping. It uses the 2.4 GHz band, which does not need licensing for low power use in most countries.

Another alternative is this 433 MHz band transmit / receive pair of modules (just 9.6 Kbps though), in case you specifically want to stay with transmit-only and receive-only designs. US$1.99 for the pair makes it pretty attractive.

Of course, in each case, you could as well build your own module starting from the IC manufacturer's reference design, and thus learn while implementing your radio functionality.

It is unlikely that the price advantage of massive volume production can be beaten, though.

There are three advantages to driving the resonant L-C tank via tranformer coupling than directly from the antenna:

1. It changes the impedance. The impedance of the signal coming from the antenna is probably in the 50-300 Ω range. Crystal radios are usually listened to with high impedance headphones, usually around 2 kΩ. Having a 100 Ω source driving a 2 kΩ load is inefficient, which ultimately means lower volume.

2. The detector diode drop is a smaller fraction of the output voltage. The transformer steps up the voltage, making it higher than what comes out of the antenna directly. The forward drop of the dector diode is fixed, so it is less relative to the signal after that signal has been increased in voltage by the transformer. This means lower strength radio stations will still produce some output, and the output you get will have less distortion.

3. Better selectivity. The relatively low impedance of the antenna dampens the resonance of the L-C tank circuit. Another way to say this is that it lowers the Q of the resonant circuit. This means its resonant peak is more spread out, which means that the radio can't be tuned as well to a single station. The effect is that strong stations will appear to spill over into nearby frequencies.