FM / MW / SW Radio as Tri-band Transceiver ( full duplex transmission )

Building a Tri-Band Transceiver (FM / MW / SW) with Full-Duplex Communication

This project explores a fascinating radio frequency phenomenon: using standard AM/FM/SW radios (either digital or analog) for full-duplex transceiver communication.

How It Works (The Local Oscillator Principle)

If you place two radios close to each other and tune them to the exact same frequency band, you will notice that both radios suddenly go completely silent at a specific tuning point.

This happens because the local oscillator (LO) inside each superheterodyne receiver emits a minute signal. In a standard superheterodyne receiver, the local oscillator mixes with incoming station frequencies to produce the intermediate frequency (IF):

• Standard FM IF Stage: 10.7 MHz

• Standard AM/MW IF Stage: 455 kHz (or 450 kHz)

Example (FM Band): If you want to receive a station at 100 MHz, the local oscillator runs at 110.7 MHz:

$$\text{110.7 MHz (LO)} - \text{100 MHz (Station)} = \text{10.7 MHz (IF)}$$

By amplifying this local oscillator frequency and adding stability, you can effectively turn standard receivers—especially car radios—into a functional transceiver setup.

Medium Wave (MW) Implementation & Mixing Math

For this specific project, we focus on the Medium Wave (MW) band using a target IF of 450 kHz. Across the entire MW band (from 522 kHz to 1650 kHz), the local oscillator frequency shifts accordingly to maintain the fixed difference:

1. Calculation 1:

   $$\text{1863 kHz (LO)} - \text{1413 kHz (Station)} = \text{450 kHz}$$

   (1413 kHz + 450 kHz = 1863 kHz)

2. Calculation 2:

   $$\text{1485 kHz (LO)} - \text{1008 kHz (Station)} = \text{450 kHz}$$

Achieving Long-Range Transmission

The entire Medium Wave band from 522 kHz to 1650 kHz maps directly to its corresponding local oscillator frequencies. To achieve long-range communication, only two RF amplifier stages are added to the local oscillator output to boost the signal strength effectively.