Transmitter and receiver

The superheterodyne receiver. The reigning architecture. The idea: instead of filtering/amplifying directly at the received frequency (hard, and it varies), first translate the signal to a fixed intermediate frequency (IF), where good filters and amplifiers can be built.

Typical chain:

Antenna → input filter (selects the band, see Circuits résonants et filtres).
Mixer: multiplies the signal by a tunable local oscillator (LO) → produces the IF. Tuning the set = changing the LO.
IF filter: provides selectivity (keep only the wanted channel).
Amplification + demodulation (Modulations : graver l'information sur une onde) → audio or data.

This is exactly the principle your SDR applies, except the last part is done in software.

Mixing traps. The mixer also creates an image frequency (on the other side of the LO) that must be rejected by filtering, otherwise two different stations overlap.

The transmitter runs the path in reverse: a stable oscillator generates the carrier, a stage modulates it with the information, filters remove harmonics and spurious emissions (a regulatory obligation — see Réglementation du service amateur), then a power amplifier drives the antenna through the feed line.