From a signal standpoint, checkout frequency hopping techniques for jam resistant communications. Specifically, Single Channel Ground and Airborne Radio System (SINCGARS) for VHF nets. Automatic Link Establishment (ALE) provides similar advantages for HF as a byproduct, but it's solving quite a different problem.
and the purpose was to establish merely a financial/technical barrier to entry for potential attackers at a time realtime grabbing/processing more than one megahertz of bw was $$K expensive.
Today $130 gets you 30MHz full duplex radio, $300 60MHz one.
