Aviation radio equipment and propagation: VHF, HF, ELT and transponder

The ROC-A exam expects you to understand the radio in front of you: what it can reach, what limits it, and the emergency equipment that works alongside it.

VHF and line of sight

Aeronautical voice uses 118 to 137 MHz, part of the VHF band (30 to 300 MHz), with amplitude modulation (AM). VHF signals travel in nearly straight lines. Terrain and the curve of the Earth block them, so range grows with altitude. An aircraft at 5,000 ft can reach a station far beyond the reach of one at 500 ft, and a drone pilot standing on the ground may hear aircraft that cannot hear them.

HF for long range

HF (3 to 30 MHz) is used for long-range communication over oceans and in the far North. HF signals can be reflected by the ionosphere as a sky wave. Lower frequencies also travel as a ground wave that follows the Earth's surface, and they pick up more static from distant thunderstorms than VHF does.

Using the radio

• Squelch mutes background noise when no signal is being received. Set too high, it cuts out weak stations.
• Aviation VHF is simplex: one frequency, one station at a time. You cannot hear others while you transmit, and two stations keying together block each other.
• A stuck push-to-talk switch blocks the frequency for everyone. If you hear one, check that it is not yours.
• Handheld radios have lower power and smaller antennas than panel radios, so expect shorter range. A low battery weakens and distorts transmissions.
• A damaged antenna or loose connector greatly reduces range; check them during the pre-flight inspection.
• e.r.p. (effective radiated power) is the power actually radiated, including antenna gain and losses. The ROC-A covers equipment up to 250 W e.r.p.

ELT

An emergency locator transmitter sends a homing signal on 121.5 MHz. Modern ELTs also transmit on 406 MHz, which the Cospas-Sarsat satellites detect along with a coded identity; satellites stopped processing 121.5 MHz in 2009. Test an ELT only in the first five minutes of a UTC hour, for no more than five seconds. If it is set off by accident, switch it off and report it to the nearest ATS unit.

Transponder

A transponder replies to radar with a four-digit code; Mode C adds pressure altitude. Press IDENT only when ATC asks. Special codes:

• 7700: emergency
• 7600: radio communication failure
• 7500: unlawful interference

With a failed receiver, squawk 7600 and transmit blind, saying "TRANSMITTING BLIND" and sending each message twice.