Transmitters & Receivers in Depth

Part II (iii)(iv) Communication systems · mic/squelch/AVC/tuning · simplex/duplex · advantages & limitations of RT

The number of frequencies a link uses, and whether both ends can talk at once, defines the mode. Aviation VHF deliberately uses the simplest and most disciplined arrangement so that everyone on a shared channel hears everyone else.

VHF aeronautical voice is simplex — a single frequency shared by all, with only one station transmitting at a time. ATIS is simplex broadcast (the ground transmits, aircraft only listen). This shared discipline is why listening-out and brevity matter so much.

1. Listen out first — make sure the frequency is clear.
2. Press the PTT and pause a beat so the first word isn't clipped.
3. Speak at a steady pace, normal volume, microphone close to the lips.
4. Release the PTT fully when finished so you can hear the reply.

Two aircraft press PTT at the same instant on the same simplex VHF frequency.

Because the channel carries AM (Chapter 10), the two carriers beat together and the controller hears a heterodyne squeal rather than either message. The controller says "two together, say again", and each aircraft re-transmits.

With FM's capture effect one would have been silently lost — which is exactly why aviation uses AM.

A microphone converts your voice to an electrical signal; noise-cancelling (differential) types reject the steady cockpit roar so only your speech is transmitted. A boom or oxygen-mask microphone keeps it positioned consistently. The headset reproduces received audio; ANR (Active Noise Reduction) headsets generate an anti-phase signal to cancel low-frequency cabin noise, reducing fatigue and improving intelligibility.

Hold the mic close and steady and speak across it, not into it, to avoid "popping". A consistent mic position is half of good readability — the other half is a steady, unhurried pace.

The audio control panel selects which transmitter the PTT keys (the transmit selector) and which receivers you listen to (the receive selectors — COM1, COM2, NAV idents, etc.). The intercom (interphone) lets the crew talk to each other — and to cabin/PA — without transmitting on the radio.

Selecting the wrong transmitter is a common cause of "transmitting on the wrong box".

Squelch — mutes the constant background hiss when no signal is present, opening the audio only when a real signal arrives. Set it just past the point where the hiss disappears.
AVC / AGC (Automatic Volume / Gain Control) — automatically holds the received audio at a steady level despite changing signal strength, so a distant station and a near one sound similar.
Volume — sets the listening level.
Tuning / frequency select — picks the operating channel, usually with a standby/active pair so the next frequency can be set ready and flip-flopped in.

The VHF COM antenna is the vertical blade — sized to a fraction of the wavelength and matched to the vertically-polarized VHF signal. HF uses a long wire or a probe (often in the fin) with an antenna coupler/tuner, because its long wavelength makes a full-size aerial impossible. A well-matched antenna transfers nearly all the transmitter power to the air; a mismatch reflects power back (high VSWR), wasting it and risking the transmitter.

At 120 MHz the wavelength is 2.5 m (Chapter 7). A common efficient length is a quarter-wave: ¼ × 2.5 m = 0.625 m — a practical blade.
An HF signal at 3 MHz has λ = 100 m; a quarter-wave would be 25 m — impossible on an airframe, so HF uses a long-wire/probe and a coupler to make a short aerial behave as if resonant.

In a steep turn the airframe can shadow an antenna mounted on the opposite side, briefly weakening the signal. It is one reason aircraft carry two COM antennas/radios and why a momentary drop in a hard turn is normal, not a fault.

A beacon that activates automatically on heavy impact (g-switch) or manually, transmitting on 406 MHz — detected by the Cospas-Sarsat satellite system, carrying a coded, registered identity so the aircraft is known — and on 121.5 MHz for short-range homing by rescuers. It is carried under Annex 6 (Chapter 3). Functional testing is restricted to defined windows to avoid false alerts. (Full detail in Chapter 22.)

ACARS (Aircraft Communications Addressing and Reporting System) is a digital datalink for short text messages between aircraft and ground — clearances, position reports, weather, maintenance and operational messages — reducing voice workload. CPDLC (Controller–Pilot Data Link Communications) carries ATC clearances as text, used especially in oceanic airspace (Chapter 18).

Hearing nothing / getting no replies? Work through: correct frequency selected (active, not standby)? · volume up and squelch not too tight? · correct transmit selector and receive selectors on the audio panel? · headset/PTT plugged in and the PTT not stuck? · circuit breaker / power in? · try the second COM. If still nothing, follow the communications-failure procedure (Chapter 20) and squawk 7600.