External Radio — VHF, HF and SELCAL

Articles 2 and 3 covered choosing the frequency (RMP) and handling transmit/receive (ACP). This article covers the external radio hardware those panels control: the near-range VHF and the long-range HF. Article 1 fixed their physical difference — VHF runs line-of-sight for the near field, HF bounces the ionosphere for range. Here we develop each one's mechanism, band, antenna and the operating restrictions the crew must know, then close with a function spanning both radios: SELCAL (selective calling) — letting the crew stop continuous listening over the ocean and be "paged" by a ground station.

1. Architecture

   VHF (near, line-of-sight, 118–136.975 MHz)        HF (long, skywave, 2.8–24 MHz)
 ┌────────┐  ┌────────┐  ┌────────┐                ┌────────┐  ┌────────┐
 │ VHF 1  │  │ VHF 2  │  │ VHF 3  │                │ HF 1   │  │ HF 2   │
 │ xcvr   │  │ xcvr   │  │ xcvr   │                │ xcvr   │  │ xcvr   │
 └───┬────┘  └───┬────┘  └───┬────┘                └───┬────┘  └───┬────┘
  blade ant.  blade ant.  blade ant.               coupler     coupler
  (fuselage)  (fuselage)  (fuselage)                 ↘        ↙  (fin root, auto-tune)
     ↑ESS         ↑BUS2      ↑BUS1                  shared structural antenna
   only VHF 1                └ normally DATA          (vertical stabiliser)
   in EMER ELEC                (VDL Mode A / Mode 2)      ↑ only HF 1 in EMER ELEC
                                 → ATSU/ACARS (ATA-46)

VHF: each system has its own blade antenna on the fuselage. HF: the two transceivers have their own couplers but share one structural antenna in the fin, interlocked so only one HF transmits at a time. On emergency power only VHF 1 (essential busbar) and HF 1 remain.

2. VHF — three independent systems, near-range line-of-sight

The FCOM gives the VHF inventory. Per FCOM DSC-23-30-10:

The aircraft has three identical VHF communication systems. Each system has a transceiver in the avionics compartment, and an antenna on the fuselage. Only VHF1 operates in EMER ELEC CONFIG. The VHF1 range is from 118.0 to 136.975 MHz.

Three fully independent systems, 8.33 kHz spacing. VHF 1's supply sits on the essential network. Per AMM 23-12-00:

The VHF1 system is supplied with 28VDC: From the 28VDC ESS BUS 401PP... In case of emergency the VHF1 system operates through the DC emergency generation.

VHF 2 hangs on BUS 2 and VHF 3 on BUS 1, neither on the essential network — hence only VHF 1 on emergency power. With article 2 (only RMP 1) and article 3 (captain/F-O ACP on the essential busbar), this composes one sentence: emergency communications = RMP 1 + VHF 1 + HF 1 + the number-1 ACP.

3. VHF 3 — normally the datalink, not a spare voice radio

Of the three, VHF 3 has a day job carrying data. Per AMM 23-12-00:

The VHF system has data link function which is defined by ARINC 750-3: Mode A [...] Mode 2. [...] This VDL mode 2 function is used to reduce the channel saturation. With this function, the rate throughput is increased ten times through a D8-PSK modulation (31.5 kbit/s).

As the VHF Data Radio (VDR), VHF 3 normally carries the ACARS/ATSU data (ATA-46). Hence a discipline the FCOM states in the how-to (developed in article 11). Per FCOM DSC-23-50:

Do not use VHF 3 for voice communications, unless VHF 1 and VHF 2 are inoperative. The datalink connection with the ATC is temporarily lost when VHF 3 is set to voice mode. The flight crew may encounter sudden disconnections from the ATC if they switch VHF 3 to voice mode too frequently.

[!warning]- VHF 3 is not a "spare voice radio" — it is the datalink radio Crews often grab VHF 3 as a third voice set. Wrong — it is normally running CPDLC/ACARS data; switch it to voice and the ATC datalink drops. Touch it only when VHF 1 and VHF 2 are both inoperative. This is the most operational face of the voice-or-data dichotomy from article 1.

4. Stuck-microphone protection

A microphone stuck in the transmit position occupies a frequency for a long time and blankets the whole channel; the system protects automatically. Per FCOM DSC-23-30-10:

The VHF has an alarm to indicate that the microphone is stuck. If a microphone is in the emission position for more than 30 s, an interrupted tone sounds for 5 s, and the emission is turned off. To reactivate the emission, the crew releases the push–to–talk button and presses it again.

The AMM pins the timeline finer: at 30 s of PTT the SDAC gives a "beep-beep" each second for 5 s and the ECAM shows COM VHF x EMITTING; at 35 s the transmission stops automatically; release the PTT and press again to resume. The HF counterpart is "transmit for more than 1 min → ECAM HF-X EMITTING" (§6).

5. HF — two systems, long-range skywave, antenna in the fin

HF reaches beyond line-of-sight off the ionosphere — the workhorse (and backup) over ocean and polar routes. Per FCOM DSC-23-30-10:

The aircraft is equipped with High Frequency (HF) communication system: HF1, HF2. Each HF is composed of a transceiver installed in the avionics compartment, and a common tuner and antenna in the vertical stabilizer. The emission range is from 2.8 to 24.0 MHz.

The physical layout (AMM 23-11-00 §2): the transceivers are in the avionics bay; the antenna coupler is at the root of the vertical stabiliser; the antenna itself is a structural shunt-type antenna in the lower leading edge of the fin — the metal fin structure is the HF antenna. Voice runs 2.8–23.999 MHz (1 kHz spacing); data 2–29.999 MHz (100 Hz); peak transmit power ~400 W. The two HF systems have their own couplers but share this fin antenna, interlocked so only one HF transmits at a time.

6. The HF antenna coupler — a new frequency needs auto-tuning

HF wavelengths are long (2.8 MHz ≈ 107 m). To make a physically short (relative to wavelength) antenna radiate efficiently across such a wide band, the coupler must match the antenna impedance to the transceiver's 50 Ω in real time. The AMM spells out the auto-tune. Per AMM 23-11-00:

The coupler is tuned in six sequences: start, reception/standby, tune A, tune B, tune C and operational position. [...] The tuning phase is initiated at HF system energization or when a new frequency is selected.

Of the six, tune A brings antenna current and voltage into phase, tune B matches the load to 50 Ω, and tune C drives the standing-wave ratio to target. Per AMM 23-11-00:

The purpose of the tune C is to complete previous adjustments and obtain a VSWR (voltage standing-wave ratio) lower than 1.3. [...] When a VSWR lower than 1.3 is obtained, the sequence counter controls start of the next sequence, i.e. operational position.

[!warning]- Why an HF frequency change takes a moment VSWR (standing-wave ratio) measures the antenna match; 1.0 is perfect, all energy radiated. The coupler must drive it below 1.3 to call it tuned. Every new HF frequency restarts the six-sequence auto-tune (servo motors driving variable capacitors/inductors), taking a second or two — the reason an HF frequency change "lags" and a tuning sound is sometimes audible. If tuning over-runs, or antenna reactance is excessive, or the coupler fails, the transceiver's KEY INTERLOCK red light comes on and transmission is inhibited.

7. HF ground and refuelling restrictions, and the datalink

High power plus a long antenna bring operating restrictions the crew must know. Per FCOM DSC-23-30-10:

When in data mode, digital information is transmitted between HF1 and ATSU. HF must not be used during refueling. The HF DATA transmission is inhibited on ground. A GND HF DATALINK pb, located on the overhead panel, may override the inhibition. The HF VOICE MODE is not inhibited on ground. The use of HF on ground must be limited to operational needs. It is important to check that nobody stays in the direct vicinity of the antennas if the HF is use on ground.

The reasons: no HF during refuelling — high-power RF near fuelling operations risks arcing/ignition; nobody near the antennas — the fin HF antenna radiates an RF-burn hazard when transmitting; ground data inhibited — the AMM adds that when the LGCIU passes "ground" status to the HF Data Radio the datalink emission is inhibited (the GND HF DATALINK pb overrides for ground testing). One emergency interaction. Per FCOM DSC-23-30-10:

Note: Only HF1 is supplied in EMER ELEC CONFIG. HF is lost when LAND RECOVERY is selected ON.

(LAND RECOVERY is an emergency landing electrical configuration that sheds load further, dropping HF — cross-reference ATA-24.) HF transmit lasting more than 1 minute raises HF-X EMITTING on the ECAM (via the SDAC KEY EVENT, AMM §3).

8. SELCAL — letting a ground station page you

Over the ocean HF is noisy and channels are congested; the crew cannot guard a headset continuously. SELCAL (selective calling) lets a ground station page the aircraft by its unique code. Per FCOM DSC-23-30-10:

Upon receiving a call code corresponding to that of the aircraft, the SELCAL system aurally and visually advises the flight crew that a ground station is calling the aircraft. The aural signal is inhibited during takeoff and landing.

Each aircraft has a unique SELCAL code. A ground station transmits it; a match triggers a cockpit buzzer plus the amber SELCAL legend flashing under the associated VHF/HF transmit key (article 3), and the crew answer. The code is set on the panel in the avionics bay. Per AMM 23-51-00:

Four thumbwheels for the selection of a code made up of four letters from amongst the following: A - B - C - D - E - F - G - H - J - K - L - M - P - Q - R - S.

[!note]- Why the SELCAL code uses just those 16 letters Four positions, each from 16 letters — deliberately skipping I, N, O (and T through Z) to avoid confusion with numbers or similar-sounding letters. Why is the aural signal inhibited on take-off and landing? Because those are the busiest, least-interruptible phases — a SELCAL buzzer could distract. It is the same "critical-phase silence" design as the cabin-call buzzer inhibit (article 7) and the HF/VHF data ground inhibit.

9. Interference avoidance

The how-to gives concrete interference notes (also in article 11). Per FCOM DSC-23-50:

If two frequencies are closer than 2 MHz (between VHF 1 and 2, or between VHF 3 and 2), or closer than 6 MHz (between VHF 1 and 3), some interference may occur. Reception of 130.275, 130.280 and 130.285 may be noisy (squelch open) on VHF 1 and/or VHF 3. In such cases, use VHF 2.

The FCOM VHF description adds an entertainment-system note: some in-flight-entertainment installations can produce audible background noise on 121.500, 135.000 and 135.005 — and 121.500 is the emergency frequency, so a hiss while guarding it may be equipment noise.

Self-test

[!note]- Q1. Which busbar feeds each VHF, why does only VHF 1 survive on emergency power, and what does VHF 3 normally do? VHF 1 = essential (401PP), VHF 2 = BUS 2, VHF 3 = BUS 1; only VHF 1 is on the essential network. VHF 3 normally carries ACARS/ATSU data.

[!note]- Q2. When may VHF 3 be used for voice, and what is the consequence? Only when VHF 1 and VHF 2 are both inoperative; the ATC datalink connection is temporarily lost.

[!note]- Q3. What happens at 30 s and 35 s of a stuck VHF microphone, and how do you resume? 30 s: interrupted tone/beep + COM VHF x EMITTING. 35 s: transmission stops automatically. Release the PTT and press again.

[!note]- Q4. Where is the HF antenna, why does a new HF frequency take a moment, and what does VSWR < 1.3 mean? A structural antenna in the fin. Each new frequency restarts the coupler's six-sequence auto-tune; VSWR < 1.3 is the match target that ends the tune.

[!note]- Q5. Give the three HF ground/refuelling restrictions and their reasons. Why does LAND RECOVERY lose HF? No HF while refuelling (arc/ignition risk); nobody near the antenna (RF-burn); data inhibited on ground. LAND RECOVERY sheds load and drops HF.

[!note]- Q6. How does SELCAL work, why is the aural signal inhibited on take-off/landing, and how many letters are in the code? A ground station pages by the aircraft's unique four-letter code (from 16 letters); a match gives a buzzer + amber SELCAL. Inhibited on take-off/landing as a critical-phase-silence design.

Key takeaways

Point	Detail
VHF	3 independent, 118–136.975 MHz, 8.33 kHz; only VHF 1 (essential) on EMER ELEC
VHF 3	Normally the datalink (VDL Mode A/2); do not use for voice unless VHF 1 & 2 inop
Stuck mic	>30 s → tone + COM VHF EMITTING; 35 s auto-stop; release + press to resume
HF	2 systems, antenna in the fin, 2.8–24 MHz; only HF 1 on EMER ELEC; lost with LAND RECOVERY
HF coupler	Six-sequence auto-tune per new frequency to VSWR < 1.3; KEY INTERLOCK on tune fault
HF restrictions	No HF while refuelling; keep clear of antenna; data inhibited on ground
SELCAL	Ground station pages by unique 4-letter code (16 letters); aural inhibited on take-off/landing

References

FCOM DSC-23-30-10 — VHF/HF inventory, EMER ELEC, stuck-mic, HF antenna/datalink/refuelling restrictions, LAND RECOVERY, SELCAL.
FCOM DSC-23-50 — VHF 3 datalink discipline; interference notes.
AMM 23-12-00 — three independent VHF, VDL Mode A/2, VHF 1 = essential busbar, 35 s auto-stop.
AMM 23-11-00 — HF band, fin antenna/coupler, HF-X EMITTING; §6.D six-sequence coupler auto-tune, VSWR < 1.3, KEY INTERLOCK.
AMM 23-51-00 — SELCAL code panel (4 positions, 16 letters).

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.