18.3 Aircraft Antennas, Frequencies & Equipment Functions

18.3 Aircraft Antennas, Frequencies & Equipment Functions

Quick Answer: VHF COM ≈ 118.000–136.975 MHz (pool: worldwide up to ~151.975 MHz). ELT: 121.5, 243, and 406 MHz. Radio altimeter ≈ 4250–4350 MHz. VOR/LOC antenna = horizontally polarized omnidirectional receive; marker beacon = balanced loop receive; Mode C = L-band monopole blade omni. 400 Hz aircraft power → magnetic parts smaller/lighter. Mics mostly dynamic; typical airborne HF ≈ 100 W, VHF ≈ 20 W. SELCAL lets ground call a selected aircraft without constant monitoring. Before ground-testing RADAR, clear personnel from the antenna beam.

Key topics 072 (Aircraft Antenna Systems and Frequencies) and 073 (Equipment Functions) are identification-heavy: match the box, the antenna, and the frequency—and know a few operational safety/function facts that show up verbatim on Element 3.

Master frequency map (civil aircraft RF)

Memorize the clusters: LF–MF (ADF), VHF NAV (108–118), VHF COM (118–137), UHF glideslope (~330), L-band DME/XPDR (~1 GHz), C-band radio altimeter (~4.3 GHz).

VHF communications band

Element 3 states the frequency range of an aircraft’s Very High Frequency (VHF) communications as 118.000 MHz to 136.975 MHz (worldwide up to 151.975 MHz). Do not assign the 108–118 MHz NAV band to voice COM, or the 329–335 MHz glideslope band to tower frequencies. Spacing has tightened over decades (25 kHz → 8.33 kHz in many regions), but the band edges are what the pool tests.

VHF COM is essentially line-of-sight; altitude helps range. Antennas are typically vertically polarized blades or whips mounted for a clear view and a solid RF ground plane (airframe skin).

Emergency Locator Transmitters (ELT)

Aircraft ELTs operate on 121.5, 243, and 406 MHz (pool wording includes all three). Roles:

An ELT automatically activates on crash impact (G-switch) or can be armed manually, transmitting a distress signal to aid search and rescue. Registration of 406 MHz beacons and correct antenna mounting are maintenance/compliance items—not optional accessories.

Radio altimeter frequencies

The frequency range of an aircraft’s radio altimeter on Element 3 is 4250 MHz to 4350 MHz (~4.3 GHz C-band). Radio altimeters measure height above terrain (AGL), not pressure altitude MSL. They typically use FM-CW or similar ranging on belly antennas. Do not confuse:

• 121.5 MHz → ELT/distress, not radio alt
• 1090 MHz → transponder replies, not radio alt
• 9 GHz-class → weather/marine X-band radar territory, not the civil radio-alt pool answer

Antenna types the pool names explicitly

VOR and localizer

What type of antenna is used in an aircraft’s VOR and Localizer (LOC) installations? A horizontally polarized omnidirectional reception antenna. VOR/LOC ground signals are horizontally polarized; the airborne antenna is receive-oriented for that polarization with broad azimuth coverage so course deviation works through turns.

Marker beacon

ILS marker beacon installation: a balanced loop reception antenna. Loop geometry favors the overhead marker radiation pattern as the aircraft passes through the beacon’s “football” of energy.

Mode C / ATCRBS

Antenna attached to a Mode C transponder installation to receive 1030 MHz interrogations: an L-band monopole blade-type omnidirectional antenna. Blades are low-drag, broadband enough for 1030/1090 MHz, and nearly omnidirectional in azimuth when mounted on a proper ground plane.

TCAS / Mode S interrogation pattern (ground-station side of the item)

When a ground station phased-array directional antenna transmits the PPM pulses in a Mode S interrogation of an aircraft’s TCAS installation context on the pool, the pattern called out is a 1030 MHz directional pattern. (Aircraft reply on 1090 MHz; keep the uplink/downlink directions straight.)

Glideslope (recap)

As in 18.2: glideslope receive antenna is typically a folded dipole reception antenna.

Placement, drag, ground plane, and isolation

Aircraft antennas are a compromise among RF performance, aerodynamic drag, structural integrity, and EMC isolation:

When diagnosing “weak VOR” or “no Mode C,” walk the antenna → connector → coax → LRU path before swapping expensive boxes. Corrosion at BNC/TNC connectors and crushed coax under floorboards are classic finds.

Equipment functions (3-K-073)

400 Hz aircraft power

Some aircraft and avionics operate with a prime power line frequency of 400 Hz. The principle advantage of a higher line frequency: the magnetic devices in a 400 Hz power supply such as transformers, chokes and filters are smaller and lighter than those used in 60 Hz power supplies. Weight and volume dominate aerospace design; higher frequency allows less iron/core for the same reactance. Pool distractors about “always draws less current,” “always more efficient/cooler,” or “much cheaper” miss the size/weight rationale.

Microphones

Aviation services use predominantly dynamic microphones. Carbon, condenser, and piezoelectric types appear in other niches; the Element 3 expected word is dynamic.

HF vs VHF transmitter power

Typical airborne HF transmitters usually provide a nominal RF power output to the antenna of about 100 watts, compared with about 20 watts RF output from a typical VHF transmitter (100, 20 on the pool). HF must close longer sky-wave/ground-wave paths; VHF COM is short-range line-of-sight with good antennas and altitude.

Ground-testing aircraft RADAR

Before ground testing an aircraft RADAR, the operator should ensure that the area in front of the antenna is clear of other maintenance personnel to avoid radiation hazards. High-power microwave energy can injure tissue and ignite hazards; hangar procedure is “beam path clear” first—not merely “measure supply voltages” or “shield the receiver.”

SELCAL

SELCAL (Selective Calling) is a type of aircraft communications system where a ground-based transmitter can call a selected aircraft or group of aircraft without the flight crew monitoring the ground-station frequency continuously. HF oceanic flights use SELCAL so crews are not locked to a noisy HF channel full-time; a chime alerts them when their code is sent. It is not encryption, continuous selectivity calibration, or airborne distance calculation.

Function-identification drill for techs

Exam-day checklist (3-K 072–073)

1. VHF COM 118.000–136.975 MHz (up to ~151.975 worldwide note).
2. ELT 121.5, 243, and 406 MHz; radio altimeter 4250–4350 MHz.
3. VOR/LOC antenna = horizontally polarized omni receive; marker = balanced loop; Mode C = L-band blade omni; TCAS/Mode S ground interrogation pattern item → 1030 MHz directional.
4. 400 Hz → smaller/lighter magnetic components.
5. Mics dynamic; HF ~100 W vs VHF ~20 W; SELCAL = selective ground call without continuous monitoring.
6. Ground RADAR test → clear people from the antenna beam first.

Topic 3-K is complete when you can name each aircraft radio navigation and surveillance system, its frequency band, its antenna class, and the one-sentence function Element 3 expects—then move on to installation, maintenance, and repair instruments in the next chapter cluster (3-L).