An aircraft antenna is a conductor that sends and receives radio waves, and most aircraft carry several, each cut for a different system: VHF communications, VOR navigation, the glideslope, the transponder, GPS and the emergency locator transmitter. The FAA names three basic designs that cover nearly all of them: the dipole, the Marconi and the loop.
This guide explains what those three designs are, which one does which job, why the metal skin matters, and what the other antennas on a typical airplane are for. It follows the FAA's Aviation Maintenance Technician Handbook for airframes and its Pilot's Handbook of Aeronautical Knowledge, with the ELT rule in 14 CFR 91.207.
Key Takeaways
Most aircraft VHF communications antennas are Marconi antennas: vertically polarized, omnidirectional and dependent on the metal skin as a ground plane.
The V-shaped VOR antenna is a horizontally polarized dipole. It's most sensitive to signals from the sides, not head-on.
A loop antenna gives no signal broadside and the strongest signal in the plane of the loop, which is how an ADF finds a direction.
The transponder, ADS-B, GPS and radar systems all work at much higher frequencies, from about 1 GHz up.
A modern ELT transmits a digital signal on 406.025 MHz. The older 121.5 MHz analog signal is no longer relayed by satellite.
Three Basic Antenna Types
The FAA's maintenance handbook says three characteristics matter most in an antenna: its length, its polarization and its directivity. The exact shape and material also change how it transmits and receives, and some non-metallic aircraft have their antennas embedded in the composite as it's built up.
The three basic antenna types in aviation and how each behaves, after the FAA Aviation Maintenance Technician Handbook. PilotPassion diagram.
A conductor about half the wavelength of the transmission frequency, fed in the center. The strongest field is perpendicular to its length. Most aviation dipoles are horizontally polarized.
The V-shaped VOR antenna is a common dipole. Each arm is a quarter wavelength.
Marconi
A quarter-wave antenna that reaches half-wave efficiency by using the conductive aircraft skin as the second quarter wavelength. Vertically polarized and omnidirectional.
Most aircraft VHF communications antennas.
Loop
Highly direction-sensitive. A wave hitting the loop broadside produces no signal. A wave in line with the plane of the loop produces the strongest signal, and the strength changes with the angle.
ADF navigation. The loop and a sense antenna are normally in a single low-profile housing.
Which Antenna Does What
Each radio system sits in its own band, and each needs its own antenna or a shared one designed for a narrow range. The handbook's chart of aviation radio frequencies places them like this.
Frequencies from the FAA Aviation Maintenance Technician Handbook (Figure 11-75) and its text on each system. The scale is logarithmic. Original diagram for PilotPassion.
Aviation radio systems and their antennas (FAA Aviation Maintenance Technician Handbook, Chapter 11)
A V-shaped, horizontally polarized dipole. Some designs share the antenna with glideslope reception.
Glideslope
328 to 336 MHz on the chart (329.3 to 335.0 in the text)
A single dipole inside a non-metallic nose is a common choice. Glideslope reception can also share the VOR dipole, and blade antennas are used too.
ADF
200 to 1,600 kHz
A loop plus a sense antenna in a single low-profile housing. The sense antenna removes the ambiguity about whether the station is ahead or behind.
DME
960 to 1,215 MHz
Has its own aircraft-mounted antenna.
Transponder
Interrogated on 1030 MHz, replies on 1090 MHz
The transponder receives interrogations at 1030 MHz and transmits replies at 1090 MHz, both in the UHF band.
ADS-B
1090 MHz (1090 ES) or 978 MHz (UAT)
The 978 MHz universal access transceiver needs an omnidirectional antenna in addition to the GPS antenna and receiver.
GPS
1575.42 MHz (L1, civil)
Needs a clear view of the sky. See below.
Radar altimeter
4.3 GHz
A transceiver with a directional antenna aimed at the ground and a second antenna that receives the return.
Weather radar
SHF, such as 5.44 or 9.375 GHz
A directional antenna usually behind a non-metallic nose cone.
Emergency locator transmitter
406.025 MHz
An independent battery-powered transmitter activated by crash forces.
Length, Polarization and Directivity
Length. An antenna half the length of the signal's wavelength is resonant, which gives the strongest radiated signal and lets a receiving antenna capture the wave with the greatest induced voltage. The handbook gives a formula for the half-wavelength antenna: 468 divided by the frequency in megahertz gives the length in feet. VHF communication radios use 118 to 136.975 MHz, and the handbook says the corresponding half wavelengths are 3.96 to 3.44 feet, which is 47.5 to 41.2 inches. That's long for an airplane, so VHF antennas are often a quarter of the wavelength, which works on a metal fuselage because the airframe supplies the missing quarter. A quarter wave on those frequencies works out to roughly 21 to 24 inches by our arithmetic.
Radios can't have a different antenna length for every channel, so the handbook says the physical length is a compromise. A capacitor in series makes an antenna electrically shorter, an inductor makes it electrically longer, and many radios use a tuning circuit to match the effective length to the wavelength wanted.
Polarization. Antennas radiate and receive in set patterns. A receiving antenna with the same polarization as the transmitting antenna gets the strongest signal. A vertically polarized antenna radiates in a donut-like pattern in all horizontal directions. A horizontally polarized one radiates in a donut-like field too, strongest at 90 degrees to its length, with no field off the end. The handbook notes that many vertical and horizontal antennas are mounted at a slight angle so a weak signal still arrives when the polarization doesn't match.
Directivity. Even omnidirectional antennas radiate a stronger signal in some directions than others. That's why where an antenna is mounted on the airframe matters.
VOR, Glideslope and ADF
The VOR antenna is the V-shaped one. Each arm is a quarter wavelength and the pair together form a half-wave antenna, fed in the center. It's horizontally polarized, so as a receiving dipole it's most sensitive to signals approaching from the sides rather than head-on in the direction of flight. The handbook says other antenna types are certified too, and installation follows the manufacturer's instructions.
The ADF is the oddest of the family because it relies on the loop's null. A signal broadside to the loop induces equal and opposite currents that cancel out, and a signal in the plane of the loop gives the strongest output. Early direction finders rotated a large loop to find the null. Modern ADF systems use a fixed loop with a ferrite core, and the most modern use two loops at 90 degrees to each other feeding a goniometer. A loop alone can't tell whether the station is ahead or behind, so a sense antenna with an omnidirectional field is added to create a single null on one side. The handbook warns that the installation of an ADF antenna is critical because it's directional: it has to be calibrated with the fuselage's longitudinal axis. Selecting ANT on the receiver cuts the loop out and leaves only the sense antenna, which suits listening to weather broadcasts.
A transponder replies to ground radar. The ground station transmits a pulse at 1030 MHz and the transponder answers at 1090 MHz with the code the controller assigned, one of 4,096 discrete octal codes. With an altitude encoder connected and altitude reporting selected, the reply can carry pressure altitude as well. Our guide to Mode C transponders explains the modes.
Collision avoidance uses the same two frequencies. The handbook says a traffic collision avoidance system (TCAS) interrogates nearby transponders at 1030 MHz and displays the position and altitude from the 1090 MHz replies. It works with a compatible transponder and its antennas. TCAS I, developed for general aviation and regional airlines, identifies traffic within a 35 to 40 mile range and issues traffic advisories. TCAS II also issues resolution advisories, which are aural commands to take evasive action, such as descend.
ADS-B Out combines GPS position with the aircraft's flight status and broadcasts it to other aircraft and ground stations. It uses either 1090 MHz, as an expanded use of the Mode S protocol, or 978 MHz with a universal access transceiver. The handbook says the 978 MHz option needs an omnidirectional antenna plus the GPS antenna and receiver.
GPS Antennas
GPS needs a clear view of the sky, and the FAA's pilot handbook is specific about what that means in practice. In many VFR installations of GPS receivers, antenna location is more a matter of convenience than performance. In IFR installations, care is taken to give the antenna an adequate clear view of the satellites, and an alternative location might let part of the aircraft block it. Hand-held receivers with suction-cup antennas on the inside of a window are limited by the aircraft structure, so signal loss may occur when the aircraft-satellite geometry blocks the view. The handbook adds that those losses, with no RAIM capability, could present erroneous position and navigation information with no warning to the pilot. The civil signal is the L1 channel at 1575.42 MHz.
Weather Radar and Radar Altimeters
Weather radar sends SHF pulses forward from a directional antenna usually located behind a non-metallic nose cone. The handbook says the radome must be painted only with approved paint, so the radio signals pass unobstructed, and many radomes have grounding strips to carry lightning and static away. It also warns technicians not to look into the antenna of a transmitting radar and not to run it in a hangar unless the space has special radio-wave absorption material.
A radio altimeter works the other way: a transceiver and a directional antenna broadcast a 4.3 GHz carrier toward the ground, and a second antenna receives the return. The time the signal takes gives height above the terrain, so the system uses one antenna to transmit and another to receive.
Ground Planes, Composites and Cables
A Marconi antenna needs a ground plane. On a metal aircraft the skin does that job. On a fabric-covered aircraft the ground plane that makes up the second quarter wavelength must be fashioned under the skin, with thin aluminum, aluminum foil, or four or more wires extended from the base of the antenna. On a non-metallic aircraft the handbook says wires, conductive plates or strips equal in length to the antenna must be installed under the skin.
Composites bring a second effect. The pilot handbook says fiberglass is transparent to radio frequencies, and carbon fiber isn't, so internal radio antennas may be found in fiberglass composites. For composite aircraft with internal antennas there must be windows in the lightning-strike mesh in the area of the antenna. Our guide to fuselage types covers composite construction.
The antenna connects to the radio through coaxial cable: a center conductor, an insulator, a braided shield and a waterproof cover. The braid shields the inner conductor from outside fields and stops its own field radiating. For best performance the impedance of the line should equal the antenna's, often about 50 ohms. The handbook says correct installation is critical to radio and antenna performance, and the airframe technician is responsible for installing, inspecting, repairing and maintaining aircraft radio antennas.
Emergency Locator Transmitters
An ELT is the one antenna system built to be used after a crash. The maintenance handbook describes it as an independent battery-powered transmitter activated by the excessive G-forces of a crash. It sends a digital signal every 50 seconds on 406.025 MHz at 5 watts for at least 24 hours. Satellites in the Cospas-Sarsat system receive it anywhere in the world and relay it to ground stations, and search and rescue is notified from there. Automatic ELTs are easily removable and often have a portable antenna so survivors can carry the working unit away from the wreck.
Modern ELTs may also transmit on 121.5 MHz, an analog signal used for homing. The handbook says that before 2009 121.5 MHz was a worldwide emergency frequency monitored by the Cospas-Sarsat satellites, and that it has been replaced by the 406 MHz standard, so transmissions on 121.5 MHz are no longer received and relayed by satellite. It adds that the FAA hasn't mandated a 406 MHz ELT, and that an older 121.5 MHz ELT satisfies 14 CFR 91.207 in all but new aircraft. The rule itself bars new installations of the older TSO-C91 type after June 21, 1995, so check the unit's approval and installation date as well as the airplane's age.
The rule itself, 91.207, requires an approved ELT on a U.S.-registered civil airplane, with exceptions that include aircraft equipped to carry not more than one person and training flights entirely within 50 nautical miles of the airport where they began. Fixed and deployable automatic ELTs must be attached as far aft as practicable. The battery must be replaced or recharged after more than one cumulative hour of use, or when 50 percent of its useful life has expired, with an exception to the 50 percent limit for batteries essentially unaffected by storage. The ELT has to be inspected within 12 calendar months for installation, battery corrosion, the controls and crash sensor, and a sufficient signal radiated from its antenna.
Technicians are cautioned not to activate an ELT, because an accidental 406 MHz signal can't be told from a real emergency and could lead to expensive searches. The handbook says testing uses direct connection or a dummy load where possible, and any test activation of a 121.5 MHz ELT is limited to the period between the top of each hour and five minutes past, for no more than three audible sweeps.
How to Read the Antennas on an Airplane
The handbooks give a few reliable identifiers. A V-shaped antenna is a VOR dipole, and some are VOR and glideslope combinations. A low-profile housing holds the ADF loop and sense antenna. A radar altimeter uses two antennas. A weather radar antenna hides behind the nose cone. Beyond those, what's fitted depends on the avionics installed, so the aircraft's equipment list is the reliable answer to what a particular airplane carries, and the installation has to follow the manufacturer's instructions.
Frequently Asked Questions
What are the main types of aircraft antennas?
The FAA names three basic types: dipole, Marconi and loop. Most VHF communications antennas are Marconi, the VOR antenna is a dipole, and the ADF uses a loop with a sense antenna.
What's a ground plane on an aircraft antenna?
A conductive surface that acts as the missing second quarter wavelength of a Marconi antenna. On a metal aircraft the skin does this. On a fabric or composite aircraft a conductive plate, foil or wires equal in length to the antenna must be installed under the skin.
Why is the VOR antenna V-shaped?
It's a horizontally polarized dipole. Each arm of the V is a quarter wavelength, and the pair form a half-wave antenna. It's most sensitive to signals arriving from the sides rather than head-on.
Why does an ADF have two antennas?
A loop antenna alone can't tell whether the station is ahead or behind. A sense antenna with an omnidirectional pattern is combined with it to create a single null on one side.
What frequency does an ELT transmit on?
A modern ELT sends a digital signal on 406.025 MHz every 50 seconds at 5 watts for at least 24 hours, relayed by Cospas-Sarsat satellites. Modern ELTs may also transmit on 121.5 MHz for homing, but satellites stopped relaying that frequency.
Does an old 121.5 MHz ELT still meet the rule?
The FAA's maintenance handbook says the FAA hasn't mandated a 406 MHz ELT and that an older 121.5 MHz ELT satisfies 14 CFR 91.207 in all but new aircraft, although the rule bars new installations of the older TSO-C91 type after June 21, 1995. The satellites no longer relay 121.5 MHz.
What frequency does a transponder use?
The ground station interrogates on 1030 MHz and the transponder replies on 1090 MHz with its assigned code. ADS-B can use 1090 MHz or 978 MHz.
Where should a GPS antenna go?
The FAA says that in many VFR installations the location is more a matter of convenience than performance, but IFR installations take care to give the antenna a clear view of the satellites. Suction-cup antennas inside a window can lose signal.
Why does a radar altimeter have two antennas?
One broadcasts a 4.3 GHz signal toward the ground and the other receives the return. The time the signal takes gives height above the terrain.
Why do composite aircraft need windows in the lightning mesh?
Fiberglass is transparent to radio frequencies and carbon fiber isn't, so a composite aircraft with internal antennas needs windows in the lightning-strike mesh over the antenna.
