An aircraft uses a range of radio frequencies to navigate and communicate with air traffic control. To do this successfully the radio equipment uses different types and sizes of antenna. Each of these antenna have their own characteristics regarding frequency and thus location on the aircraft. Even the connection between the antenna and avionics has its own set of specifications.

In this second article on aircraft antenna we will continue our discussion with navigation, GPS, transponder and ELT type antennas.

NAVigation

Aircraft navigation uses more frequency ranges than communications does. LORAN-C in the LF range and LORAN-A in the MF band, VOR ILS and marker beacons in VHF and Transponders/DME, ILS Glide path and GPS in the UHF frequency bands. They can all be used at the same time. The reason for using all these frequency bands grew somewhat historically.

LORAN
LORAN-C in the LF band isn't used that much anymore, although it can be more reliable as the transmitters have much more power than spaced based GPS. The frequency band used is 90 - 110 kHz. Read more on this subject here.

VOR - ILS
VOR radio stations can be found in the VHF band between 112 and 118 MHz (ILS localizers are between 108 and 112 MHz. Glide slope frequencies are three times the VOR frequency, around 328 - 335 MHz UHF). The signals are horizontally polarized and therefore you will need a horizontally installed antenna. A good example can be seen on many aircraft where you see the dipole on the vertical fin but I have seen aircraft were the antenna is installed on the bottom of the tail.
This antenna is somewhat bend, backwards or forwards, and this makes it sensitive in all directions. Another advantage is that by bending the radiators it is suitable for 50 ohm coax cable. A normal open dipole has 73 ohm impedance and should not be connected to a 50 ohm cable and radio. This mismatch results in poor performance. Earlier on this page we described a Balun (transformer) to correct this situation.

Tactical Air Navigation Aid and VOR (TACAN - VORTAC)
TACAN is a military UHF radio navigation system giving combined azimuth and distance information. The DME element of the TACAN is available for civil use. At some military airports VOR and TACAN are co-located, these installations are called VORTAC, short for VOR and TACAN.

Markerbeacon
Marker beacons are part of the ILS landing system. They transmit on 75 MHz and receivers are sometimes built into the intercom system (Garmin, Bendix King and others). The antenna used is usually a boat type and located under the aircraft.

GPS
GPS frequencies are in the UHF band and there are a number of frequencies in use in between 1176 and 1379 MHz. The signal is transmitted from 24 satellites orbiting the earth. This signal is circular polarized and the antenna needs to be installed on top of the aircraft so that it can 'see' the sky around it. Reception is quite undisturbed by weather or ionospheric influences.

These antenna are 'active', meaning they have an amplifier build-in to enhance the signal from the satellites. Any good UHF cable, like LMR-400 would do well. The amplifier is fed through this cable.

Transponders and DME

The transponder in an aircraft is a radio receiver and transmitter in one box which receives the interrogation on 1030 MHz and gives its reply on 1090 MHz. An example is shown to the right, sometimes blade type antenna are used. These have a wider frequency range and can be used by the distance measuring equipment too (DME). Disadvantage is slightly more weight and the need to mount it correctly so it will not act as a small rudder.

DME operates by sending an interrogation signal to a ground station and receiving/ processing the reply. DME uses the frequency range between 962 MHz to 1213 MHz, and the frequencies 1030/1090 MHz used by the transponder falls in the middle of this band. This is the reason the same antenna can be used (blade types and not the monopole I showed above) for DME and transponder.

DME frequency channels are paired with the VOR selections, many radios require that the VOR be selected to set the frequency of the DME receiver.

Emergencies, ELT

ELT, Emergency Locator Transmitter, historically used 121.5 MHz (Military aircraft on 243 MHz, it is a 3rd harmonic frequency) in the VHF band for its transmissions. Due to limitations the use of 121.5 MHz is being fased out from satellite service and the use of 406 MHz ELTs is recommended. The ELT should be installed in the tail section of the aircraft for greater survivability. The antenna is a small whip type usually included with the ELT.

As of Feb 1st, 2009 the satellite system Cospas-Sarsat will quit monitoring 121.5 MHz and 243 MHz. From this point on only 406 MHz will be monitored by satellites. More info can be found on the site of Cospas-Sarsat.

Quote Wikipedia: "COSPAS is an acronym for the Russian words "Cosmicheskaya Sistyema Poiska Avariynich Sudov" which translates to "Space System for the Search of Vessels in Distress". SARSAT is an acronym for Search And Rescue Satellite-Aided Tracking."

Do not tamper with this device, any false alarm could result in very high fines. Be sure to follow the installation instructions to the letter. If you wish to test this device then do so within the first five (5) minutes of the hour, keep a radio nearby on 121.5 MHz and transmit no more than three (3) sweeps.

It would be wise, if your aircraft is equipped with more than one receiver to tune the other to 121.5 MHz. This way if any alarms are received you can report this to the nearest ATC unit. Likewise after a flight, just before switching off the radio's, tune to 121.5 MHz to make sure your ELT was not activated by accident due to a somewhat rough landing.