Navigation Antenna, I
An aircraft uses a range of radio frequencies to navigate to its destination and communicate with air traffic control. To do this successfully, the onboard radio equipment uses different types and sizes of antenna's, each for their own frequency band.
Each of these antenna's have their own characteristics regarding frequency and application and thus location on the aircraft. Even the connection between the antenna and avionics has its own set of specifications.
Some probably wonder if you can paint the aircraft antenna's. That depends on the type of paint, frequency and bandwidth of the antenna, check with the manufacturer of the antenna or just don't paint them.
On this page (and others) we will continue our discussion with VHF navigation, GPS, transponders / DME and ELT and the antenna's used for these devices.
Aircraft navigation uses more frequency ranges than voice 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-C in the LF band isn't that common anymore in aviation, 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. The application is mainly maritime nowadays.
ADF - NDB
These systems use low frequency signals (LF band: 30 - 300 kHz) thus the antenna's they use are quite large compared to VHF communications antenna. The antenna system consist of a long wire somewhere between wing and tail and a sense antenna which looks like a loop. The signals of these are then electronically combined so that the receiver is able to locate and indicate the direction of the NDB station.
Modern ADF antenna systems use a complete unit to house these two antenna's. See image to the right of a King KI-44B.
VOR - ILS
VOR radio stations can be found in the VHF band between 112 and 118 MHz (ILS localizers are in 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 (omni). Another advantage is that by bending the radiators it is suitable for 50 ohm impedance 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 poorer performance. Earlier on 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, which is short for VOR and TACAN.
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 model and located under the aircraft in the middle of the fuselage. The transmitters are located along the centerline of the runway at predetermined distances.
GPS frequencies are located in the UHF band and usually between 1176 and 1379 MHz. The signal is transmitted from at least 24 satellites orbiting the earth at low level in about 1,5 hour (there are more for backup purposes). 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 caused by solar flares or day / night changes.
A GPS antenna is 'active', meaning they have an amplifier build-in next to the antenna to enhance the signal from the satellites and to overcome cable losses. Any good UHF cable, like LMR-400 would do well. The amplifier is fed through this cable make sure that you do not short it. You will find this antenna on top of the aircraft so it can 'see' the GPS satellites.