Navigation Antenna, II
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 of you 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. In case of doubt: 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.
Transponders & DME
The transponder in an aircraft is a radio receiver and transmitter in one box which receives the ATC interrogation on 1030 MHz and transmits its coded reply on 1090 MHz. An example is shown to the right, sometimes blade model antenna's are used. These have a wider frequency range (bandwidth) and can be used by the distance measuring equipment too (DME). Disadvantage is slightly more weight and the need to mount it correctly in line, 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 as shown above) for DME and transponder.
DME frequency channels are paired with the VOR selections, many radio's require that the VOR be selected to set the frequency of the DME receiver.
The location for this antenna is either on the back or belly of the aircraft away from other transmitting antenna or sensitive receivers. TCAS systems usually use two antenna's for a complete coverage around the aircraft, being the top and bottom of the fuselage.
ELT, Emergency Locator Transmitter, historically used 121,5 MHz (Military aircraft on 243 MHz, it is a 3rd harmonic of the base frequency) in the VHF band for its transmissions. Due to limitations the use of 121,5 MHz is fased out from satellite service and the use of 406 MHz ELTs is mandatory.
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 and located near the tail so it arrives last on the scene of the accident and has the best chance of surviving the crash. More detailed information about installing an ELT in our homebuilt section.
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 (to check the low level homing signal) and transmit no more than three (3) sweeps.
As of Feb 1st, 2009 the satellite system COSPAS-SARSAT has stopped 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.
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.
After the flight
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. Most new 406 MHz ELTs contain a 121,5 MHz homing beacon so this technique does work.