In July 2019, an American passenger aircraft was unable to land at Dallas airport due to loss of communication with air traffic control (ATC). As a result, the aircraft had to remain in air for 30 minutes thus delaying all successive flights that were scheduled to take off. Airline companies face huge losses due to such delays. Loss of communication with ATC for few minutes is common. In the USA, 5-6 incidents are reported every month. This often occurs due to poor design of the antenna systems.
How do we design reliable antennas?
To answer this question, we need to understand how aircrafts communicate with each other and with Air Traffic Control (ATC). This is possible due to Very High Frequency (VHF) antennas. These antennas are placed on the body of the aircraft and they transmit electromagnetic signals to and from ATC. Transceiver systems at both ends process and convert these signals into sound. However, sometimes antennas fail to capture signals from the ATC causing loss of communication. This could arise due to improper design and placement of antennas or signal interference between multiple antennas (referred to as co-site interference).
In order to eliminate the possibility of such undesirable incidents, we designed and simulated VHF Antennas using SIMULIA’s Electromagnetic tools. We carried out the following steps.
- Selection of Appropriate Antenna.
- Optimization of Antenna Dimensions to achieve Target Center Frequency.
- Optimum Antenna Placement for Minimum Cross-Coupling.
- Elimination of Co-Site Interference between Onboard Antennas.
Selection of Appropriate Antenna
The operating frequency range of VHF antenna systems is 118 to 137 MHz with 127.5 MHz as the center frequency.
There are couple of different design options for VHF antennas –
- Monopole Antenna
- Dipole Antenna
- Loop Antenna
- Notched blade Antenna
Notched blade antenna was selected for its bandwidth and aerodynamic performance. The blade can be mounted directly on the airframe, which provides grounding to reflect waves, improving the radiation.
A blade VHF antenna is readily available in Antenna Magus, Which is SIMULIA’s antenna design and modelling tool. A radome, which is a dome like structure to protect radar equipment and is made from material transparent to radio waves was designed and placed upon the antenna to make it weatherproof.
Optimization of Antenna Dimensions to Achieve Target Center Frequency
The time-domain electromagnetic simulation that we performed allowed us to determine the actual center frequency of this blade antenna – it was above 130 MHz whereas our target center frequency was 127.5MHz.
Using optimization tools the width and height of the antenna were optimized to achieve the target center frequency.
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