Spare a thought for the little white bump sitting on top of an aircraft. What you are looking at is probably a radome – a cover for the sophisticated satellite connectivity system sitting underneath it.
But what exactly is a radome and why are they so special?
The radome has two basic functions. The first is to protect the delicate satellite antenna and its moving parts from the extremes of heat, pressure and humidity that would otherwise shorten their life.
Given that it must work from zero to around 40,000 feet, and in temperature extremes that range from perhaps -60 degrees to +50 degrees Celsius, you start to see some of the technical challenges associated with manufacturing radomes.
A radome’s other function is to be as transparent as possible to the incoming and outgoing microwave signals to and from a satellite.
This latter function is more difficult that it sounds. Materials used in radome construction must be free of carbon fibre conductive particles. These would attenuate the signals to such an extent that your connectivity system wouldn’t work.
Most radomes are glass, quartz, polyethylene or aramid-based and are designed for transmission efficiency. The materials used have to possess low dielectric constants and low moisture absorption qualities. This allows increased efficiency from the antenna and very low resistance to electromagnetic waves.
The exact material will depend upon the frequency being used. L-band systems (such as Inmarsat SwiftBroadband) use frequencies around 1.6MHz, but Ku-band (as used by Panasonic) is about 12-18GHz (12-18,000MHz) and Ka-band (Viasat Exede and Inmarsat GX) is up around 26.5-40GHz (26,500 – 40,000MHz).
Every material will respond differently to these different frequencies, so a radome is generally designed for one particular band, although some can cope with all frequencies.
Let’s look at a quick case study.
When LiveTV was looking for a new radome for its new Ka-band system that uses ViaSat’s satellites, contractor General Dynamics Armament and Technical Products (GDATP) selected a quartz fibre/epoxy material from TenCate Advanced Composite in California.
Quartz would have given the best transmit performance, but the TenCate epoxy resin brought cost savings to the project.
The triband radome had to maintain the lowest possible profile to preserve aircraft aerodynamics and meet a strict weight requirement, yet provide ample clearance for the antenna hardware underneath it.
GDATP used computer modelling to determine what the RF characteristics of the radome might be before committing to a design.
The company analysed several radome profiles that LiveTV and ViaSat had identified as possible solutions, along with options developed in-house.
The programme concluded with testing to validate its performance and FAA certification. This includes bird impact and lightning strike tests.
The FAA mandates that bird strike tests are required to demonstrate that a flight can be successfully completed with any structural damage sustained if a radome is struck by a four-pound bird at speeds of more than 400 miles per hour.
And according to statistics published by the Royal Canadian Air Force, a plane can be struck by lightning on average every 1,000 to 3,000 flight hours. For commercial aircraft, that’s equivalent to one strike per aircraft per year.
Radomes are tested with multi-megavolt generators and real-life lightning can produce up to 200,000 amp currents.
The tests were fine and the rest is now history with LiveTV, now part of Thales, launching its Ka-band service on JetBlue and United Airlines.
So next time take a closer look at the little white blob on the top of your aircraft’s fuselage. There’s more to it than meets the eye.