Avionics News September 2012 - 21

duplexers, serve multiple VHF radios from a single antenna solution. Drag is the biggest thing at risk here; fewer crops in the antenna farm means less drag and a cleaner, faster airplane. Antennae: Connecting Devices Separated by Vast Distance Among the simplest ways to understand the functions our antennae provide is to think of them as a jack plugged into a conductor – air, in this case – that completes its circuit at another antenna plugged into the same circuit. Yes, at the most-basic level, antennae connect distant circuits to one another. So, a comm antenna connects the transmitter to the receiver at the other end of an exchange and back again. The tiny pole-type transponder antenna completes a circuit connecting the transponder to the ground-based radar pinging it – and back again. The GPS antenna is the circuit connection between the navigator and reference satellites thousands of miles above. Antennae differ considerably across the range of functions for one simple reason: they must match up with the frequency spectrum they service. As avionics technicians know, they get smaller or shorter as the frequency ranges increase and the wavelengths shorten. Aircraft systems fit into one of three frequency ranges: very high frequency, or VHF; microwave; and long wave. Long wave radios were aviation’s earliest navigation system: the non-directional beacon transmitter and the automatic direction finder, or ADF. Build in the commercial-radio AM spectrum, and the lowfrequency NDB gave pilots a signal to track to and from. Early on, the stations tracked were predominantly high-power broadcast radio stations augmented later by stand-alone NDBs commissioned by the Civil Aeronautics Administration – the Federal Aviation Administration predecessor. Most of what aviation adapted after the NDB operates in the shorter-wave VHF spectrum: comm radios, nav radios, glideslope, localizer and our little friends, the marker beacons. In the shortest-wave microwave spectrum operate

our transponders, spherics devices (Strike Finder and Stormscope), satellite-weather transmitters and receivers, and GPS’ navigation-network satellites – plus cellphones and sundry other items increasingly common in the 21st century. On larger turbine aircraft, in-flight entertainment systems and in-flight Internet access bring their own specific antenna challenges. Numbers Add Up Antennae needs once were modest and minimal – the loop and wire of an ADF, the whiskers of a four-way radio range or the similar sensors for receiving VOR broadcasts. Only a couple of decades ago, aircraft tended to sport at least one, often two VHF antennae for voice communications; one each for VHF VOR navigation and another for the localizer signal, one for the VHF nav’s glideslope function, one for each transponder installed and a special wire-andloop antenna package for the ADF radio. It sounds like a lot, but spread as they were around the airplane’s exterior, we seldom viewed them as numerous. About the same time, lightning-detection technology began to advance requiring yet another dedicated antenna. Adding such spherics devices frequently posed the greatest challenge for an installing tech, given the need to map the skin to find the area of least interference from other electrical sources. Not long after the first Stormscopes arrived came the tandem revolutions in area-navigation technologies, which brought new and different antennae needs – a long whip for the Loran C receivers popular from the mid-1980s until the second revolution in space-based navigation from the Global Positioning System – a receiver that needed a smallfootprint patch antenna. Following close behind came the advances in collisionavoidance and traffic-alert systems and – yes, you guessed it – another antenna sprouting on the antenna-farmland of the aircraft airframe. More recently, we have seen the emergence of datalink weather systems, some ground-based, some satellite-based,
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Avionics News September 2012

Table of Contents for the Digital Edition of Avionics News September 2012