Product Focus: Antennas

Phase arrayed, blade, whip, internal, external, trailing wire, collapsible, invisible, multispectrum, electronic scan, tunable, high gain, low gain, stealthy and multifunctional: all are descriptions of antenna types or attributes. All antennae (the plural in Latin for those unfortunates who only suffer a technical education) have one or both main functions. They either receive or transmit information through the electromagnetic spectrum. Some do both.

Like the man whose dental work turned him into a human radio receiver, antennas (English plural for those of us who tire of the insufferable pseudo superiority of people still insisting on using terms from a long-dead language) can bring unintended emanations into complex avionics. It is not the antenna’s fault; they are just made that way. Two examples: during the TCAS Implementation Program, the ARINC SC 147 committee (the one charged with bringing traffic alert collision avoidance systems to the public) received reports that the initial units received far more than their share of questionable conflict resolution advisories around certain airports.

The TCAS Implementation Task Force, under the direction of U.S. Federal Aviation Administration (FAA) manager, Tom Williamson, discovered that the errant calibration of antennas at instrument landing system (ILS)-equipped runways were inappropriately interacting with the traffic alert collision avoidance system logic. We called the calibrating antennas "parrots on a pole." The task of the FAA was to determine a method of "killing the parrot’s squawk" without destroying its main function–making the ILS beam run straight.

At other airports, the team discovered that the U.S. Navy ship transponders sometimes showed up through aircraft TCAS systems as collision risks. More than one airliner "took it around" in cloudy conditions when the TCAS indicated a target on Final at a deep-water port.

The fix to both problems was simple. It involved changing the internal registers within the Mode-S transponders in the "parrot" and the ships to less than sea level. The airborne TCAS antenna would still "see" the target; but the logic would disgard it as non-threatening.

Much of the current thinking in future air traffic control (ATC) systems involves the use of Global Positioning System data to supplement or eventually replace Mode-S antenna received data. Automatic dependent surveillance (ADS)-based schemes have been conceived that potentially can enhance both the safety of the airspace environment and dramatically increase its capacity.

Many of the advanced technology ATC systems were first launched as a means around the antenna limitations of the original TCAS II implementation. The experts in industry and government in the early 1990s felt that antenna-based resolution advisories were simply too inaccurate to support horizontal anti-conflict strategies in a highly dynamic, multi-aircraft environment. So, TCAS III died a stillbirth at RTCA, and ADS-B absorbed its fraternal twin. The antenna was the weak link.

TCAS II has since evolved into a commodity. Brand competition no longer dominates its market. Now the emphasis is on price and systems integration.

The big money in antennas today is in the consumer empowerment area. Jennifer Hoover of Ball Aerospace & Technologies Corp. states, "Passengers can remain connected to their lives and businesses using high-speed data antenna technology. Even thousands of miles from land, a high-gain satcom antenna can use Inmarsat satellites to give the passenger access to the entire Internet and e-mail."

The same antenna system can make "transoceanic" phone calls a reality. If a passenger needed to, he or she could conference call with a management team while cruising at 37,000 feet, some thousand miles from anywhere over any body of water on the globe. If the home office in Kentucky needs to connect with a key player who is en route from Hong Kong to Kuala Lampur, a satcom phone call to his/her seat on the airliner is made possible through the efficiency of the high-gain antenna system.

But not all antennas are made equal. The environment that they must operate in is unforgiving at best. At 37,000 feet on a typical day, the outside static air temperature will be near to -59� C. The same antenna is expected to function faultlessly sitting on a heat-soaked ramp at sea level with a reflected air temperature of more than 49� C. The antenna is required to remain fixed to the airframe in crosswind gusts of hundreds of miles per hour.

Some antennas are designed to resist delamination at supersonic speeds. The FAA discussed limiting the Concorde’s access to its airspace until a suitable TCAS antenna could be designed. Engineers rose to the challenge.

Not all environmental hazards to antennas are airborne. Some antennas must be designed to be resistant to "ramp rash," the name given to the continual assault by foreign objects to the airframe while on the tarmac. Ramp rash is not just the net result of jet blast or wind-blown trash. Much of it is caused by vehicle impact with the aircraft structure. Service trucks, belt loaders, and baggage carts regularly ding the originally pristine aluminum. Sometimes they take out antennas.

Ever wonder why the air phones don’t seem to work on some planes? The usual cause is that all of the installed connections are in use. Most small jets–MD-80/B-737 size–have only three air phone antennas. If all are in use at one time, the fourth passenger who tries to make a call must wait until one of the connections is free. Larger jets typically have more connections available. But none of them have a separate connection for each passenger even if there is a phone in each seat back. And, yes, sometimes the phones don’t work because someone broke the antenna blade off.

Antennas are exciting. New military applications include tunable VHF and UHF voice and data multiban antennas that can frequency-hop with their associated frequency-agile radios. Such equipment does have a civilian use. It could be the basis for a secure ADS surveillance and communication system that would protect the ATC system of the future from unauthorized access. When the world is ready for such systems, antenna engineers and their companies will be more than able to step up to the plate.