Business & GA, Commercial, Military

Avionics System Design: Antennas and Feedlines

By Walter Shawlee 2 | April 1, 2002
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All aircraft start out looking sleek and graceful. Then comes the avionics department, and their sleekness diminishes because every surface seems to have an antenna attached. Large or mission-specific airframes, especially, are outfitted with a broad range of radio systems, most of which require dedicated antennas. Installing communication antennas poses a significant challenge, as they not only receive radio frequency (RF) signals, but transmit them.

The wavelengths of communication systems tend to demand physically big antennas to maintain performance. The antennas often take the form of a quarter-wavelength monopole (whip or blade), loaded against the grounded airframe skin, which forms the other half of a dipole structure.

VHF and UHF radios (with wavelengths of 3 to 0.5 meters) have considerable difficulty sharing a common antenna. Couplers allow antenna sharing but preclude simultaneous operation, often defeating the reason for carrying multiple radios. So the airframe tends to have an antenna for each radio.

An antenna’s loading and position on the airframe are critical for satisfactory performance. HF radios may employ long-wire, tubular or loaded structure antennas and achieve proper loading and matching with an active or passive coupler system. This active antenna tuning technology commonly is used with low-band FM (30- to 50-Mhz) radios and a monopole/blade antenna in the 6- to 9-meter band.

Blade antennas are preferred over simpler whips on faster aircraft, mainly because of the speed and icing environment they encounter. (Generally, a whip-style antenna can withstand no more than a 200-knot speed.) Inside many blades is a simple monopole, which is surrounded by structurally reinforced fibreglass to withstand stress loading. Some blades incorporate more elaborate, lower-inductance structures to give wider band response with less peaking, or erratic behavior. They may even contain active tuning with PIN (positive intrinsic negative) diode switching or multiband elements.

Any low-loss metallic object not surrounded by a grounded shield will work as an antenna. The question is one of performance. Much creative thinking goes into air transport and military HF systems, which require considerable antenna length (wavelengths of 10 to 160 meters). In such instances, parts of the airframe (often the vertical stabilizer or tail boom) are excited (supplied a signal) directly via a coupler, which attempts to achieve something resembling a 50-ohm load for the transmitter.

Serious problems arise when an antenna fails to serve as a good launch site for the RF energy it receives. Without proper grounding, the power bounces back into the transmitter. Whether open or shorted at the antenna end, the feedline to the antenna suddenly will experience zero return loss, i.e., all power is reflected back into the radio and airframe, where it can cause significant interference to other onboard systems. Ideally, designers hope for better than 30 dB of return loss so that 99.9 percent of the energy is radiated away from the airframe. Only when the entire feedline has low loss and the antenna is properly resonant against the airframe–resulting in a 50-ohm impedance–is this level of return loss possible. Frequently, due to mechanical realities–the antenna is too short or is masked by parts of the aircraft–systems must be fielded with unattractively low return loss. As a result, high levels of unwanted RF are reflected back through the system wiring, and interference problems begin to appear in other electrical systems.

If you want to examine how your antenna system is performing, excite the feedline at the radio end using a directional coupler, scalar analyzer and sweep generator. A quick sweep test, which is valuable for developing new and better systems, will show the continuous system performance regarding both forward and reflected power.

Many installers routinely use a watt meter as the definitive test for antennas, but it is a blunt tool that reveals nothing about antenna radiation effectiveness. It can detect some catastrophic faults but also lets many marginal problems due to the installation escape.

AM transmissions are especially problematic when power reflects back into the airframe wiring. They generate severe audio interference in adjacent wiring and may cause upset events in low-level logic wiring. Lights may go on, navigational readings may change, and other unanticipated system problems may arise. Because it is now present as high-level standing waves on the feedline’s outer shield, the AM signal is readily coupled into any adjacent wiring. A secondary shield (a non-current-carrying outer triaxial cable shield) over the RF feedlines is strongly recommended to help reduce this interference and often can produce significant improvements in marginal installations.

AM com radios also are problematic when the antenna is mounted near composite structures, as the RF is radiated directly back into the aircraft, often at high levels. This could generate problems ranging from audio interference to electrical system failure. One should never assume that transmission near an inadequate ground, composite area or window is trouble free; the RF levels are so high that they can cause transmitted audio reception in the aircraft on a headset that is not even plugged in.

Engine exhaust and the issues of fuselage structure and ground clearance preclude antenna placement on much of an aircraft’s surface. Using the remaining real estate almost always results in unwanted coupling problems unless the airframe is the size of a B737 or larger. To work properly, the quarter wave, monopole antenna requires a ground disk equal in radius to the length of the monopole–a setup that requires a lot of aircraft surface.

In addition, antennas must be positioned so as to prevent line-of-sight interference. On a B737, a com antenna can be mounted on the top of the fuselage and one on the bottom. The fuselage then serves as a shield between the systems, and provides plenty of ground to load the antenna correctly. Installers mounting antennas on smaller aircraft seek the best compromise.

Next month, we will continue discussing the trials and tribulations of radio antennas and their feedlines.

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