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Catch a Wave

Aircraft antennas keep you on top of the world

Those poor antennas, they live a terrible life. Out in the wind, rain, ice, and sludge. Any postal worker could identify with them. They are probably the most overlooked part of an avionics system, yet the most important. Except for a few boxes (such as autopilots), avionics rely on antennas to talk with the outside world. Knowing the position, function, and limitations of antennas may help explain some weird problems that may appear.

Most pilots can be dated by their terminology for these protrusions and lightning rods. The old-timers call them aerials, but the term is gaining strength again. Today they come in many different shapes and sizes. Different antenna manufacturers use slightly different shapes, but each antenna is basically formed by its function. Often, a well-equipped airplane will have an antenna farm on the belly, and trying to figure out what each antenna does can be confusing. But taken one by one, those antennas are easier to understand. The frequencies at which they operate and directional qualities usually determine their shape and placement.

Communication antennas

Communication antennas (Figure 1) are basic in operation and have relatively few problems, except for delamination (more on that later). Each com transmitter has its own antenna, mostly for redundancy and a couple of technical issues. The antennas can be mounted on either the top or bottom of the aircraft, but each installation is susceptible to shadowing from the fuselage.

Shadowing is caused by structure, such as fins or gear doors, in the transmitting path of the antenna. Know where your antennas are and how shadowing may affect their range and coverage. If you have com antennas on the top and bottom of the aircraft, it helps to determine which antenna feeds which radio. The radio that feeds the top antenna would be better for communications while the aircraft is still on the ground, and the antenna on the bottom would be better for communications while airborne, having a clear shot of the ground antenna site in each case. Some older Cessna twins have a com antenna buried in the vertical stabilizer, which limits their range and coverage.

Loran antennas

The loran antenna is similar in size to the com antenna and sometimes the exact same shape, but it is different inside. Most modern loran antennas have an amplifier built into the base to boost the signal, while some older systems use a small amplifier mounted just inside the skin. A loran antenna can be either top- or bottom-mounted, but the receiver must be configured for the antenna position. Often a loran signal problem can be isolated by bumping the antenna slightly with your hand (no baseball bats here) while watching the signal levels on the loran display. A drop in signal levels signifies a defective antenna or amplifier.

Loran systems are also susceptible to P-static interference, caused by a buildup of electrical charge as the aircraft flies through rain or dust in the atmosphere. P-static can be greatly reduced by proper bonding of antennas and airframe structures, especially the grounding straps between a control surface and the structure. Those static wicks on the wing tips and tails are used to dissipate the static buildup, but are susceptible to bonding and deterioration problems. They may not look deteriorated to you, but to an electron, a little bit of corrosion can render them useless.

Those vinyl stickers on the vertical fin are another little-known problem that often sends an avionics technician into the aspirin bottle. The stickers have a tendency to attract static buildup and cause all kinds of interference.

Loop antennas

Loop antennas (Figure 2) are shaped, as their name suggests, in a continuous loop. They have the ability to determine which direction a signal is coming from; hence, they are also called directional antennas. Most have two or three separate coils of very thin wire wound at varying angles to each other in the shape of a bagel laid flat. The signal is received at different strengths between the coils, and the receiver uses those different signal strengths to determine the direction from which those signals originated. The ADF uses this type of antenna, as do lightning detection systems.

Although most of the older ADF receivers also need a long wire antenna, called a sense wire, to resolve any 180-degree errors inherent in loop antennas, the newer loop antennas combine the loop and the sense wire in the same package. Because of the flat-bagel shape, most loop antennas are wide and short and usually live on the bottom of aircraft, but they can be mounted on the top. These antennas suffer from delamination because they commonly hold water and oil that collect within the case. A good seal job before water can accumulate goes a long way toward increasing the life of these antennas.

Lightning detection antennas (such as those for Stormscopes and StrikeFinders) have special mounting problems because their job is to detect and amplify electrical noise, any noise, including that from ignition systems, alternators, loose skin panels, and of course the atmosphere (lightning). The intent of these lightning detection systems is to filter out the nonatmospheric noises, but sometimes aircraft-produced noises overwhelm the detection systems and the system displays "ghost storms" that seem to always follow the aircraft wherever it goes. So antenna placement is critical, even to the point that the entire aircraft should be electrically mapped for noise before mounting this type of antenna. Sometimes the source of noise needs to be located and repaired, often requiring the replacement of the entire ignition system.

Marker beacon antennas

Marker beacon signals are highly directional, which means you have to be almost directly over the transmitting ground station to receive them; therefore, marker beacon antennas need to be on the bottom of the aircraft. There are a few different types of marker antennas; the more common types look like little canoes about 10 inches long (Figure 3). These antenna systems are relatively simple and reliable, but they have the same delaminating problems as other antennas.

For some installations, Cessna has used flush antennas that appear to be flat plates under the empennage. It also has used an antenna that consists of a thick wire that protrudes straight down out of the empennage and then makes a turn toward the tail (Figure 4). Both of those types have very few problems.

UHF antennas

UHF antennas are commonly used for transponders and DMEs and are always found on the bottom of the aircraft. They are about four inches long, and the same antenna is often used for both systems because the transponder frequency is in the middle of the DME frequency band. Two types are commonly used, spike (Figure 5) and blade (Figure 6) antennas. The spike should only be used for transponders, because the antenna length is tuned to one frequency, the transponder frequency. The blade antenna is also called a broadband antenna because it is tuned for a range of DME frequencies. A spike would not work very well for a DME; the blade antennas are preferred because the radiation pattern is better and ice formation is less likely to break them.

The spikes are prone to caking up with oil, reducing the transmitting range. Often, just cleaning a spike antenna doubles your transponder range and gets rid of those intermittent Mode C problems. The reason is that the ground secondary radars need only one sweep to determine your squawk code (Mode A), but they need two good sweeps to determine altitude information (Mode C). Hence, a dirty antenna may not conduct a good signal reliably. This goes for all antennas; a dirty antenna does not perform up to its potential.

The spike antennas are also susceptible to breaking from an errant scrub brush. After the antenna is broken from its mount, it can be reinserted, leaving the owner none the wiser and wondering why the replies are intermittent.

Landing-gear doors often shadow these small antennas (Figure 7), so check your aircraft to determine if your DME might drop out from the side or front when the gear is down. The blade antennas are susceptible to delamination, which tends to detune the frequency response and distort the transmitted signal. That's why the biannual transponder check is so important. A detuned transponder signal will be rejected by the ATC radar receivers and lead to intermittent problems.

Nav antennas

The nav antenna is almost always mounted on the vertical tail. Among the exceptions are some Beech Bonanzas that use a top-mounted combination antenna that contains both a nav and com antenna (Figure 8). There are three types of nav antennas: the cat whisker, the dual blade, and the towel bar. The cat whisker consists of a couple of rods jutting out from each side of the vertical stabilizer at a 45-degree angle (Figure 9).

The cat whisker antenna is poor at receiving signals from the side, and was developed for aircraft that fly low and commonly track either directly to or from a station. The dual blade is just that, two blades, one on each side of the tail (Figure 10). The towel bar resembles the common bathroom fixture, one on each side of the tail. The blade and towel bar antennas are both "balanced loop" designs, which have equal receiving sensitivity from all directions. A balanced loop antenna is required for area navigation (RNAV) systems, which rely on receiving stations much to the side of the aircraft.

A single nav antenna almost always feeds both nav receivers and sometimes the glideslope as well. Therefore, a failure in the nav antenna system would cause both systems to go down. In rare instances this same antenna also feeds the marker beacon, but problems are created when the horizontal tail shadows the antenna from the marker beacon transmitter.

GPS antennas

The GPS satellites transmit less than five watts of power, so by the time the signal reaches you, it is very, very weak. Because of this, the GPS antenna has a built-in amplifier to boost the signal for the receiver. Additionally, the GPS frequency is so high (in the gigahertz band) that the signals travel in a line-of-sight manner. This makes receiving the signal susceptible to airframe shadowing, thus mandating that a GPS antenna be mounted at the very top of the fuselage.

The communications radios can cause a lot of interference with GPS as a result of the proximity of the panel units or their antennas. The reason is that GPS signals are so weak, and com signals are so very powerful (in relation). In fact, an approach-approved GPS is tested for com interference during installation. Most barely pass. Therefore, it is important that the com and GPS antennas be mounted as far apart as possible. Sometimes a com antenna must be relocated to the bottom of the aircraft.

Suction-cup antennas, common with handheld GPSs, are shadowed by the aircraft structure when placed in the window. Even if enough satellites are being tracked in level flight, a turn may cause some satellites to drop out. The suction cup also tends to fall during turbulence. This is just one of the many reasons why IFR certification with a handheld GPS will be difficult, if not impossible, to achieve (but the future is always surprising).

Emergency locator beacon antennas

Hopefully, you'll never get to use an emergency locator beacon antenna, but in case you do, they are especially designed to survive an "unscheduled" landing. They are almost always on the upper skin of the empennage and are made of a flexible material (Figure 11). There are a few exceptions, though; some may be buried in the vertical tail or look like small com antennas.

Radar altimeters

Radar altimeter antennas are simple, comprising either a single or dual antenna system. They look like plates about six inches square and live on the bottom of the aircraft. The radar signal is transmitted straight down to bounce off the ground. The time between transmitting and receiving the signal is measured and used to determine the distance above the ground. Because of the high frequencies involved, a good electrical bond with the aircraft skin is important; a poor bond may allow the system to talk to itself, which causes false readings.

Mounting and bonding

Many factors influence antenna performance. Of course the physical condition of the antenna plays an important role. If the antenna is cracked or the paint worn off, water may enter and cause delamination (a separation of the composite layers), which may render the antenna useless. Another problem deals with the underlying structure and electrical metal bonding. If the antenna base is not structurally strong, the antenna will vibrate from the slipstream and cause the skin to fatigue. Eventually this causes cracks and may rip the antenna off the fuselage, especially if there is ice buildup on the antenna. A riveted doubler plate under the antenna base prevents the vibration and work-hardening of the skin.

The antenna also must be electrically bonded (grounded) to the airframe so a good electrical connection is maintained. If some corrosion gets underneath the antenna, this bond may be compromised and the antenna's efficiency may degrade to the point that a com may only transmit a couple of miles. Without a seal around the antenna, water creeps underneath and causes corrosion in a very short time. This bonding is practically nonexistent if a mechanic uses a sealant under the antenna, trying to curb corrosion. The proper bond is a bare metal skin to antenna mating surface with sealant around the base of the antenna.

In addition to a skin doubler and good electrical bonding, a transmitting antenna also needs an effective ground plane. To transmit correctly, a quarter wave antenna (the antenna is one-quarter the length of the electromagnetic wave) needs an electrically bonded structure around it with a radius equal to the antenna length. In other words, lay the antenna down and scribe a circle. This is the amount of metal (ground plane) that the antenna needs around it to work properly. Those of you with composite aircraft still need this ground plane, which often consists of a metal plate just inside the skin. For instance, if a com antenna is two feet long, it must have two feet of metal around it. Just a few of those transmitting antennas are the com, DME, transponder, ELT, radar altimeter, satellite telephone, and HF.

Another problem is paint. Antennas should never be painted over their original coatings. Any paint buildup reduces the efficiency of an antenna. Transmitting antennas are particularly sensitive to paint problems, especially when covered with metallic paint.

The exceptions

There is always the exception to the rule when it comes to antenna shapes and their mounting, especially in the kitbuilt aircraft arena. There are some interesting innovations for hiding antennas under those fiberglass skins. Real estate is very scarce on an aircraft, and sometimes there is very little left for antennas. Helicopters and seaplanes have precious little area to work with and antenna placement is very important. Every antenna location is a compromise between a solid mounting, shadowing, other antenna interference, ground planes, and aerodynamics. But with a little knowledge of their limitations and some care, they will live a life that any postal worker would envy.

So if you're contemplating a new avionics installation, don't skimp on the antennas; they're your link to all those good waves out there in the atmosphere.

Paul Novacek, AOPA 855200, of Yorktown, Virginia, is a research project manager for several NASA projects in the AGATE/SATS and aviation safety programs. He has more than 900 hours in 20 years of flying.

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