4-1 CHAPTER 4. ANTENNA INSTALLATION
4.1 Purpose. The purpose of this chapter is to describe antenna installation methods and practices. An antenna that is installed on an aircraft must function properly and may not adversely affect other systems or equipment.
4.2 Hazards and Warnings.
4.2.1 Follow antenna manufacturer’s instructions and recommendations when they are available and appropriate and not contrary to the instructions of the aircraft manufacturer.
4.2.2 Extension of the landing gear or flaps may impact belly-mounted antenna performance.
4.3 Additional References. For further information concerning acceptable methods, techniques, and practices concerning alteration involving specific structures, refer to the appropriate chapter of AC 43.13-1, Acceptable Methods, Techniques, and Practices―Aircraft Inspection and Repair, and Civil Air Regulation (CAR) 6, Rotorcraft Airworthiness; Normal Category.
4.4 Structural Support.
4.4.1 The antenna’s structural load, plus any required allowances, may not exceed the design capacity of the structure intended to support it. It is important to understand the operational characteristics of the aircraft and consider forces that occur during flight (dynamic loading) as well as those that occur when the aircraft is not in motion (static loading). For example, an aircraft designed without flaps may employ a side slip procedure to lose altitude, during which the direction of airflow across the fuselage is not in line with the aircraft longitudinal axis. Antenna mountings on these aircraft need to be designed and evaluated for the direction of airflow that occurs during such an operation.
4.4.2 Whenever possible, an antenna should be mounted to a flat surface. Minor aircraft skin curvature can be accommodated with the use of an appropriate gasket, but if gaps over
0.020 inches appear between the base plate and mounting surface, use of a mounting saddle is recommended.
4.4.3 Since antenna systems typically require a ground plane (this may be a conductive surface that the antenna mounts to), any separation of an antenna from its ground plane may impact performance. Contact the manufacturer for recommendations if a gasket or mounting saddle is needed.
4-2 Figure 4-1. Antenna Mount With Saddle
4.4.4 Mounting screws must never be over torqued in an attempt to distort aircraft structure to reduce gaps between the antenna base plate and aircraft-mounting surface.
4.4.5 Consider the factors of flutter, vibration characteristics, and drag load. The approximate drag load an antenna develops may be determined by the following formula:
D = 0.000327 AV2 (The formula includes a 90 percent reduction factor for the streamline shape of the antenna.)
D is the drag load on the antenna in pounds.
A is the frontal area of the antenna in square feet.
V is the never-exceed speed (VNE) of the aircraft in miles per hour (mph).
Example: Antenna manufacturer specification frontal area = 0.135 square feet and the VNE of the aircraft is 250 mph.
D = 0.000327 x .135 x (250)2 = 0.000327 x .135 x 62,500 = 2.75 pounds
4.4.6 The above formula may be adapted to determine side load forces by substituting the apparent frontal area value for A when the aircraft motion and antenna orientation are not the same.
4.5 Physical Interference.
4.5.1 Antennas should be located where they will not interfere with the operation of the aircraft or other aircraft systems. One such example of interference could be the obstruction of the visibility of a navigation position light or beacon.
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4.5.2 Antennas should be located so that they do not obstruct or limit airflow to areas of the airframe that require airflow. Care should be exercised so that an antenna is not located where it will be damaged by heat from engine exhaust, fumes from battery vents, or fuel/fluid drains.
4.5.3 Antennas may accumulate ice that can then depart and damage areas behind them.
Special attention should be paid to areas of the airframe near pitot-static ports and sensors, and near flight controls since the antenna may alter airflow characteristics.
4.5.4 Antennas should be located in such a manner that they are not susceptible to damage from misuse, such as near a door where they might be mistaken for or used as a handhold.
4.6 Antenna Selection. The selection of an appropriate antenna will include consideration of system requirements and aircraft characteristics. The size and shape of an antenna vary with frequency, power rating, and maximum design speed of the antenna. See Figures 4-7, ADF Loop, through 4-19, DME/Transponder Blade, for pictures of typical antennas.
4.7 Antenna Location.
4.7.1 In general, antenna locations on an aircraft that provide unobstructed line-of-sight views of the transmitted or received signals are preferred. Objects located in the path of a signal may cause a blanking of antenna coverage and impact the performance of the system.
Note: Global Positioning System (GPS) antennas will not receive a signal if a line-of-sight view of a satellite is not available. Do not mount a GPS antenna on the underside of an aircraft.
4.7.2 The acceptable and unacceptable spacing between an antenna and an obstacle or the permissible interval between antennas is dependent upon operating frequency and system characteristics. When in doubt, contact the antenna manufacturer or system designer for further information.
4.7.3 Antennas should be separated as far as possible from interference sources (other radiating antennas, ignition noise sources, etc.). When known interference sources are present, it may be advisable to temporarily position an antenna and check the location for suitability prior to mounting the antenna.
4.7.4 Very high frequency (VHF) Com 1 should be mounted on the top of the aircraft since this will provide the best unobstructed location. VHF Com 2 can also be mounted on the top, provided there is at least 1/2 wavelength (of the antenna operating frequency) distance available between antennas.
4.7.5 If Com 2 is mounted under the aircraft, a bent whip may be required to provide ground clearance. Bent whips may not provide the best performance because of proximity to the aircraft skin. Signal reflection and obstruction is more of a problem with such locations.
4-4 Extension of the landing gear or flaps may also impact belly-mounted antenna performance.
4.7.6 Antennas need to “see” with a direct line-of-sight to the source. Antenna patterns can be disrupted by landing gear or vertical stabilizers, for example. When mounting antennas, try to locate them in areas where line-of-sight view is not obstructed.
4.7.7 As a rule of thumb, maintain 36 inches as a minimum distance between antennas. Refer to the manufacturer’s installation guidelines for specific system limitations and requirements.
4.7.8 Antennas should be located such that the cables that run between the antenna and equipment are as short as practical. Signal loss of a cable is dependent upon cable design, length, and frequency. In some cases, cables must be specific lengths to provide a required capacitance, attenuation, or signal transmission time. VHF omnidirectional range (VOR)/Localizer (LOC)/glideslope (GS) blade or towel bar type antennas require the cables to their coupling assembly to be the exact same length to maintain a phase relationship.
4.8 Antenna Bonding.
4.8.1 The electrical bonding of the antenna to the aircraft surface is extremely important. The conductive skin of an aircraft is an electrical part of the antenna system. If an antenna is not properly bonded to the aircraft, its pattern may be distorted, and nulls in coverage may appear.
4.8.2 The electrical bonding of the antennas to the aircraft skin of a metal aircraft is best accomplished by direct metal-to-metal contact of the antenna base to the skin. A resistance of no more than 0.003 ohms between the antenna base plate and skin should be achieved.
Note: To achieve this electrical bonding, the aircraft paint in the mounting area will need to be removed and the surface covered with an oxide film
(i.e., aluminum conversion coat) to protect aluminum against corrosion in accordance with MIL-DTL-5541.
4.8.3 An alternate method for providing electrical bonding to metal aircraft skin is through the antenna mounting screws, which attach to a backing plate inside the aircraft, making electrical contact with the backside of the skin. To ensure good contact, remove any interior paint in the area where the backing plate is placed and coat this area in accordance with MIL-C-5541B to minimize corrosion.
4.8.4 Composite or fabric-covered aircraft that do not provide a conductive mounting surface generally require fabrication of a conductive surface (ground plane) and bonding through the mounting screws.
4.8.5 Antenna performance can be severely degraded from corrosion caused by moisture accumulation where the antenna electrically bonds to its ground plane. It is advisable to
4-5 apply room temperature vulcanizing (RTV) around the antenna edges to seal the antenna bond. However, always ensure chemical compatibility before using any sealant.
4.9 Electromagnetic Interference (EMI).
4.9.1 Since an antenna’s purpose is to either receive or transmit radio frequency (RF) energy, or both, it is essential to consider EMI.
4.9.2 Antenna mounting positions that are as far as possible from an EMI source should be selected. In special cases, it may be possible to employ filters or select an antenna that has been specifically designed to be resistant to EMI.
4.9.3 EMI test procedures are found in Chapter 3, Communication, Navigation, and Emergency Locator Transmitter System Installations.
4.10 Mechanical Installation.
4.10.1 Mounting Hardware.
4.10.1.1 Typical antenna installations employ either #8-32 or #10-32 stainless steel mounting screws.
4.10.1.2 Some designs require a pan head screw. For others, a counter sunk screw is required.
4.10.1.3 Mounting screw length will vary based on each particular installation requirement.
4.10.2 General Practices.
4.10.2.1 Before drilling holes in the aircraft skin, refer to the installation drawing to determine the proper size and spacing.
Note: When replacing antennas, it is important to match the original mounting holes. Previous mounting holes that are not reused with appropriate hardware must be repaired, and the mounting location must be returned to its design strength.
4.10.2.2 It is highly recommended to use a structural backing plate. Backing plates strengthen the immediate point of attachment, but if they are not attached to a load-carrying structure, they do not provide structural load support.
4.10.2.3 To properly secure the antenna, mounting screws should be secured with stainless steel nuts with flat washers and lock washers, or with flat washers and lock nuts.
4.10.2.4 Sandwich the aircraft skin between the antenna base plate and the internally mounted backing plate. Before securing the antenna to the aircraft, make sure
4-6 all the cables are connected to the unit and fit through the connector holes in the aircraft.
4.10.2.5 Gently tighten the mounting hardware so that uniform stress is placed on each side of the antenna. For #8-32 screws, do not exceed 20 in/lbs of torque, and for #10-32 screws, do not exceed 23 in/lbs of torque. Refer to the fastener manufacturer’s torque guidelines to confirm that these recommended settings do not exceed the chosen fastener’s torque limits.
4.10.2.6 Once the antenna is mounted, any minor gaps between the base plate or gasket and aircraft skin should be filled with RTV silicone adhesive sealant.
4.10.2.7 Double-check that a reading of 0.003 ohms between the antenna base plate and ground has been achieved.
4.11 Ground Plane Requirement (Wood/Fabric and Composite Aircraft).
4.11.1 When the antenna is not mounted to a conductive surface capable of providing a required ground plane for operation, a ground plane must be fabricated.
4.11.2 Most antennas require a ground plane size of approximately 24 inches by 24 inches.
While the rule of thumb is to provide a minimum of 1/4 wavelength of the operation frequency, larger is better, and ground plane symmetry is critical. Gaps in antenna coverage or performance may occur if a ground plane is not symmetrical.
Table 4-1. Wavelength in Feet = 984/Frequency in Megahertz (MHz) Frequency Wavelength (feet) 1/4 Wavelength (inch) 75 MHz 13.1 40 125 MHz 7.9 24 1,000 MHz 1 3
4.11.3 Wire mesh is the best material to use when a solid plate is not practical. Heavy aluminum foil can also be used. In all cases, electrical continuity from the ground plane to the airframe ground is essential.
4.11.4 Be sure the ground plane is well attached to the airframe with cement or epoxy if not otherwise supported. This will prevent noise problems or erratic operation that could occur if the plane moves. Capacitance will occur as an antenna base is separated from its ground plane. This may distort antenna coverage or operation.
Figure 4-2. Antenna Ground Plane for Nonconductive Mount WHIP
ANTENNA C= ====
METAL FOIL UNDER
FABRIC OR WOOD SKIN Figure 4-3. One Means to Provide Adequate Antenna Bonding Through a Composite Airtrame → ANTENA (Length to meet installation requirements) Flathead screw stainless steel _ Seal Aluminum Grounded Foot Plate _Composite Aircraft -Aluminum Ground Plate -Ground to Aircraft (4 olaces) -Doubler Plate Elastic Stop Nut Stainless Steel or equivalent 4-7
4-8 Figure 4-4. Use Bushings for All Screws N ote: Carbon fiber composite material, while conductive, has not been found to be adequate as a ground plane.
4.11.5 Refer to AC 43.13-1, Chapter 3, Fiberglass and Plastics, for acceptable methods, techniques, and practices applicable to fiberglass and plastics.
4.12 Antenna Feed Line Balun.
4.12.1 Antenna cables (electrical feed lines) may be designed to be electrically balanced or unbalanced. Certain antenna designs incorporate dual elements with a requirement for balanced input. In these cases, standard cables that are unbalanced are generally employed for the cable run, and a balancing transformer is located at the antenna feed connection.
4.12.2 A balun is a device that converts an unbalanced feed into a balanced input and may include a transformer that matches feed impedance to provide maximum signal transfer.
Follow the manufacturer’s installation procedures when a balun is required. Some balun designs require the balun to be grounded to the airframe.
4.12.3 Refer to AC 43.13-1, Chapter 11, Aircraft Electrical Systems, for bonding practices.
4-9 Figure 4-5. Typical Dipole Antenna Assembly
4-10 Figure 4-6. Typical VOR Balun
4.13 Antenna Repair. Painting an antenna or applying protective coatings or devices that are not approved are not allowed under this AC. Paint is an RF detuner. If an antenna is painted in the field, paint type and paint thickness present uncontrolled variables that will affect an antenna’s performance and may result in the antenna no longer meeting its specifications or Technical Standard Order (TSO).
4-11 Figure 4-7. ADF Loop Fi gure 4-8. ADF Combined Sense Loop Fi gure 4-9. Communications (COM) Whip
4-12 Figure 4-10. COM Whip Bent Fi gure 4-11. Emergency Locator Transmitter Fi gure 4-12. Global Positioning System