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
4-13 Figure 4-13. Glideslope Fi gure 4-14. Marker Fi gure 4-15. Combined COM/VOR
4-14 Figure 4-16. VOR Rabbit Ear Fig ure 4-17. VOR Blades
4-15 Figure 4-18. Transponder/DME Probe Fig ure 4-19. DME/Transponder Blade
5-1 CHAPTER 5. ANTICOLLISION AND SUPPLEMENTARY LIGHT INSTALLATION
5.1 Purpose. This chapter gives procedures and standards to be used when replacing older rotating beacon assemblies and wing lights with strobe or other anticollision systems.
This chapter assumes that the newer units have FAA approval in the form of a Parts Manufacturer Approval (PMA) or Technical Standard Order (TSO) and not an experimental or aviation unit.
5.2 Hazards and Warnings. When installing anticollision lights, take care to ensure the unit is properly grounded, the airframe structure can support the new unit, and the aircraft wiring is of the correct size. Mechanics should take special precautions to protect their eyes when testing the new unit. Strobe lights are especially hazardous in dark or darkened hangars when activated.
5.3 Regulations and Other References. The operational requirements for anticollision lights can be found in 14 CFR § 91.205. Information regarding installation requirements for nontransport category aircraft is included in 14 CFR §§ 23.1401 and 27.1401. Additional information can be found in AC 20-30, Aircraft Position Light and Anticollision Light Installations; AC 20-74, Aircraft Position and Anticollision Light Measurements; and Civil Air Regulation (CAR) 6, Rotorcraft Airworthiness; Normal Category. Aircraft for which an application for type certificate (TC) was made before April 1, 1957, may conform to either the above regulations or to the following standards:
5.3.1 Anticollision lights, when installed, should be installed on top of the fuselage or tail in such a location that the light will not impair the flight crewmembers’ vision and will not detract from the conspicuity of the position lights. If there is no acceptable location on top of the fuselage or tail, a bottom fuselage or wing tip installation may be used.
5.3.2 The color of the anticollision light must be either aviation red or aviation white in accordance with the specifications of 14 CFR § 23.1397 or 14 CFR § 27.1397, as applicable.
5.3.3 The arrangement of the anticollision light system (i.e., number of light sources, beam width, speed of rotation, other characteristics, etc.) must give an effective flash frequency of no less than 40 but not more than 100 cycles per minute. The effective flash frequency is the frequency at which the aircraft’s complete anticollision light system is observed from a distance and applies to each sector of light, including any overlaps that exist when the system consists of more than one light source. In overlaps, flash frequencies may exceed 100 but not more than 180 cycles per minute.
5.3.4 The system must consist of enough lights to illuminate the vital areas around the aircraft, considering the physical configuration and flight characteristics of the aircraft. The field of coverage must extend in each direction within at least 75 degrees above and 75 degrees below the horizontal plane of the aircraft. The minimum light intensity and minimum effective intensities are given in 14 CFR §§ 23.1401 and 27.1401, respectively.
5.3.5 Supplementary lights may be installed, in addition to position and anticollision lights required by applicable regulations, provided that the required position and anticollision