Sunday, February 26, 2012

Cessna 172R ELT Antenna

The ELT antenna must be mounted in accordance with the requirements. Locate the antenna at least 30 inches (75 cm) away from other antennas, wires, vertical stabilizer, etc. to minimize distortion of the radiated field and interference with other equipment. The antenna must be installed VERTICALLY (within ±15º of the vertical plane is acceptable). Artex has no performance data for installations that deviate from the stated requirements.
The ME406 is certified to be used with the following antennas (automatic fixed “AF” configuration):
• Artex Whip Antenna, 121.5/406 MHz, P/N 110-773
• Artex Rod Antenna, 121.5/406 MHz, P/N 110-338
The antenna requires a ground plane. On aircraft constructed with non-conductive materials, such as composite materials or fiberglass, a ground plane must be added. Ideally, the ground plane should extend out from the antenna mounting point at least 24 inches (60cm) in every direction. Many times this is not possible, but an effective plane can be constructed as follows:
A ‘doubler’ layer of sheet metal, such as aluminum, can be mounted under the aircraft skin. Alternatively, four or more ‘radials’ fastened to the underside of the fuselage skin can be used to fashion a ground plane. Each radial can be metallic tape, 22 AWG wire, etc. Tape should be at least 1 inch (25.4 mm) wide and each radial 24 inches (60 cm), minimum. The ground plane connects to the shield of the RF antenna connector. Resistance between the ground plane and shield connection should be maintained at 0.003Ω maximum. A star washer should be used between the antenna connector housing and ground plane. Take precautions to guard against corrosion, loosening, etc.
The antenna must be mounted on the top of the aircraft to assure maximum ‘visibility’ of satellites (406 MHz). The best location is the upper aft portion of the fuselage. It should be mounted vertically and away from projections such as a propeller, tail surfaces, or the shadow of larger antennas.

1. Prepare the surface for antenna installations in such a manner to ensure a ground contact of less than 0.003Ω. If bare metal surfaces are needed for surface preparation they should be treated with Alodine®1200 (or similar compound) to eliminate aluminum oxidation.
2. Drill 0.562” (14.3 mm) hole in aircraft skin.
1) Metal adapter plates should be used if the curvature or compound radius of the aircraft skin is such that antennas cannot be directly installed vertically with their plates mounted flat to the aircraft outer surface.
2) Backing plates or doublers should be installed to ensure adequate support for associated air loading during flight. Refer to AC 43.13 for complete information.
3) Remove the ½-28 hex nut and external tooth lock washer from the base of the antenna.
4) Insert antenna connector through mounting hole, make sure the “O” ring remains in the base of the antenna connector flange groove and that the connector has sufficient clearance through the aircraft skin.
5) Place the lock washer and the hex nut on the inside of the aircraft and sandwich the aircraft skin between the base of the antenna and lock washer followed by the hex nut. Tighten the hex nut to between 25 to 30 inch lbs (280 to 340 Newton-cm).
6) Apply a small, smooth fillet with RTV sealant around the periphery of the antenna base to seal out moisture.
7) For maximum signal strength, the length of the antenna coax to the ELT should be as short as possible (use of the standard 6-foot (1.8 M) coax is recommended when possible).

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