Antenna Grounding; Teardrop Footprint Antenna Installation (Ga 55 And Ga 56); Preparation Of Doubler; Teardrop Footprint Antenna Doubler Design And Installation - Garmin GDU 37X Installation Manual

3.3.6 Antenna Grounding

The antenna is grounded through the mounting hardware and the coax connection. The mounting
hardware (washers and nuts) and doubler plate should make contact with an unpainted grounded surface
ensuring proper antenna grounding. It is important to have good conductivity between the coaxial shield
and the ground plane. The bottom of the antenna does not need to make contact with the ground plane (i.e.
the surface may be painted). The antenna will capacitively couple to the ground plane beneath the paint or
aircraft cover.

3.4 Teardrop Footprint Antenna Installation (GA 55 and GA 56)

This section describes the structural mounting of the teardrop footprint antenna installation.
An acceptable installation method is to use Garmin P/N: 115-00846-10 doubler plate with the GA 55 or
GA 56 stud mount antennas. Another acceptable method is to fabricate and install one of three doublers
(Figure 3-4, Figure 3-5, and Figure 3-6), depending on the thickness of the skin. The three doubler designs
vary only by number of rivets and hole preparation for installation with flush rivets. Table 3-5 provides a
summary of design and installation details for selecting the appropriate antenna doubler/backplate.
Figure 3-7 shows an example of the doubler installed between stringers on the top fuselage skin, just off
centerline. The location should be flat, with no gaps between the skin and doubler, to keep from deforming
the skin during installation.
Table 3-5 Teardrop Footprint Antenna Doubler Design and Installation
Aircraft Skin Thickness
Doubler Design (Figure)
Number of Rivets Required
Type of Rivets Required
Skin Preparation for Rivets
Doubler Preparation for Rivets
Skin Cutout Detail (Figure)
Doubler Installation (Figure)
Notes:
1. Rivet length determined at installation, dependent on thickness of material (rivet length = grip length +
1.5 * rivet diameter)
Refer to Figure C-2.X for Garmin Antenna installation drawings.

3.4.1 Preparation of Doubler

1. Use Garmin P/N: 115-00846-10, or refer to Table 3-5 for guidance on selecting the appropriate doubler
drawing based on the thickness of skin at the antenna location. Make the doubler from 2024-T3
Aluminum (AMS-QQ-A-250/5), 0.063" sheet thickness.
2. For installation in aircraft skins of thickness less than 0.051", countersink the rivet holes in the doubler
for use with flush head rivets (MS20426AD4-x).
3. When using Garmin P/N: 115-00846-10 doubler, sixteen rivet holes exist in the part. For installation
of Garmin P/N: 115-00846-10 in skins of thickness between 0.032" and 0.049", only the rivets
identified for use through the skin cutout detail (Figure 3-8) and doubler installation (Figure 3-11) are
required.
GDU 37X Installation Manual
190-01054-01
0.032" to 0.049"
Figure 3-4
12
1 MS20426AD4-x
Dimple
Countersink
Figure 3-8
Figure 3-11
0.049" to 0.051" 0.051" to 0.063"
Figure 3-5
16
MS20426AD4-x MS20426AD4-x
Dimple
Countersink
Figure 3-9
Figure 3-12
Figure 3-6
16
Countersink
None
Figure 3-10
Figure 3-13
Page 3-7
Revision C