Garmin GDL 69/69A Installation Manual - Page 32
GLcomp in dB = XMgain - Antenna + Cable, GLcomp, XMgain, Antenna, Cable
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Installation Procedure 2.6.4.2 Determining GDL 69/69A Gain/Loss Component Value The GDL 69/69A has a zero to 10dB variable attenuator that is used to balance the gain/loss component between its RF input and internal XM Receiver. The gain/loss components can be adjusted between +6dB and -4dB to balance the Antenna input to XM receiver 20dB gain requirement as specified for the XM system. The Gain/Loss component is factory preset with a default value of -0.5 dB. Using the Signal Gain Equation shown in Figure 2-9 and solving for the GDL 69/69A component the equation becomes GLcomp in dB = XMgain - Antenna + Cable Where: GLcomp - GDL 69/69A Gain/Loss Component XMgain - XM specified gain from antenna input to XM receiver input (20dB) Antenna - Antenna Gain Cable - Cable Loss Example: If the cable loss calculated in the previous example is used, the GDL 69/69A loss component is: GLcomp = 20dB - 25dB + 3.61dB = -5dB + 3.61dB = - 1.39dB The GDL 69/69A gain/loss component will be programmed into the configuration module during post installation checkout. 2.6.5 Coaxial Cable Installation 1. Choose the correct coax: RG-400/U has good characteristics for loss, size, and flexibility. NOTE The cable loss of the antenna cable is critical to the performance of the GDL 69/69A operation. To accommodate this, the GDL 69/69A has the ability to be configured for the amount of antenna cable loss. To reduce the need to configure the GDL 69/69A for cable loss, the GDL 69/69A is factory-preset with a cable loss of 4.5 dB, which is equivalent to 12 to 16 feet of RG-400/U with two properly terminated TNC connectors. If the cable loss is different than the default value, the cable loss must be calculated or measured and the loss value programmed into the GDL 69's configuration module. Refer to Section 4.2 for additional information on determining antenna cable loss value and how to program the configuration module. NOTE To avoid programming the configuration module, use a coax cable length that is within the factory default cable loss value (refer to Section 2.6.4.1). When using the default cable loss value, additional cable after routing through the aircraft may be coiled and secured as needed. Do not coil the cable tighter than a one foot diameter. 2. Trim the coaxial cable to the desired length and install TNC connectors at each end per the cabling instructions listed in Figure 2-10. For routing convenience, one end of the coaxial run can be terminated prior to installation. 3. With the GDL 69/69A receiver and antenna installed, route and clamp the coaxial cable in position. Secure cable in accordance with AC 43.13-1B, Chapter 11. Page 2-16 Revision E GDL 69/69A Installation Manual 190-00355-02