Garmin GDL 69/69A Installation Manual - page 31
Installation Procedure
GDL 69/69A Installation Manual
Page 2-15
190-00355-02
Revision E
Table 2-7. XM Gain/Loss Component Calculation
Antenna Gain (1)
+
Cable Loss (2) (3)
-
GDL 69/69A Gain/Loss (4)
+6dB
> x > -4dB
+/-
Total Gain
Antenna/Receiver
= 20 dB
Note:
(1)
Garmin GA 55, GA 55A, GA 57 XM antenna typical gain 25 dB. For antenna gain for other
antennas, see manufacturer’s specifications.
(2) If 12-16ft of RG-400/U cable is used, a value of 4.5dB can be used. See Section 2.6.4.1 for
explanation of calculation.
(3) If an antenna with increased gain is installed (antenna other than the Garmin antennas listed in
Table 1-5), additional cable length may be required to be coiled to compensate for the required
cable loss. Alternately, an external attenuator may be used to obtain the desired antenna cable loss.
However, installation of the external attenuator is beyond the scope of this STC. Additional
manufacturer’s data may be necessary and FAA approval may be required to cover the installation
of an external attenuator.
(4) The GDL 69/69A Gain/Loss component must be between +6dB and -4dB. If the GDL 69/69A
gain/loss component is outside this range, additional gain or loss must be added between the
antenna and the GLD 69/69A RF input. The factory default setting for the internal GDL 69/69A
component is -0.5dB. The variable attenuation value required to attain the GDL 69/69A gain/loss
component will be programmed into the configuration module during post installation checkout.
See Section 2.6.4.2 for explanation of calculation.
2.6.4.1
Determining Antenna Cable Loss Value
The GDL 69/69A is factory preset with a default cable loss value of 4.5 dB, which is equivalent to 12 to
16 feet of RG-400/U cable with two properly terminated TNC connectors. If the installed antenna cable is
within this length, use this value in Table 2-7. If the cable is different from the default cable, use the
following formula to determine the cable loss value to use in Table 2-7.
Loss in dB = (Length x Loss) + (0.5 x #Connectors)
100
Where:
Length – Cable length in feet
Loss – Specified cable loss per 100 feet at 2332-2345 MHz
Connectors – Number of connectors on cable
For example:
If an RG-400 coax cable is 10 feet long with 2 TNC connectors, the cable loss component is
Loss = (10 x 26.1) + (0.5 x 2) = 3.61 dB
100