ADSB-GPS Install

[Dayton Murdock]

Hi All
I am installing a Free flight ADSB in my -4. currently I have my GPS antenna under the cowl and my question is can I split the GPS antenna lead and operate two radios form 1 antenna? I want to connect my AFS 4500 and the ADSB to one GPS antenna can this be done?

[Walt]

Originally Posted by Dayton Murdock

Hi All
I am installing a Free flight ADSB in my -4. currently I have my GPS antenna under the cowl and my question is can I split the GPS antenna lead and operate two radios form 1 antenna? I want to connect my AFS 4500 and the ADSB to one GPS antenna can this be done?

No... a seperate antenna is required for each GPS.

From the FF install manual:

1.2.4 GPS Antenna Requirements
The FDL-978-XVR requires a TSO-C144 (DO-228) or TSO-C190 (DO-301) complaint
GPS antenna. Installation of a TSO-C190 antenna is only recommended for anticipated
future upgrades to TSO-C145c Class Beta 2, 3, or 4 navigation sensors and avoids the
need for a GPS antenna change at that time.

The performance of the FDL-978-XVR internal GPS is affected by the gain, noise figure,
impedance, and frequency selectivity characteristics of the antenna. FDL-978-XVR
should be used only with the recommended antenna and cable (See 1.4.3 and 1.5.1).
Use of other antennas or cables may not meet all the performance characteristics
required by TSO-C145c.

[BobTurner]

Walt,

Do those TSOs or their instructions specify external mounting?

[Jordan1976]

Bob,
Likely your first issue is not the fact that it's under some fiberglass, it's that it's really hot. You need to read the specs for the antenna you plan to use and make sure that under the cowl never gets hotter than the specs require. The hotter an antenna gets, the worse the noise figure.

I'd imagine most antennas top out at 60-70C, which is only about 165F, and is totally reasonable for an antenna only designed to be on the outside of a GA plane, but is way below the temps under your cowl after a shutdown.

[Walt]

This particular antenna supplied with the FF unit is from Aero Antenna Technology and is rated from -55C to 85c (185F).

One thing many folks may not be aware of is that GPS antennas are "active" antennas which mean they contain a built in amplifier which is powered from the center conductor of the coax. Different antennas have different input voltages, gains and associated amplifier specs which for the most part makes "sharing" them with other GPS units impossible.

More info from the install manual:

2.12 GPS ANTENNA INSTALLATION
The antenna should be installed according to the manufacturer’s instructions.
Selecting appropriate GPS antenna locations is critical to the proper performance of the FDL-978-XVR/RX. The following considerations should be taken into account when selecting the Antenna location.
Shadowing by aircraft structure (and in some cases by rotorcraft blades) can adversely affect operation of the GNSS equipment.
1. Typically, a GNSS antenna is located forward or aft of the wings on top of the fuselage.
2. GNSS antennas must be installed to provide the widest, unobstructed view of GNSS satellites.
3. Antenna placement on the airframe should be optimized to ensure the sensor can take full advantage of a 5 degree mask angle. This includes consideration of antenna location with respect to blockage from all aircraft components at typical level cruising attitude, as well as minimizing the effects of aircraft shadowing during typical maneuvers.
4. The antenna should be separated as much as practical from transmitting antennas. For small aircraft, the antenna also should be separated as much as practical from the windscreen to prevent case-to-antenna coupling.
5. For installations on rotorcraft, the effects of the rotor blades on antenna performance should be considered. This may require additional equipment functional flight evaluation or analytical evaluation to confirm the suitability of antenna location.
6. For multiple-sensor installations, the installer should reduce the likelihood that a single lightning strike affects all the sensors.