GPS Antennas and Garmin Avionics

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Lotro
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GPS Antennas and Garmin Avionics

Post by Lotro » Wed Oct 24, 2018 4:12 pm

I'm working with my AME to put a GTX335 XPDR in a certified PA18. This will make me ADSB compliant for flight in the states. This installation requires an externally mounted GPS antenna.

I'd also like to install a Garmin G5 EFIS to use as an VFR artificial horizon (and thus do-away with my vacuum system). This installation calls for an externally mounted GPS antenna.

I'd also like to make use of a hand-me-down Garmin Aera 796 (panel mount with Air Gizmo) and a GDL-39-3D which don't technically require an externally mounted GPS antenna, but I think it would be a good idea.

Does anyone know if I require 3 externally mounted GPS antennas for this setup?

Thanks,

~Lotro

P.S. If there was an avionics place within 100 miles of me, I'd happily have someone into the shop for a look. Problem is the plane is in the process of restoration and currently not flyable.
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photofly
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Re: GPS Antennas and Garmin Avionics

Post by photofly » Wed Oct 24, 2018 5:43 pm

Both the G5 and the GTX335 can both receive and transmit GPS data via their respective serial ports, so I would expect you to be able to connect the antenna to one of them (your choice which) and feed the other with serial position data.

The AERA probably not, as it’s not a fixed install and so can’t be plumbed in to permanently installed systems.
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AirFrame
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Re: GPS Antennas and Garmin Avionics

Post by AirFrame » Thu Oct 25, 2018 6:50 am

I have a panel-mounted Aera 660 that feeds GPS data to my Dynon D10, my 327 transponder, and my MGL fuel flow meter. So a single GPS source *can* feed all of the units you have.

The 335 needs a WAAS certified source for ADS-B, so i'd suggest wiring those directly to each other so the 335 best meets ADS-B requirements. From the 335, there should be a serial port that will push NMEA on the TX line. Use that single onput to feed all of your devices on their serial RX line. I think Photofly is right, they can all communicate over serial, and in my experience you can split that serial line into three and feed directly to each device... I also put diodes on each line to protect each unit from the others.

Another option: The 335, G5, and 796 may all be new enough that they support Garmin's Connext interface, which is another option.
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ahramin
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Re: GPS Antennas and Garmin Avionics

Post by ahramin » Thu Oct 25, 2018 8:52 am

You would have to check the STC. Installing a transponder is Specialized Maintenance and requires an AMO. Is your AME working under an AMO?
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Re: GPS Antennas and Garmin Avionics

Post by Lotro » Thu Oct 25, 2018 7:24 pm

Thanks everyone for the insightful replies, I really, truly appreciate it. I've had a hard time getting straight answers and your answers really take a load off my mind.
ahramin wrote:
Thu Oct 25, 2018 8:52 am
You would have to check the STC. Installing a transponder is Specialized Maintenance and requires an AMO. Is your AME working under an AMO?
My honest answer is "I don't know". He maintains his own AME license, and may, or may not, work under another shop's AMO as required with permission. I'm satisfied of his competency and regulatory compliance, but you raise an important issue that I was not aware of. The local AMO doesn't have an avionics person on staff or available.

FWIW what I'm struggling with is the availability of a good avionics technician, not a willingness to pay one to have the work done. I'd probably have to fly someone in, or fly the plane somewhere, which at this stage isn't possible.

Ahramin - your question is valid, and I appreciate you raising the issue. I intend to address it accordingly.

Thanks everyone for your help.
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Re: GPS Antennas and Garmin Avionics

Post by photofly » Thu Oct 25, 2018 7:33 pm

To answer your original question though - no, you can only connect a GPS puck to one receiver.
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Re: GPS Antennas and Garmin Avionics

Post by ahramin » Fri Oct 26, 2018 8:26 am

Section 5.4 on page 91 of the G5 Certified Install Manual shows how to send GPS position from a GTX3X5 to the G5. The G5 does not require its own antenna. If your STC uses the same manual you're good with one antenna for both. I don't know about the Aera, but an antenna on the glareshield usually works well.

Where did the G5 and the transponder come from?
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AirFrame
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Re: GPS Antennas and Garmin Avionics

Post by AirFrame » Sat Oct 27, 2018 8:18 am

photofly wrote:
Thu Oct 25, 2018 7:33 pm
To answer your original question though - no, you can only connect a GPS puck to one receiver.
If the puck is just an antenna, this is correct, as the puck is only passing a radio signal down the wires to the receiver in the device (transponder, g5, gps). You can't reliably "split" that signal to multiple devices I don't think.

If the puck also contains a receiver, then the signal coming in on the wiring is digital (NMEA or Garmin format for example) and you can split that signal out to as many devices as you want until the signal strength isn't high enough that the devices can reliably receive it.
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Re: GPS Antennas and Garmin Avionics

Post by photofly » Sat Oct 27, 2018 11:27 am

AirFrame wrote:
Sat Oct 27, 2018 8:18 am
photofly wrote:
Thu Oct 25, 2018 7:33 pm
To answer your original question though - no, you can only connect a GPS puck to one receiver.
If the puck is just an antenna, this is correct, as the puck is only passing a radio signal down the wires to the receiver in the device (transponder, g5, gps). You can't reliably "split" that signal to multiple devices I don't think.

If the puck also contains a receiver, then the signal coming in on the wiring is digital (NMEA or Garmin format for example) and you can split that signal out to as many devices as you want until the signal strength isn't high enough that the devices can reliably receive it.
The GPS puck for Avionics is an active antenna, with filtering and amplifying, I believe. And it’s provided power by the unit it’s connected to. But it’s not the entire GPS receiver with NMEA output, it’s still an analogue RF signal output down the co-ax.

In theory you could split the signal to two receivers, like you can with VHF antennas (and eat the 3dB signal strength loss), but you’d still have to provide the power sourcing, and because it’s aviation you’d need an approved signal splitter of which I don’t believe there are any.
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Kirk: This is a dangerous mission. Likely, one of us will die. The landing party will be me, Spock, McCoy, and Ensign Ricky.
Ensign Ricky: Aw, crap.

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AirFrame
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Re: GPS Antennas and Garmin Avionics

Post by AirFrame » Sun Oct 28, 2018 9:05 am

photofly wrote:
Sat Oct 27, 2018 11:27 am
In theory you could split the signal to two receivers, like you can with VHF antennas (and eat the 3dB signal strength loss), but you’d still have to provide the power sourcing, and because it’s aviation you’d need an approved signal splitter of which I don’t believe there are any.
Right. This is why I fly an amateur-built...
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