Inexpensive DIY solution to GTX345R Bluetooth problem - with pics

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Chris B
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Inexpensive DIY solution to GTX345R Bluetooth problem - with pics

Post by Chris B »

One of my close friends happens to be a professional RF guru, and helped find a simple solution to the GTX345R Bluetooth problem in the DA40. The total cost is ~$30, is passive, requires no permanent change to the airframe, while providing excellent connectivity - even with two people and lots of luggage in the back! :thumbsup:

Chris

***********

The GTX345R location in the DA40 is almost diabolically designed to prevent Bluetooth reception. The transmitter is under the GARMIN label on the GTX345R, as shown by the black oval added with a Sharpie:
Image

The entire rear shell, flange and deck cover of the extended baggage compartment rests directly on top of the GTX345R. And is carbon fiber. Which is fantastic for weight, but blocks RF, leaving only small "transparent" gaps around the perimeter of the shell. It is remarkable that any Bluetooth signal reaches the front seats.
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This solution re-broadcasts the GTX345R output beyond the carbon fiber cocoon. Bluetooth uses the same frequencies as low-band Wi-Fi (2.4 GHz), which simplifies finding parts.

The only required tools are a #2 Phillips screwdriver to remove the baggage deck cover, and a 3mm hex driver to remove one shelf support screw. The latter is necessary to route an RF cable behind the shelf support.

The following parts list is a good starting point, but alternatives should also work fine. Just ensure that the connectors mate correctly. It is easy to confuse RP-SMA with SMA. :oops: I re-purposed a whip antenna with integrated extension cable (link), but that antenna is discontinued and the RF cable is too long anyway. A better approach is probably: ----plus---- ---- or ----
Microstrip antenna positioned on the GTX345R. Perpendicular to the oval slot provided the strongest signal.
Image

Installation with the rear baggage deck cover removed. This is the only access required. Plan to chat with my IA about drilling a small hole in the rear bulkhead ("minor mod") near the top of this photo (under the shelf support) to simplify routing.
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Completed installation, but will replace the tape with a plastic clamp using the left trim screw:
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Steve
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Re: Inexpensive DIY solution to GTX345R Bluetooth problem - with pics

Post by Steve »

Chris:

That is an ingenious approach! It would be interesting to measure the radiated bluetooth signal strength in the front cockpit with and without your antenna extender in place, perhaps with an app like this:

https://itunes.apple.com/us/app/bluetoo ... 78131?mt=8

I suppose that the FAA could object, reasoning that this antenna works in both directions, and could potentially feed higher bluetooth signal levels (from phones, etc.) to the inside of the GTX345R shielded case...

Steve
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Chris B
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Re: Inexpensive DIY solution to GTX345R Bluetooth problem - with pics

Post by Chris B »

Steve wrote: Mon Feb 25, 2019 2:20 pmIt would be interesting to measure the radiated bluetooth signal strength in the front cockpit with and without your antenna extender in place, perhaps with an app like this:

https://itunes.apple.com/us/app/bluetoo ... 78131?mt=8
Hi Steve -

We used a similar app from Google Play:
https://play.google.com/store/apps/deta ... rveillance
This works better with my phone & tablet. ;)

The app's RSSI readings are uncalibrated and don't relate to any specific dBi. They also bounce around, possibly because of how Bluetooth LE negotiates, but improved from about -100 to - 70. This is how we tuned the microstrip antenna location. We originally tried my friend's expensive RF spectrum analyzer, but Bluetooth uses frequency hopping in a crowded band, which makes it difficult to track any specific transmitter.

The bidirectional nature of this approach is essential. The GTX345R must also be able to "hear" the portable device to communicate. This is the same reason that boosting Wi-Fi AP transmitter power is ineffective. The remote device can't "shout back."

Chris
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Re: Inexpensive DIY solution to GTX345R Bluetooth problem - with pics

Post by TwinStarScott »

Chris: That is an ingenious approach! wrote:
Ditto Steve! Here are three additional thoughts / comments:

1) for future 345R installations - some might want to experiment with a longer cable run, by placing the antenna under the glare-shield. Then test for even better reception, before making a final decision and buttoning everything up. Although it might be somewhat difficult to get an avionics shop to sign off on this workaround, brilliant as it is. So might be best done after the fact? In hindsight, this remote antenna fix is something DAI should have requested Garmin incorporate into the initial release of the 345R. I believe Garmin already did something similar to this several years ago - only for the GMA 35C remote audio panel. Just our luck. . . .

2) the only possible long term drawback to this DIY antenna is that the 345 does not support the flight plan transfer feature - should DAI ever make GDU 15.xx available to legacy owners. Then if and when - the pre-wiring Chris's avionics shop has already done for adding a FlightStream up front, only the FS itself has to be installed to find out if legacy systems would have access to this feature. Here's a video that makes it seem likely that it would, as this link documents the flight plan transfer does in fact work with the 430/530 products (and not just the GTN series) and further documents that a FS210 is also required:

https://www.youtube.com/watch?v=HPNF3OaRDE4


3) Another possible advantage of adding “the Chris Antenna”, is with a FS installed, then theoretically two more electronic devices could be connected to access ADS-B IN features. As the FS and 345R - are both limited to two devices each. Yet in practice, if more than two are desired, adding a portable Garmin ADS-B receiver might be the easier option [furthermore, my understanding is Appareo receivers use WiFi and can connect up to 5 devices].
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Re: Inexpensive DIY solution to GTX345R Bluetooth problem - with pics

Post by Chris B »

It turns out that the tiny U.FL connectors need to be well-supported. IME, the U.FL <-> RP-SMA adapter connection is easy to accidentally disconnect. :oops:

After consulting with my RF guru friend, the new plan is to substitute a mini-whip antenna in place of the microstrip antenna. This requires using an SMA to RP-SMA adapter instead of the U.FL/RP-SMA adapter, but the connections are now all robust. My friend expects no material performance difference. The gap from the top of the GTX345R to the carbon fiber cover is ~10mm (3/8"), which provides plenty of room for the small whip antenna.

Since others may be implementing this solution, I wanted to update this thread even though I have not yet received or installed the new components:
My IA has also OK'd drilling the small access hole in the bulkhead for routing, lined with a rubber grommet. More pics after installation, but this could be a few weeks.

Chris
Last edited by Chris B on Wed Mar 06, 2019 3:49 am, edited 1 time in total.
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Chris B
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Re: Inexpensive DIY solution to GTX345R Bluetooth problem - with pics

Post by Chris B »

ScottSutton wrote: Tue Feb 26, 2019 1:35 am ... with a FS installed, then theoretically two more electronic devices could be connected to access ADS-B IN features.
I don't think this will work. Or at least is not supported.

The GTX 3X5 installation manual (Rev 4) indicates in Appendix C, page 9, table C-8:
Garmin wrote:Flight Stream 110/210
Both the RS-232 input and RS-422 output are required; internal Bluetooth must be disabled on the GTX with a FS 110/210 installation.
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Re: Inexpensive DIY solution to GTX345R Bluetooth problem - with pics

Post by Pehu »

Just received my new DA40NG (n399) from factory few days back and now I am trying toe figure out this bluetooth thingy.

I have GTC345R and G1000Nxi. But I do not see any bluetooth devices I can connect, not anywhere on the A/C. Garmin pilot app just talks about some manual, that I do not have or cannot find (45R pilot manual).

So how do I get the bluetooth paired to my ipad? How do I get bluetooth even on? :)

Also, contemplating buying FS-510. It should work with Nxi nowadays right?

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Re: Inexpensive DIY solution to GTX345R Bluetooth problem - with pics

Post by TwinStarScott »

Pekka,

In your airplane turn on your master and avionics switches, then:
1) open "Settings" in your ipad
2) Turn on "Bluetooth"
3) Look for "GTX-345R" under My Devices and tap on "Not connected"
4) a spinning icon will appear next to a blue circle
5) it should connect in a matter of a few seconds
If it doesn't:
6) open up the rear canopy door, fold down the left rear seat and try holding your ipad near the rear bulkhead and close to the baggage compartment floor.
7) once the ipad connects, then open up Garmin Pilot and notice the bluetooth icon in the upper right of the screen or go to the connext page within the app for even more detailed info.

Let us know what you find out.

Regarding the FS510, you're going to have to wait until DAI releases Phase II to be able to install this (from memory and per CFI Dave).

If the above fails and in the interim, you might want to consider purchasing a portable ADS-B unit - and if you're running Garmin Pilot the GDL-50 series are great units. I particularly like the GDL-52 because of the XM Wx option it provides (subscription only).

PS - Garmin's OEM sales manager to DAI recently told me Garmin is aware of reception issues with the GTX-345R in Diamond airframes and it's possible a fix will be released at some point in time.
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Re: Inexpensive DIY solution to GTX345R Bluetooth problem - with pics

Post by Pehu »

Scott, thanks for the info.

I do not see GTX device in my bluetooth devices on my Ipad. I tried putting the pad to aft compartment too. Waited for some time, refreshed the screen. Nothing.

I was able to find the pilots guide, it says the transponder is always in pairing mode.... ah well. Nothing there anyhow.

What I am looking at is anyways the flight plan transfer option (and DB updates), so I guess I have to wait for DAI to fix this. Or just use FS210, and update the databases in the old way.

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Re: Inexpensive DIY solution to GTX345R Bluetooth problem - with pics

Post by Pehu »

By reading some other postings, getting the "phase II" software would also require me to upgrade to GIA64 and new audio panel and whatnots. Correct? :)

So quite a costly upgrade to get FS510 working. Better option might be to install FS210.
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