GPS 2020 mounting location

B

BlueCH750

Low and Slow
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Apr 11, 2016
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I have read the posts, but would appreciate a forum opinion or Dynon input.

I would like to mount the the puck on top of the fuselage. I can achieve over three foot separation from my VHF antennae, but would place the puck within a foot of my ELT whip.  Typically, the ELT would not be an "emitter" (transmit) during normal flight so does anyone see an issue with locating the GPS puck in near proximity to the ELT antennae?  Thanks for all thoughts and input.

Dave
 
I thought there were some reports of ELT antennas picking up some RF from COM and reradiating them, interfering with GPS...it wasn't the GPS antenna's fault, it was the ELT, but I could swear I saw reports of this issue...
 
I thought there were some reports of ELT antennas picking up some RF from COM and reradiating them, interfering with GPS...it wasn't the GPS antenna's fault, it was the ELT, but I could swear I saw reports of this issue...
Click to expand...

I was told that Garmin is aware of a known problem with some GTN-650 installations. A lot of factors involved. On my first GTN-650 install when transmitting on the GTN-650 the GTN-650 would loose GPS signal and drop out - usually right when you really want it on the approach. The SkyView GPS was not at all affected. Working with the Garmin approved avionics shop he walked me through disconnecting the ELT antenna to see if that was the problem or not (it was not). For me the issue was related to the Comm #2 install even though I was not transmitting on that radio. When I replaced Comm #2 with the Dynon radio the problem went away.

Not applicable to the Dynon GPS install.

This was three years ago so I don't know if this is still an issue with GTN-650s or not.

Carl
 
Yes, there can be an issue with very old 121.5 ELT's capturing the COM radiation and re-radiating on 121.5 where the 13th harmonic is near the GPS band. This also has dependence on your COM-ELT distance.

However, this isn't an issue with a 406 ELT (even if it has 121.5 as well), and given this is a new plane, I assume a modern ELT.

Some COM radios will also take out a GPS because of the same harmonic issue. They make notch filters to solve all of these issues if you run into them.

On top of all of this, the Dynon GPS is much more modern than most Certified GPS designs and is pretty robust against a single frequency aggressor, so it would likely not be effected even if the issue existed.
 
All,

Thanks for the discussion.  The issue Steve referenced was my main concern...discussed in depth on the RV site a couple year ago my some builders and the exact same issue Dynon just addressed in reply #5.  My ELT is indeed a more modern one (ACK model E-04) along with GPS 2020.  Between antennas, I have 40" spacing VHF to ELT and 15" from ELT to GPS.  VHF to GPS approx 48"  I feel better about location now!!  Thanks Dynon and others who posted!!

Dave
 
Yes, there can be an issue with very old 121.5 ELT's capturing the COM radiation and re-radiating on 121.5 where the 13th harmonic is near the GPS band. This also has dependence on your COM-ELT distance.

However, this isn't an issue with a 406 ELT (even if it has 121.5 as well), and given this is a new plane, I assume a modern ELT.

Some COM radios will also take out a GPS because of the same harmonic issue. They make notch filters to solve all of these issues if you run into them.

On top of all of this, the Dynon GPS is much more modern than most Certified GPS designs and is pretty robust against a single frequency aggressor, so it would likely not be effected even if the issue existed.
Click to expand...

Excellent discussion, thanks! And glad to hear that you guys are building rock-solid equipment like that. FWIW, I've never seen either my GPS-250 or the GSP-2020 I recently replaced it with EVER drop out (and usually, they'll gt a fix inside of my metal hangar, while my Garmin certified antenna has to have a clear view of the sky!).

Does remind me to go through all those frequencies that Garmin lists in their install manual, though, and check the SV sat reception, just in case...

And yeah, I'm one of the troglodytes who put in an old-style 121.5 MHz ELT LOL! I do have a PLB on the pilot's harness, though...
 
Something else to consider with Garmin GPS antennas that I have not yet confirmed. A friend spoke with a rep that said they recommend at least 10' of cable between the antenna and the receiver (e.g. 430W).

Anyone else heard this? Seems to contrast with typical cable length recommendations.
 
Here are some of the GTN-650 GPS antenna install instructions:

3a. The GPS antenna should be mounted no closer than two feet from any VHF COM antenna or
any other antenna which may emit harmonic interference at the L1 frequency of [ch61472]
1575.42 MHz. An aircraft EMC check (reference VHF COM interference check in Post
Installation Checkout procedures) can verify the degradation of GPS in the presence of
interference signals. If an EMC check reveals unacceptable interference, insert a GPS notch
filter in line with the offending VHF COM or the (re-radiating) ELT transmitter.
3b. The GPS antenna should be mounted no closer than two feet from any antennas emitting more
than 25 watts of power. An aircraft EMC check can verify the degradation of GPS in the
presence of interference signals.
3c. To achieve the best possible low-elevation antenna gain (by minimizing pattern degradation
due to shadowing and near-field interaction), the GPS antenna must be mounted with
clearance from other antennas, including passive antennas such as another GPS antenna or
XM antenna. When practical, installers will use 12 inch center-to-center spacing between
antennas. If 12 inch spacing is not practical, installers will use the maximum center-to-center
spacing from adjacent antennas, but never less than 9 inch center-to-center spacing. Spacing
less than 9 inches center-to-center results in unacceptable GPS/SBAS antenna pattern
degradation.
4. To maintain a constant gain pattern and limit degradation by the windscreen, avoid mounting
the antenna closer than 3 inches from the windscreen.

From my experience, the GTN-650 GPS signal dropping out problem occurred when transmitting on the GTN-650 itself. Transmitting on Comm #2 caused no GPS signal degradation. When this problem went away after replacing Comm #2 with the Dynon radio I concluded the issue were harmonics from the GTN-650 when transmitting were coming up into the Comm #2 via its antenna feed line - and Comm #2 (MicroAir) itself re-radiating the interference into the GTN-650 antenna. I note that on the initial install the Comm #2 radio (radio, not the comm #2 antenna) was physically close to the GTN-650 GPS antenna. This, I suspect, aggravated the issue. The only change made for this problem to go away was changing out the Comm #2 radio. All antenna (comm and GPS) mounting remained the same.

I can only assume the root issue here is the GTN-650 comm section is not as clean as it should be, and all the install guidelines provides mitigation.

Note - no problems ever encountered with the Dynon GPS-250 install.

Bottom line - mount your antennas where you want but with some common sense. For many people meeting all the guidelines will be challenging.

Carl
 
Something else to consider with Garmin GPS antennas that I have not yet confirmed. A friend spoke with a rep that said they recommend at least 10' of cable between the antenna and the receiver (e.g. 430W).

Anyone else heard this? Seems to contrast with typical cable length recommendations.
Click to expand...

Yes...from the installation manual

NOTE
GPS antennas in Table 1-3 require a cable loss between 3 dB and 7 dB.  The coaxial connectors and adapters, such as TNC to BNC, add additional loss to the cable and should be considered when computing the cable loss. A typical loss of 0.2 dB can be used for each connection. To maintain integrity of the WAAS signal, the GPS antenna coaxial cable must have a minimum of two shields (e.g. RG-400 or RG-142B).
NOTE
If RG-142B or RG-400 is used, 1.5 dB equates to a length of approximately 6.5 feet of cable with a connector on each end. RG-142B or RG-400 cable can be used as long as the length is less than 35 feet. For longer lengths, use low-loss double or triple shielded 50[ch937] coax.
For very short runs, where the loss is less than 1.5 dB, additional cable should be used to increase the loss to within 1.5 dB to 6.5 dB. This additional cable may be coiled, taking into account the minimum bend radius of the cable.

BTW, this is a little surprising that a field rep wouldn't know that a) the information is in the installation manual, and b) it's not "at least 10'". Just goes to show you...RTFM, don't listen to sales guys.
 
As an FYI on the tech background with the cable length- the Garmin GPS is designed to assume there would be cable, and most planes need 10+ feet. So they put a lot of gain in the system, assuming there would be loss. So to maximize performance for the average plane (10+ feet) they designed the system so that less than 3dB of loss would overpower it. It's a pretty simple solution where you can just throw more cable at it if you are closer than that.

Of course, the Dynon solution of no RF cable anywhere is a bit easier (and much cheaper) still ;)
 
Something else to consider with Garmin GPS antennas that I have not yet confirmed. A friend spoke with a rep that said they recommend at least 10' of cable between the antenna and the receiver (e.g. 430W).

Anyone else heard this? Seems to contrast with typical cable length recommendations.
Click to expand...


BTW, this is a little surprising that a field rep wouldn't know that a) the information is in the installation manual, and b) it's not "at least 10'".  Just goes to show you...RTFM, don't listen to sales guys.
Click to expand...

Sounds to me (based on the Dynon response and the manual excerpt) like the > 10' thing may be legit, though not scientific. The way I heard it was that seemed to be the length that worked best.

As always, YMMV.
 
No issues here with ELT and GPS-250 close together. About 800 hours sense late 2011. (Photo Attached).
 

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Here is where I ended up mounting by top antennae(s).  Gives me 44 inches from the VHF to GPS 2020 and 14 inches from the ELT to the GPS 2020. Thanks for all the discussion on the forum!! Dave
 

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FWIW, I used 7ft of RG400 cable with a 90degree TNC and a 90degree BNC connecting the Garmin 430W antenna to the unit. The G430W antenna is about 2ft from my DYNON GPS250 antenna both under the glare shield with a large piece of Plexiglas providing the unobstructed view of the sky. Has been in operation since March 2013 with absolutely no problem. The G430 is quick to get a lock on but the DYNON GPS is still even faster.

67-GlareShieldwithGPSAntennas2_zpsf5daa35e.jpg


8-)
 
FWIW, I used 7ft of RG400 cable with a 90degree TNC and a 90degree BNC connecting the Garmin 430W antenna to the unit. The G430W antenna is about 2ft from my DYNON GPS250 antenna both under the glare shield with a large piece of Plexiglas providing the unobstructed view of the sky. Has been in operation since March 2013 with absolutely no problem. The G430 is quick to get a lock on but the DYNON GPS is still even faster.

67-GlareShieldwithGPSAntennas2_zpsf5daa35e.jpg


8-)
Click to expand...

Interesting approach. How are your satellite signals on the status pages for each device? It appears that you are masking a pretty sizable angle of view to the open sky, which would limit the number of satellites from which you get a signal usable for the PVT solution.
 
Interesting approach.  How are your satellite signals on the status pages for each device?  It appears that you are masking a pretty sizable angle of view to the open sky, which would limit the number of satellites from which you get a signal usable for the PVT solution.
Click to expand...
Every time I have checked I have at least 6 strong satellites in view on the ground and most of the time while flying I get 9 or more. Except for once that the area I was flying in was NOTAMED for GPS outage, I have never lost GPS on either device. Caveat; I have a fiberglass airplane so that makes a difference for the very low angle satellites. IMHO for this approach to antenna location in a metal airplane you must have a fiberglass glare shield instead of an aluminum one as I have and even that may not be good enough, especially in a high wing "spam can".

8-)
 
Question: I have an all-wood aircraft (GP-4). I have followed the designer's plan of keeping all antennae inside airframe structure and used copper strips of specific size and layout for all my equipment. Can I place the Dynon GPS 2020 antenna without restriction inside my fuselage? Is the copper tape an issue?

John
 
Can I mount the Dynon GPS2020 antenna under the fiberglass engine cowl of my RV-7A, just in front of the firewall, about an inch or two below the fiberglass? And can I mount a GNS430 GPS antenna right next to it?

Will the GPS antenna's receive signal okay thru fiberglass?
Will engine compartment temperatures be too high for GPS antennas?
Will two GPS antenna's installed next to each other cause interference with each other?

Sorry, if you answered this already. I can't find it.
I'm building a bracket now. Advanced Sys. is working on quick panel.

thanks
 
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