GPS antenna for DYnon and Garmin 175

[Cap_Ax]

I am installing these two on a high wing Cessna, my ideal location allowed for 6” apart. I don’t see anywhere where Dynon specifies distance between these two, perhaps I miss it, however, I just read that Garmin recommends minimum 9” separation.

I already have holes drilled and antenna mounted already at 6” centers .

I was wondering if anyone has installed these closer than 9” minimum and has seen any issues?

Thanks!

 

[Rhino]

I always try to follow manufacturer recommendations, but numerous builders have had to implement changes that violate those. Sometimes they work. Sometimes they don't. The general rule in the electronics world is that distance between antennas should be more than 1/4 wavelength. For GPS, that's a little over two inches (6.11cm at 1.22Ghz to be exact). That doesn't necessarily mean Garmin is wrong. They may be considering factors that don't conform to the general rule, which is really intended for vertical polarization, and GPS isn't. But they could also be exercising excessive caution. Since the holes are already drilled, I personally would continue the installation just to the point necessary to allow operation, roll it out of the hangar, and test it to confirm. But that's just me. Maybe someone else has real world experience that would give you a better answer.

 

[Cap_Ax]

Thanks @Rhino !

 

[Rhino]

Found more antenna installation guidance from Garmin. Most of their separation guidance is based on the possibility of RF interference from transmitting antennas, and 24 inches minimum separation is recommended. Separation guidelines for non-transmitting antennas such as GPS are mainly based on the effects of physical shadowing. For GPS it's shadowing at elevations of about 5 degrees. The taller the antenna, the greater the chance of shadowing. The standard rule for Garmin GA 35, 36 and 37 antennas is a minimum recommended separation of six inches. And I highlighted recommended because it isn't mandatory. Based on this I'd be even more convinced to continue as is, if I were in your shoes, and just correct as necessary later.

I should probably also add the qualifier that I have absolutely no idea whatsoever how any of this might or might not conform to any regulations in New Zealand. The burden to determine that is unfortunately on you.

 

[Rhino]

By the way, please report your results here, good or bad. Many people come and search this forum for such information, even without becoming a member. The results of your efforts could potentially save someone else a lot of grief someday.

 

[GalinHdz]

What does the STC for your airplane say, if anything, about the installation?

 

[Rhino]

Does New Zealand have STCs?

EDIT: The STC I found for the GPS 175 didn't address antenna separation, though I can't be completely sure it's the most current STC. It tells you to reference the antenna requirements for antenna installation specs, which is why I went to the Garmin guidelines for the specs I referenced above. I believe that was also from an STC, but I'd have to find it again to confirm that.

 

[Rhino]

Okay, this is from the Garmin STC for GPS antennas:

To achieve the best possible low-elevation antenna gain (by minimizing pattern degradation due
to shadowing and near-field interaction), the GPS antenna shall be mounted with clearance from
other antennas, including passive antennas such as another GPS antenna or XM antenna. When
practical, installers shall use 12 inch center-to-center spacing between antennas. If 12 inch
spacing is not practical, installers shall 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/WAAS antenna pattern degradation.

But there's a preamble to that section, which says:

The installation guidelines presented here meet the intent of AC 20-138A section 16. The greater the variance
from these guidelines, the greater the chance of decreased antenna performance. Approach procedures
with vertical guidance are the most sensitive to these effects. LNAV only approaches, terminal
operations, and enroute operations may also be affected. Because meeting all of these installations
guidelines may not be possible on all aircraft, these guidelines are listed in order of importance to achieve
optimum performance. Items 3a, 3b, and 3c below are of equal importance and their significance may
depend on the aircraft installation. The installer should use their best judgment to balance the installation
guidelines.

https://static.garmincdn.com/pumac/190-01284-00.pdf

The separation guidance is in item 3 out of 4 under that preamble, so it isn't the most important. The preamble also allows for the existence of variance, and best judgement of the installer is allowed.

From looking at it, I would say real world antenna dimensions would affect the possibility of shadowing, as I alluded to earlier. The teardrop antenna has a higher vertical profile than the ARINC 743 antenna, so it would presumably present a higher probability of shadowing. Also keep in mind this guidance is only specifically designed for separation between Garmin GA 35, 36, and 37 antennas. It doesn't cover antennas from different manufacturers. I don't know what antenna Cap_Ax is using for his Dynon (the GPS-2020 presumably), so it's questionable how much that guidance applies. He may not even be using a GA 35, 36, or 37 antenna for his Garmin. He didn't say. New Zealand also doesn't have WAAS, which these antennas are designed for, and WAAS considerations may have something to do with the conditions in their separation guidance.

All of that is why I would still just continue as Cap_Ax is doing right now, were I in his shoes, and just make changes if the subsequent operational tests indicated a need for them. I would consider the ground test just a preliminary though, just to provide a better comfort level and to allow changes when it's easier to make them. The final decision would only be made with inflight testing after the full install, to ensure shadowing doesn't present a problem. But that's just me. And what do I know? I'm just a homebuilder. YMMV.

New Zealand only has a little over 40 STCs, and none are for Cessna or Garmin products, nor are any antenna specific. How manufacturer STCs produced for the US do or do not affect installations in New Zealand, I have no clue. I'm somewhat sure AC 20-138A section 16 doesn't apply there either, though I assume they have some similar regulation. That's why I made the disclaimer above. I have a hard enough time figuring out US restrictions and how they apply. I won't attempt to do that for New Zealand.

 

[Bud_Keil]

Don't have any experience on certified A/C but I installed a GPS175 antenna and a SV-GPS-2020 receiver/antenna on my RV-4 with a 7" spacing with no adverse effects. There just isn't much room on the RV-4 to meet ALL of the installation spacing requirements. I also installed a pair of Garmin antennas (a GPS and an ADSB) under the cowling of an RV-7, again with no performance issues.

 

[Cap_Ax]

By the way, please report your results here, good or bad. Many people come and search this forum for such information, even without becoming a member. The results of your efforts could potentially save someone else a lot of grief someda

*Update*
left the antenas where they were, about 6” center to center. Both GPS reported excellent reception, Garmin connected to 12 sats while taxing even before I reached the runway.

Flights was also uneventful as far as GPS connectivity.

Thanks everyone!

 

[pacecapt]

Spacing should not be an issue for GPS receive only antennas. The biggest issue with GPS is LOS to the satellites. They don't broadcast any RF.

 

[Rhino]

It can be an issue, just much less of one. Bleedover can always be a problem, as can shadowing on an antenna that depends on low angle reception like a GPS antenna does. But this particular scenario was unlikely to be a problem, and fortunately it wasn't.