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GPS Antenna - on or under the cowling

Flying Canuck

Well Known Member
Patron
I finally got my GPS 175 that I ordered in November but have not been able to decide where I'm installing the GA 35 antenna. It's down to 2 options separated by 2 or 3 inches vertically. As far forward on the center of the cowling that I can, aft of the rear baffles. Only question is if I take the easy route and install it visible on top or if I come up with a bracket and put it underneath the cowling.

I know that both have been done and are feasible, is there anyone with experiences that might help me decide? I think I'm leaning towards under just because it will look so much better, but any performance concerns?

Thoughts anyone?
 
Well....

There's some anecdotal evidence that active electronics suffer shortened life spans due to the heat under the cowling.

I chose to make fiberglass panels for the "boot cowl" and undermount the GPS & SXM antennas.
 

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That's an interesting idea. I've already the access panels there, I could mount on top of one - although I'd have to watch for clearance from a GPS antenna on my glare shield. For putting it under a new fiberglass panel, I'd be concerned about two things, I'm not great with fiberglass fabrication and the fact that only the panel above is fiberglass, would that limit reception from angles that don't go through the fiberglass? That would be only shallow angles, so maybe it's fine. Do you have this installation flying?

I like the toaster attachment in your last picture.
 
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I like the toaster attachment in your last picture.

I was working on the kitchen counter -- which my wife just loves :D

GPS reception is solid, 12+ satellites with >3 bars each & 3d Diff. I have over 1000 flight hours now with GNS430W and GTN650Xi in two different RV-7s with this solution.

IIRC there is a <1° masking to the front, 5° above the horizon to the sides, and 25° to the rear. However, with 31 satellites in the constellation, that small of a masking doesn't present an operational problem.
 
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There's some anecdotal evidence that active electronics suffer shortened life spans due to the heat under the cowling.

I chose to make fiberglass panels for the "boot cowl" and undermount the GPS & SXM antennas.

Do you mind if I steal this idea??
 
When mounting GPS antennas, remember that they do not only look straight up to see satellites but also look at angles to pick up satellites closer to the horizon. So you don't want to shade the areas around the antenna and block off part of the search area. Make sure your panel opening is large enough to permit this.
I prefer mounting on a shelf in front of the FW as close to the underside of the cowling as possible, and about 6" in front of the FW, especially for WAAS antennas. This minimizes the shading from the fuselage behind the antenna.
 
Since you (the OP) are in Canada you may wish to check with Transport Canada. I know some of your IFR rules are different than the US's, e.g., 2 separate navigation boxes. If TC says you must install the 175 in accordance with the Garmin installation manual, then you're out of luck with any of the hidden locations previously mentioned, as, IIRC, that manual doesn't authorize them. Here in the US the FAA doesn't seem to care, at least not for EAB.
 
My reading of the CARs didn't catch any mention of installation requirements. The GPS175 installation manual gives the following guidance for antenna installation and nothing much more.

The GPS antenna should:
1. Be installed as near to level as possible with respect to the normal cruise flight attitude of the aircraft.
2. Be installed in a location to minimize the effects of airframe shadowing during typical maneuvers.
3. Be installed a minimum of two feet from any VHF COM antenna or any other antenna which may emit harmonic
interference at the L1 frequency of 1575.42 MHz.
4. Be installed a minimum of two feet from any antennas emitting more than 25 watts.
5. Be installed a minimum of nine inches (center to center) from other antennas, including passive antennas such as
another GPS or XM antenna.
6. Be installed a minimum of three inches from the windscreen.
7. Have a twelve inch center to center spacing between GPS antennas.

This doesn't say anything about hidden installations as long as airframe shadowing is minimized (#2).
 
Claude,

I’ve three gps antennas installed on the firewall directly below the cowl and meet the criteria you referenced above. Two non-WAAS units were installed by the builder for the dual 430s in 2008. Two years ago I installed a WAAS antenna between the non-WAAS antennas for my Garmin 335 transponder. Haven’t had any trouble with them at all.

I do recommend you make sure the mounting brackets you use are secured to the firewall using nut plates so it will be easier to remove them for access to things like your battery, hose replacement, wiring or cables. I really like the setup I use.

Jim
 
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Below is a direct quote from the 'Garmin GPS175 Maintenance manual with instructions for continued airworthiness, STC.....'
(I can't see the installation manual since Garmin won't publish it)
-----------------
"Check GPS antenna installation, connections, and cable routing. The GPS antenna must be mounted on the top of the aircraft."
----------------
As I said, here in the US we must use "Approved" gps boxes for IFR, and the FAA doesn't seem to be taking a hard stand on adherence to the TSO'd manuals. Just make sure TC is on the same page.
 
Here is the CARs reference (points a-i omitted for brevity)
605.18 No person shall conduct a take-off in a power-driven aircraft for the purpose of IFR flight unless it is equipped with


(j) sufficient radio navigation equipment to permit the pilot, in the event of the failure at any stage of the flight of any item of that equipment, including any associated flight instrument display,

(i) to proceed to the destination aerodrome or proceed to another aerodrome that is suitable for landing, and

(ii) where the aircraft is operated in IMC, to complete an instrument approach and, if necessary, conduct a missed approach procedure.

I also found this in CAR 551.108, looks similar to what you cited, but vague enough to be read in multiple ways (should, not must).

(a) Installation of Radio navigation equipment required by sections 605.14, 605.15, 605.16 and 605.18 of the CARs should be carried out according to manufacturers' instructions and tested and evaluated using Chapter 5 of FAA Advisory Circular AC 23-8B, as amended from time to time, as guidance.
(amended 2005/12/01)

(b) In addition, according to section 551.03 of the Airworthiness Manual, the installation of equipment must meet the applicable standards of the certification basis of the aircraft.
(amended 2009/12/01)

The GPS installation menu does have a drawing after the text I quoted earlier that shows example placements. I've attached it. It does say there that antenna must be on top of aircraft, but it does so pointing to an installation under the plane. Yes, I'm sure that could be read simply as you say to mean exterior on top. It's no legal document, probably room for interpretation. I'm not at all sold on the idea of an internal mount, but these references don't seem to be rigid enough to prohibit this. Again, just my opinion.
 

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Here is the CARs reference (points a-i omitted for brevity)

. It does say there that antenna must be on top of aircraft, but it does so pointing to an installation under the plane.

Notice the circle with the line thru it, like a U turn sign with a line thru it, meaning "No U turn". Circle with a line thru it means "Not allowed here".
 
Notice the circle with the line thru it, like a U turn sign with a line thru it, meaning "No U turn". Circle with a line thru it means "Not allowed here".

No question, you are not allowed to put the antenna under the plane. Since this doesn't point to a hidden installation on the top side of the plane, it can't really say anything about it. Just a loophole, not necessarily my opinion.
 
I mounted the GA-35 antenna for my GDL82 as high as I could on the windshield brace of my RV-7A. It passed FAA ADS-B performance test on the first try and hasn't generated any errors since. I plan to connect it to a GPS175 when it's installed and then feed the GDL82 through the GPS175. If you zoom in, you can see it's mounted crossways. The coax cable is tie-wrapped along the rollbar and is mostly hidden.
 

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I've just had a look at the Staff Instruction document that is the key to getting Transport Canada to remove the VFR Only operating limit. This is somewhat more strongly worded. A couple of highlights, bolding is mine.

An application for the removal of the “VFR Only” operating condition of a Special Certificate of Airworthiness – Amateur-built shall be evaluated on the basis of the following principles:
(a) The evaluation is first and foremost an evaluation of the data, i.e., the documentation that supports the installation, testing and calibration of the IFR equipment;
(b) This procedure applies both to traditional and non-traditional IFR equipment installations;
(c) The aircraft has to be equipped in accordance with CAR 605.18;
(d) The IFR equipment does not need to be certified in accordance with TSO standards, and hence does not require authorised release certificates;
(e) Any measuring device or test equipment used during the IFR equipment installation must meet the specifications of the manufacturer of the measuring device or test equipment with respect to accuracy;
(f) The IFR equipment installation does not need to be approved by Transport Canada National Aircraft Certification;
(g) The IFR equipment installation must conform to technical data acceptable to the Minister; sources of acceptable data include, but are not limited to:
(i) the equipment manufacturer’s installation, testing and calibration instructions, drawings and recommended methods;
(ii) Transport Canada advisory documents;
(iii) FAA Advisory Circular 43.13-1 and -2, UK CAA Civil Aircraft Inspection Procedures (CAIP), JAA Advisory Circulars, (ACJ) and publications issued by
recognized authorities on the subject matter concerned;
(iv) owner devised own data, which need not be approved, but must be subject to an appropriate level of review or analysis, or be shown to comply with recognized industry standards, or commonly accepted practice;
(h) The installation, testing and calibration of the IFR equipment can be performed by the aircraft owner or other person authorised by the owner. However, if it is determined that the complexity of the installation, testing and/or calibration, which includes the operation of the test equipment required for the completion of the work, is beyond their capability, an AME and/or AMO should be consulted and/or utilized; and
(i) The aircraft owner or an AME can sign the maintenance release in respect of the installation, testing and calibration of the IFR equipment.

After reading that, I don't think it would be prudent to try to make a hidden installation work. Ultimately Bob you are spot on with your first take on this.
 
I finally got my GPS 175 that I ordered in November but have not been able to decide where I'm installing the GA 35 antenna. It's down to 2 options separated by 2 or 3 inches vertically. As far forward on the center of the cowling that I can, aft of the rear baffles. Only question is if I take the easy route and install it visible on top or if I come up with a bracket and put it underneath the cowling.

I know that both have been done and are feasible, is there anyone with experiences that might help me decide? I think I'm leaning towards under just because it will look so much better, but any performance concerns?

Thoughts anyone?
Does it work? yes Are there some heat issues? sometimes!!

DAR Gary
 
Another idea...

This is where I put the third GPS antenna (G3X Touch, PFD.) I fabbed a small "L" shaped shelf and used two of the canopy attach screws for "permanent" mounting.

As an aside - there is guidance about where other antennas (VOR/LOC/GS, ELT 121.5/243/406) should (shall, must, ...whatever...) be mounted and none of the Archer stuff conforms to it, nor does the Van's factory ELT antenna location in the RV-14 (antenna inside the canopy, behind the roll bar...).

We learned through scientific method, experimentation, and data analysis, that we could locate such protuberances inside/behind non-RF opaque substrate -- the composite guys, and the military, have been locating antennas out of the boundary layer for 50+ years.
 

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There's some anecdotal evidence that active electronics suffer shortened life spans due to the heat under the cowling.

I chose to make fiberglass panels for the "boot cowl" and undermount the GPS & SXM antennas.

Great solution! I haven't rivet the forward skin on my RV8 yet so I think I will give it a try .
 
I have two thoughts on this:
- Garmin has told people not to mount antennas in the engine compartment (no source, sorry) and has denied warranty on antennas that have overheated and failed because of said placement (also no source.) not all antennas will fail in this way, but it’s been known to happen.

- My IFR GPS failing can be a significant safety of life issue. As such I don’t play games with putting antennas or other components in areas not specifically called out as recommended. I’m not gonna fall out of the sky if my antenna fails in IMC, but I will notify atc (as per far) and my workload will spike at least momentarily as I sort out what to do next. I’m happy to “be experimental” in many other areas but not with IFR equipment.

Just my 2 pesos and worth less than I got paid for it ;)
 
glareshield?

Just curious - why not the glareshield? Probably a good reason, not trying to second-guess you, but that's a good location for me, and working pretty well. Doesn't bother me in the least, and it's very close to my electronics.
 
Just curious - why not the glareshield? Probably a good reason, not trying to second-guess you, but that's a good location for me, and working pretty well. Doesn't bother me in the least, and it's very close to my electronics.

If that was a question for me (the OP), I already have a GPS antenna on my glare shield (AVMAP Ultra) that I would need to relocate. It also might not meet the condition of being 3" away from the windscreen (I'd have to measure). I have a tip up canopy, the glare shield moves up with that. This might shorten the lifespan of the antenna cable with the regular movement.
 
There's some anecdotal evidence that active electronics suffer shortened life spans due to the heat under the cowling.

I chose to make fiberglass panels for the "boot cowl" and undermount the GPS & SXM antennas.

This is a beautiful work obviously but I wonder if a best solution for a hidden antenna? Beside the level of effort that I will take to do this, the added weight and chance of water leaking when there are other places like wingtip is available. Also, please note there is minimum length of the Coax that is needed for the antenna.
 
When I built my -8, I hid the GPS antenna for my 430 under the cowl, and it worked great - until I upgraded to a 430W, at which points I suffered lots of drop-outs due (I was told by experts) to the fact the view of satellites to the rear was obstructed by teh firewall, and forward by the rear engine baffle. Makes sense - the WAAS GPS is just more sensitive to blockage.

It was never really a problem until ADS-B came along, and now the FAA KNOWS any time I am in coverage and my GPS loses “integrity” - so I moved the antenna up to the turtle deck and haven’t had any further issues.

So its not just about IFR capability anymore - have enough ADS-B dropouts, and you can get a letter.....
 
I chose to make fiberglass panels for the "boot cowl" and undermount the GPS & SXM antennas.


Is this going to create a reception issue? Driving with a friend who was having issues with XM radio reception in his truck, after phone calls to tech support with no help, we figured out the fiberglass canoe that was straddling the antenna was creating interference.

Still trying to figure out where to put mine. I think behind the baggage on top. There will be enough clearance with my slider, and be really close on the 6" clearance edge to edge
 
It's good to hear you'll have clearance for the antenna. I'd more concerned about water ingress while flying in rain. In my -7A slider, it seems no matter how much I've tried to seal off the rear canopy skirt from water ingress, I always get some dripping onto the rear baggage.
 
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GPS antenna x 2 installed under the cowl like Steve says below. 1800 hours and 14 years later, still going strong. Easy, peasy.


When mounting GPS antennas, remember that they do not only look straight up to see satellites but also look at angles to pick up satellites closer to the horizon. So you don't want to shade the areas around the antenna and block off part of the search area. Make sure your panel opening is large enough to permit this.
I prefer mounting on a shelf in front of the FW as close to the underside of the cowling as possible, and about 6" in front of the FW, especially for WAAS antennas. This minimizes the shading from the fuselage behind the antenna.
 
Under the cowl

Make a tray and mount 4 to 5 inches forward of the firewall at the top works great. Seems to have a good view looking rearward and of course forward. Temperature sensors in this area show less than 120 F max. Sensor Push is a nice portable mini sensor if once wants to check temps.
 
Just curious - why not the glareshield? Probably a good reason, not trying to second-guess you, but that's a good location for me, and working pretty well. Doesn't bother me in the least, and it's very close to my electronics.

On the first RV-10 I did both the SkyView GPS antenna/receiver and the GTN-650 GPS antenna on the glare shield, one on each side. Worked just fine. Just mind the minimum length of coax for the GTN-650 antenna.

For the new RV-10 I’ll again mount the SkyView GPS antenna on the glaresheild but move the GTN-650 antenna on top of the cabin, feed wire going up the hollow center brace.

Carl
 
My thoughts... ADS-B antenna put anywhere you like, ADS-b dropouts don't cause me an issue and don't really care.
IFR navigators always external mounted, these are critical to safety.
 
Canada

Transport Canada won't care about the actual installation - put antenna anywhere you wish.

The requirement for 2 nav sources however is correct if you want IFR CofA.

For the antenna mounting the GPS in the tray pictured above will work fine. The XM antenna will not work as well given our higher latitude (will be fine in Texas :)) - I had to move mine due to repeated XM signal lost messages.
 
The XM antenna will not work as well given our higher latitude (will be fine in Texas :)) - I had to move mine due to repeated XM signal lost messages.

Correct -- the mask angle created by the mounting obscures visibility of the XM satellites (probably North of 45°, heading North, would be worse case with this mounting.)

Just out of curiosity, did this improve after the launch/commissioning of XM-5?

Another reason to live in Texas :)
 
Claude:

I've mounted two GPS antennas under the cowl on a little shelf attached to, and forward of, the firewall.

Coming up on six years and just shy of 600 engine hours, both still perform as expected...

(VFR operation only)

John
 
Claude:

I've mounted two GPS antennas under the cowl on a little shelf attached to, and forward of, the firewall.

Coming up on six years and just shy of 600 engine hours, both still perform as expected...

(VFR operation only)

John
Same - 4-5 years, 4-500 hrs, non metallic paint, non WAAS. One antenna. No issues.
 
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Claude:

I've mounted two GPS antennas under the cowl on a little shelf attached to, and forward of, the firewall.

Coming up on six years and just shy of 600 engine hours, both still perform as expected...

(VFR operation only)

John

Same - 4-5 years, 4-500 hrs. One antenna. No issues.

I’m getting ready to make the decision on mounting my Garmin GPS WAAS antennae.

We’re either of yours WAAS?

Also, doesn’t the ADS-B get its position from the WAAS antenna?

I’m thinking historical president might not apply to WAAS and ADS-B :confused:

As an aside, how did you deal with the minimum cable length? Just coil it up?
 
I’m getting ready to make the decision on mounting my Garmin GPS WAAS antennae.

We’re either of yours WAAS?

Also, doesn’t the ADS-B get its position from the WAAS antenna?

I’m thinking historical president might not apply to WAAS and ADS-B :confused:

As an aside, how did you deal with the minimum cable length? Just coil it up?

Sent you a PM
 
GPS under cowl

Have VFR RV-6 with two GPS antenna under the cowl about 6" forward of the firewall. A Dynon SV-GPS-250 was there for 700+ hours then a few months ago quit became intermittent and a couple months ago totally quit. It has been replaced with same new one and works fine again. The other antenna is a Trigg TA70 which has 100+ hours on it. Other than the Dynon 250 quitting both antenna have been trouble free with good reception.
 
GTN 650 IFR Navigator WAAS antenna

When I upgraded the avionics I removed the GNS 430 and installed a GTN 650 and used the existing gps antenna bracket under the top engine cowl for the new WAAS antenna. Occasionally I get GPS dropouts for a couple seconds. I have a black gps antenna mounted on the glareshield for the backup Aera 660 gps. If I swap these two antennas I would have a big bright white antenna right in my face. Right now I never even see the black antenna. If I swap them is there a specific material I could use to cover the white antenna?

Someone also stated that it needs to be 3” aft of the front windscreen, is this critical?
 
16 years with a 430W antennae under the cowling. It sits up high about half way between the back baffle and the firewall.
 
Someone also stated that it needs to be 3” aft of the front windscreen, is this critical?

You are probably referring to the Garmin GPS 175 Installation Manual Antenna Location guidelines that I posted. I've attached the whole page.

This is an ordered list, so this is the 6th most important location consideration.
 

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A little thread drift

Wouldn’t under a painted cowl be like painting the GPS antenna. Garmin say do not paint the antenna so I taped it with non reflective black tape and put it on my glare shield so it wouldn’t reflect off the wind screen - kind of ugly. Would it work to paint it flat back?
 
Wouldn’t under a painted cowl be like painting the GPS antenna. Garmin say do not paint the antenna so I taped it with non reflective black tape and put it on my glare shield so it wouldn’t reflect off the wind screen - kind of ugly. Would it work to paint it flat back?

Paint containing metal solids, carbon, tend to absorb and/or reflect RF at the frequencies used by GPS, thus reducing the effectiveness of the antenna. Easier to say "No Paint" than to maintain an exhaustive list of approved/disapproved paints.
 
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My Plan

My plan is to wrap it in black cloth and put on glare shield.
The Garmin manual shows a glare shield mount that is recessed so that the top of the antenna is flush with glare shield profile. I might do this instead.

Last option is under cowl; don't really like that idea since I plan to be capable of IFR, and I want my XM music when I visit Antarctica.
 
Bjdecker question re: metallic solids ?

I have often wondered when guys confirm acceptable component life and performance under the cowl, does that include metallic paint ?
 
GPS antenna

With the old non WAAS antenna for the GNS 400 I used to have I never had an issue with GPS reception. As stated in my earlier post, I have been getting GPS dropouts occasionally with the new WAAS antenna for the new GTN 650. Based on what someone said about needing some rearward view of the WAAS antennas for gps reception I looked at my antenna mount. Yep- the WAAS is mounted butted right up against the firewall and actually sits a little underneath the mounting lip for the top cowl.

Thanks to Ironflight, “I suffered lots of drop-outs due (I was told by experts) to the fact the view of satellites to the rear was obstructed by the firewall”- this is exactly what happened to me.

I think I’ll leave it on top of the engine bay just under the top cowl but modify the bracket and move it forward.

Thanks for the as always great info from the forum
 
I have been getting GPS dropouts occasionally with the new WAAS antenna for the new GTN 650.

Out of curiosity, what does the GPS Status page on the GTN 650 show when you get a GPS dropout? Is it some or all satellites?
 
Brian,

The dropouts are usually only about 10” or so and by the time I get to the GPS status page it’s back online. Maybe I’ll fly around with that page called up sometime and see. I suspect this won’t happen again once I reposition the antenna.
 
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