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GPS Antenna - on or under the cowling

Why wouldn’t aft of the canopy be the default position?

Seems to me that aft of the canopy on top of the airplane would be best spot for the GPS antenna. Other then running the coax back there, what would be the downside? Getting the necessary separation from the com antenna that will likely be back there also shouldn’t be an issue.
 
Another option

I mounted mine on top of the crossbar which supports the seat backs in the RV 7. In other words, just aft and inboard of the pilots right shoulder. You’ll see the spot. Hope that makes sense. Great performance and easy install, FWIW.
Jim
 
Finally braved the cold (still around -22C) and went out to the hangar to decide on my antenna mounting location. I've decided to mount it on top of the left front access panel. Will make for an easy installation and check all of the boxes from Garmin's recommendations. I'll install it on Monday along with my GPS 175. Already mounted my SV-ARINC-429 module. Then comes the documenting for the purpose of removing the VFR only operating limit.

Thanks for the lively discussion here, it was very interesting. Not quite primer wars, but close.
 
I mounted mine on top of the crossbar which supports the seat backs in the RV 7. In other words, just aft and inboard of the pilots right shoulder. You’ll see the spot. Hope that makes sense. Great performance and easy install, FWIW.
Jim

If it works, it works, but it seems like a really big chunk of sky would be masked by the aft top skin, rear bulkhead, and a bit of the sides where the skins curve downwards. Which antenna? The SV-250 or -2020?
 
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

When you guys say "dropouts", do you mean loss of all signals? Or degradation of position accuracy? The raw GPS position calculation only needs 4 satellites to get XYZt. I suppose maybe the rearward mask might block the view to a WAAS satellite, which could mess it up, but I'm having trouble envisioning why the GPS solution as a whole would "drop out". Even if you masked a sizeable chunk of the rear view, there are always plenty of GPS sats in view overhead, forward and to the sides to get a fix...

Just piqued my curiousity a bit....
 
When you guys say "dropouts", do you mean loss of all signals? Or degradation of position accuracy? The raw GPS position calculation only needs 4 satellites to get XYZt. I suppose maybe the rearward mask might block the view to a WAAS satellite, which could mess it up, but I'm having trouble envisioning why the GPS solution as a whole would "drop out". Even if you masked a sizeable chunk of the rear view, there are always plenty of GPS sats in view overhead, forward and to the sides to get a fix...

Just piqued my curiousity a bit....

In my case “dropouts” include everything from a message on the IFD 440 saying “GPS Integrity Lost” with continued good position data for the pilot to complete loss of GPS signal and the IFD going into “Ded Reckoning” mode. Now to be completely up front, this most often happens when I have done something that the FAA “system” didn’t expect, like going upside down, which is perfectly legal, but the guys who designed ADS-B didn’t;t appear to anticipate..... ;)

Paul
 
In my case “dropouts” include everything from a message on the IFD 440 saying “GPS Integrity Lost” with continued good position data for the pilot to complete loss of GPS signal and the IFD going into “Ded Reckoning” mode. Now to be completely up front, this most often happens when I have done something that the FAA “system” didn’t expect, like going upside down, which is perfectly legal, but the guys who designed ADS-B didn’t;t appear to anticipate..... ;)

Paul

Wait, are you saying that if I fly inverted I can go into ADSB "private" mode and still be legal? Time to sell my 9 and buy a 7. Inverted cross countries, now there is a trick.
 
Some thick paint (not just metallic paint) and even dirty rain when just sitting on the ground can detune these narrow-band antennas. Dirty rain is not a problem in the air because it washes off. So says a personal friend who was an antenna design engineer for Rockwell Collins for 30 years.
 
Some thick paint (not just metallic paint) and even dirty rain when just sitting on the ground can detune these narrow-band antennas. Dirty rain is not a problem in the air because it washes off. So says a personal friend who was an antenna design engineer for Rockwell Collins for 30 years.

These are broadband, high gain, amplified, high frequency antennas...

L1 => 1575.42MHz, and the channel width is ~30MHz.

...and yes, debris on the face will attenuate the signal...
 
In my case “dropouts” include everything from a message on the IFD 440 saying “GPS Integrity Lost” with continued good position data for the pilot to complete loss of GPS signal and the IFD going into “Ded Reckoning” mode. Now to be completely up front, this most often happens when I have done something that the FAA “system” didn’t expect, like going upside down, which is perfectly legal, but the guys who designed ADS-B didn’t;t appear to anticipate..... ;)

Paul

Yep...my 430W does the same thing when I go upside down, BUT the Dynon seems to chug right on through it (at least, if it loses lock, it recovers it so quickly it's never noticed or alarmed).
 
The more modern systems use looking at multiple solutions, instead of RAIM, to assure ‘integrity’. 4 satellites gets you a solution; 6 or more gets you ‘integrity’.
 
The more modern systems use looking at multiple solutions, instead of RAIM, to assure ‘integrity’. 4 satellites gets you a solution; 6 or more gets you ‘integrity’.

Interesting. Do they do multiple LSQs on sets of 4+ sats (or perhaps all combinations of n-1 satellites available) and then some sort of averaging of the results (or choose the solution with the lowest error)? Waaaay back when I wrote GPS software, in the days pre-WAAS, we just ran an LSQ using all available satellites to reduce DOP, but that was in the stone age by comparison to today's equipment. :)
 
Interesting. Do they do multiple LSQs on sets of 4+ sats (or perhaps all combinations of n-1 satellites available) and then some sort of averaging of the results (or choose the solution with the lowest error)? Waaaay back when I wrote GPS software, in the days pre-WAAS, we just ran an LSQ using all available satellites to reduce DOP, but that was in the stone age by comparison to today's equipment. :)

"It was my understanding there would be no math..." -- Chevy Chase as Gerald R. Ford.
 
Having trouble finding a decent spot for a Dynon 250 as I have two top mounted antennas. Under the top cowl would be a candidate but I'm wondering about possible interference from Lightspeed ignition coils which sit rear and centre. Anyone had performance issues with this arrangement?
 
Almost convinced myself

I have almost convinced myself to put my two gps antennas recessed in the glareshiel. With a fabric covering, it will be hard to see them but they will have a good view of the sky. This is similar to the illustration in the Garmin G3x manual. The manual says they can be close to each other (6”?) but not next to transmitting antennas( they wont be.). As in a previous post, my stretch goal is to get XM weather in Antartica. :)

I am concerned about under the cowl because of the possible losses going thru the cowl, but the fitgher jets shot thru the radome, dont they?
 
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Having trouble finding a decent spot for a Dynon 250 as I have two top mounted antennas. Under the top cowl would be a candidate but I'm wondering about possible interference from Lightspeed ignition coils which sit rear and centre. Anyone had performance issues with this arrangement?

I mounted the Dynon GPS antenna/receiver on the glare shield on three RVs. I cover it with dark cloth to eliminate glare. Works perfect.

Carl
 
Thanks for the replies.This is an upgrade on a tip-up so I think I'll try the glare shield first as it is an easy placement. It's close to zero work to put it there. If the performance isn't up to scratch I'm no worse off having to move it than I am now.
I'd still like to know if proximity to electronic ignition gives any trouble with under the cowl installs.
 
Thanks for the replies.This is an upgrade on a tip-up so I think I'll try the glare shield first as it is an easy placement. It's close to zero work to put it there. If the performance isn't up to scratch I'm no worse off having to move it than I am now.
I'd still like to know if proximity to electronic ignition gives any trouble with under the cowl installs.

Jim -

PM sent...

B
 
I have had very good success at mounting the GPS antennae beside the support member that goes between the roll bar and the top of the baggage bulk head. They are out of the way and always have a good view of the sky.
 
That's a position I haven't seen before. Easy to mount without being a contortionist but how do you route the plumbing and cable ? Do you run it straight forward?
 
The cables run under the longeron then up thru the roll over and out thru a hole drilled in the roll over channel where the brace attaches. You must route the wires from the top of the roll over channel and out the bottoms where the large access hole is. It would be a bear going the other direction. Make small angle brackets to hold the GPS ant. I have done 7 this way and it is very easy.
 
I have had very good success at mounting the GPS antennae beside the support member that goes between the roll bar and the top of the baggage bulk head. They are out of the way and always have a good view of the sky.

You're back to that antenna blocking problem though both to the rear and one side with the above location. The antenna is a critical device for an IFR box, best suggestion is to as close as possible follow the manufacturers advice, they've already done the testing and performance validation. If you don't do any IFR then I guess it doesn't matter, but then why install an IFR navigator is the first place. Do you think you are going to go faster with the antenna inside?
 
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There is no blockage of the GPS signals unless you are upside down. I guess if you wanted to you could mount an antenna on the belly so that when inverted you wouldn't loose the signal.
 
There is no blockage of the GPS signals unless you are upside down. I guess if you wanted to you could mount an antenna on the belly so that when inverted you wouldn't loose the signal.

Not sure how that would be possible in the position you suggested. Here's some info on GPS antenna mounting parameters.
GPS reception is not just straight up but to within 20 deg of the horizon in all directions.

Just because it 'works' does not mean the installation is ideal.

http://manuals.spectracom.com/VS/Content/ASG/AntennInst.htm
 
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Unless the GPS antenna is mounted outside and no obstacles are in the way of a clear view of the horizon in all directions all installations are a compromise. I originally said that I had good luck putting the GPS antenna in that position never said it was perfect.
 
Not sure how that would be possible in the position you suggested. Here's some info on GPS antenna mounting parameters.
GPS reception is not just straight up but to within 20 deg of the horizon in all directions.

I'm with you Walt, but the required mask angle above the horizon is more like 5 degrees. (See AC 20-138D, section 13-1.1; this is also mentioned in the installation manual for the GTN) Is this a problem in, say, Texas? Perhaps not, but when you are attempting an LPV approach into Anchorage those WAAS satellites will be less than 20º above the horizon.

I put my GPS antennas (and my ELT antenna) on the outside of the airplane, following the manufacturer's installation guidance. Your mileage may vary.
 
I was think of putting one of my GPS antennas under the cowl and I was think of putting one under the canopy behind the passenger on the turtle deck, will that location work? Its an RV8.
 
I've had good performance with a 796 and an EKP 5, portable GPS using a glareshield antenna. Never had a drop out, however if I were planning to go IFR I would definitely stick to the guidelines for antenna placement. If I didn't already have two antennae on the top I would also follow the guidelines. Retrofits are often a compromise.
 
see in person

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.

Brian, any chance to come by and see your installation in person - i'm in Georgetown and building a 6

Chris Rogers
 
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