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  #11  
Old 06-11-2021, 07:01 AM
WA85's Avatar
WA85 WA85 is offline
 
Join Date: Apr 2010
Location: Huntsville, AL
Posts: 157
Default GPS antenna operating environments

A few examples of GPS antenna temperature operating limitations;

Garmin's GA 35 / 36 are manufactured and tested to meet TSO575-93G. The temperature operating limits are -67 F to 185 F ( -55 C to + 85 C) with no significant degradation of performance.

Garmin's GA 37 is manufactured and tested to meet TSO2300-126G. The temperature operating limits are -67 F to 185 F ( -55 C to + 85 C) with no significant degradation of performance.

Dynon lists their GPS 2020 antenna operating limitations as -40 F to 140 F (-40 C to 60 C) with no significant degradation of performance.

NOTE, one of the TSO's lists a maximum variation of dB gain with temperature variations.

Several Dynon GPS 2020 antennas have deformed from heat while mounted on the glare shield.

The highest temperature that one might expect with no degrading of performance is 185 deg F.

One might suppose that the inflight FWF temperatures are slightly below 185 deg F. Could one guarantee that the post shutdown FWF environment won't exceed 185 deg F on a hot Texas summer day?

Its all good until it isn't.

Fly Safe
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  #12  
Old 06-11-2021, 07:24 AM
swjohnsey swjohnsey is offline
 
Join Date: Nov 2018
Location: Kingsville, TX
Posts: 339
Default

I put a cotter pin from Lowe's in my tailwheel!
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  #13  
Old 06-11-2021, 08:38 AM
terrye terrye is offline
 
Join Date: Feb 2010
Location: Vancouver, BC, Canada
Posts: 944
Default Miserable Quality Loves Company - Part 2 Safety Margins

Thanks for posting the temperature limits from the TSO documents. I re-read Garmin's GA 35/36/37 Antenna Installation Instructions 190-00848-00 and 1.3.4 Environmental Specifications does list these TSO documents, but refers the 'installing agency' to the 'Dealer's Only' portion of their website to be able to access these documents. I don't have a dealer login.

Section 2.2 details the antenna mounting location. On the 2 seat RVs I'm not sure it is possible to comply will ALL of the guidelines. In particular 1. which states as close to level as possible (violated by aft fuselage mounting), 2. which states farther away from the tail section to avoid airframe shadowing.

So except for the temperature requirement (not explicitly stated in the Antenna Installation Instructions), the under cowl location best satisfies all of the guidelines.

The only other place I can think of that satisfies all of the guidelines for a side by side RV would be on a tray under the canopy just ahead of the baggage bulkhead. Anyone care to post pix of this installation and report on performance?
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  #14  
Old 06-11-2021, 08:42 AM
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wcalvert wcalvert is offline
 
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Location: Anacortes Wa
Posts: 232
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Wow, this drifted off hard into the land of GPS antenna location arguments ... !!

Back to the OP's original discussion point please.

And that cotter pin comment! Tail's for sure gonna fall off!
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  #15  
Old 06-11-2021, 08:48 AM
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airguy airguy is offline
 
Join Date: Oct 2006
Location: Garden City, Tx
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Default

Quote:
Originally Posted by WA85 View Post
One might suppose that the inflight FWF temperatures are slightly below 185 deg F. Could one guarantee that the post shutdown FWF environment won't exceed 185 deg F on a hot Texas summer day?
That would be "storage environment" versus "operating environment". Nobody cares if the variance exceeds limits when the unit is powered down - as long as it returns to acceptable limits after cooling again.

And no, I have not measured the under-cowl temperatures after shutdown - but I certainly don't dispute that they get HOT. I open my oil access door on shutdown to let it breathe, that certainly helps.

5 Texas summers so far for me (and just starting the 6th) with 1 Dynon and 2 Garmins under the cowling, 850 hours. So far it appears any variances disappear when temps come back down.
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Built an off-plan RV9A with too much fuel and too much HP. Should drop dead any minute now.
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  #16  
Old 06-11-2021, 09:12 AM
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Carl Froehlich Carl Froehlich is offline
 
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Location: Dogwood Airpark (VA42)
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A data point on under the cowl temps. This tape records the highest temp reached. Readings on both pMags match.

Carl
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  #17  
Old 06-11-2021, 09:19 AM
spatsch spatsch is offline
 
Join Date: Nov 2014
Location: Denison, TX
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Default

Quote:
Originally Posted by wcalvert View Post
Wow, this drifted off hard into the land of GPS antenna location arguments ... !!

Back to the OP's original discussion point please.

And that cotter pin comment! Tail's for sure gonna fall off!
Clear violation of AC 43.13 I think so we should form a mob and make sure Stan gets grounded instantly!

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  #18  
Old 06-11-2021, 09:31 AM
wawrzynskivp wawrzynskivp is offline
 
Join Date: Feb 2018
Location: Incline Village Nv
Posts: 71
Default 360 View, level, no EMI

First time trying to attach pic, hope it works.
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Last edited by wawrzynskivp : 06-11-2021 at 09:39 AM.
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  #19  
Old 06-16-2021, 09:47 AM
Canadian_JOY Canadian_JOY is offline
 
Join Date: Aug 2007
Location: Ontario, Canada
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Quote:
Originally Posted by wawrzynskivp View Post
First time trying to attach pic, hope it works.
Keeping to the "safety margins" theme, this installation may at first blush appear to meet the manufacturer's installation recommendations. However, when checking the TSO documents we'll find the antenna has been tested for lightning only when installed in the swept stroke zones of an aircraft. Normally this means somewhere on top of the fuselage itself.

Is there a degradation of safety margins associated with this installation? One could argue that an increased probability of the antenna becoming a lightning attach point represents a degradation of safety margins.

Equally, one could argue that, by placing the antenna in a locale free from other negatively-impacting factors (temperature, obstruction, being level with the horizon, etc) this location actually provides an enhancement in overall safety margin for the majority of the time.

There is no truly black-and-white answer to this question, only varying shades of grey, with each of us applying our own personal 'quality factor' to the variables before we produce our final assessment as to whether this particular shade of grey suits our own personal safety margins. This last point is critical - what's a show stopper to one person is no big deal to another person.

Coming back to earlier comments about GPS antennae mounted in the engine bay, an earlier poster provided temperature ranges for various antennas along with the caveat that operation within these temperatures would cause no significant performance degradation. Given that GPS antennas contain active pre-amplifiers, it's the noise contributed by those pre-amplifiers that's of concern. Generally speaking, the warmer the amp, the noisier it becomes (referred to as gain over temperature or g/t). Simply put, keeping the antenna cool means the GPS receiver is getting a better, more reliable signal. As with all things, you, the builder, must decide how much of the safety margins built into the GPS equipment you are comfortable eroding by intentionally choosing a consistently hot antenna mounting location.

Oh, one other interesting data point... When a G3X-equipped Grumman AA5B Tiger was being shown off at Oshkosh at the Garmin booth I noted the glaringly-white GA35 antenna was mounted on the glareshield. Now how much safety margin was eroded in that installation by placing that glaring white blob of plastic in front of the pilot's eyes? It's all a trade-off where we get to choose the compromises with which we are comfortable.
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  #20  
Old 06-16-2021, 01:14 PM
wawrzynskivp wawrzynskivp is offline
 
Join Date: Feb 2018
Location: Incline Village Nv
Posts: 71
Default Lightning Swept Zones

My rated navigation antenna is TSO 190C compliant when installed in 'Lightning Zones 1C(tail), 2A or 3' It's not in this pic but the TSO'ed antenna is probably in zone 2A (undetermined as far as I know) in the typical location on the back. The antenna in the pic isn't TSO rated.

Anyone know whether the RV-7 has undergone the FAA lightning zone determination process? My guess was no, so I didn't actually pursue where the available zones are. But aircraft dorsals aren't exclusively examined and rated. They do the whole airplane. Without the actual engineering I would guess my location in the pic is 1A, or maybe 1C if the nonconducting section of the tail fairing was sufficient to justify that. There is a chapter in the attached link that talks about non-conducting fairings. So, 1C (the tail) would be a TSO 190C compliant location if it were so determined, but to my knowledge there are no Swept Zones determined for the RV-7. Interesting reading about the swept zones if anyone is interested: https://www.faa.gov/regulations_poli...2-12111992.pdf

Yes, emphatically! The discussion about safety margins is always a discussion of tradeoffs.

Last edited by wawrzynskivp : 06-22-2021 at 12:55 PM.
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