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Feedback on antenna placement?

rdamazio

Well Known Member
Hi all.

As I'm about to start building the tailcone, I did some research on all the antennas I may need for my 10 and how to position them (wrt ground planes, interference, etc.).

My plan is to have this or something equivalent to this:
  • IFR+VFR GPS (e.g. G3X+GTN)
  • NAV1/NAV2
  • COM1/COM2
  • Transponder + ADS-B In/Out
  • Stormscope
  • Maybe, just maybe a TAS (since I will fly outside the US)
  • ELT

(I'm still far from buying the actual avionics, so the exact equipment may change over time, but the antennas for them are less likely to change)

I looked at antenna models, and am leaning towards this set:
  • 2 GPS antennas (those don't change much between manufacturers)
  • 2 Delta Pop COM antennas (one top, one bottom)
  • 1 Bob Archer wingtip NAV antenna
  • 1 Delta Pop Transponder
  • 1 ADS-B (e.g. GDL39R)
  • 1 L3 Stormscope (antenna is NY-163)
  • If I get the TAS, 2 directional antennas
  • ELT antenna

To make this a bit harder, I'm also considering adding the MotoPOD hardpoints to have some extra luggage room when needed - but when the pod is on, it takes up most of the belly.

After reading through all the documentation I could find for these types of antennas and the installation restrictions, I made a first CAD draft of what those would look like on the aircraft:

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Thoughts? Am I overthinking this? It's obviously impossible to respect all the ground plane sizes (the circles), but I tried to get close (and followed this post about the TAS antenna ground plane specifically). I'm also assuming that it's OK to not have the TAS when the pod is on.

I'm also unsure about all the antennas being laterally aligned with the longitudinal axis - is that the recommended practice? (I suspect that the UAT antenna may be too close to the battery mount and may need to be shifted anyway)

(UWB on there is one of the Delta Pop UWB antennas, which I'm still totally undecided about, but placed anyway - also OK to be unavailable with the pod on)

Any feedback appreciated!
Rodrigo
 
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If you are going to mount the XPDR antenna that far aft then recommend selecting a transponder that does not mount in the panel. That way you can mount the transponder itself closer to the antenna.

For the ELT, mount it horizontally under the empennage fairing, the base of the antenna attached to the bulkhead just forward of the fairing.

For the ADS-B receiver antenna, mount it anywhere you want but here again mount the ADS-B receiver with it. Note that there will be minimum spacing requirements between the ADS-B antenna and transponder antenna. There is no concern with proximity to the battery(s). Delta Pop also makes a nice inexpensive ADS-B receive antenna.

The cabin top is problematic for comm antenna mounting. if you go this way, recommend moving it aft to perhaps center the two bulkheads aft of the canopy top. If you leave it on the cabin top, add a ground plane (several small wires glassed into the bottom of the top or a thin aluminum plate). Note also the overhead area aft of the baggage a common area to mount the EFIS ADHARS modules so keep in mind any spacing requirements.

Recommend you consider the mission and decide if you really need the pod or not. As is the RV-10 can hold an amazing amount of stuff - enough to easily hit gross weight limits.

Carl
 
I would put the transponder just aft of the firewall in the center tunnel. Then you can put the ADS-B antenna just aft of the rear baggage wall for spacing (wouldn't hurt to mount the ads-b receiver there as well. For now, I wouldn't worry almost at all about antenna placement. That can all be done later when you make your final decisions on the avionics. You actually make end up with a transponder/ads-b receiver in one that may not require 2 antennas, since they are virtually the same antenna anyway.

The NAV antenna can feed 2 NAV radios, so you should be good there.

The ELT antenna under the tail one fairing is my recommdation as well. You can mount the ELT there also.

For now, I would get at least 2 of Van's ELT mount kits for the tailcone side walls to be able to mount an ADS-B receiver and possibly other equipment when you get there.

I have never mounted a COM antenna under the wing, but I don't see any rof Aston why that wouldn't work, except possible the yaw effect of the drag created. A low drag antenna would certainly be the best there. My main concern here would be the chance of hitting something with the antenna when taxiing in tight spaces. Under the fuse has to be clean because of the wheels.
 
For thought, with the G3X you will have the magnetometer. I believe most are mounting that in the tail. Mine will be locatied at the top of the second station aft of the baggage bulk head. You will need to be concernered with magnetic interference. I installed the cat wisker VOR/ILS antenna on the tail, under the horizontal. My GPS antennas, GTN 650 and one G3X same spot as yours, with a third dash mount for the second screen. Transponder and ELT in area that Jesse said, mine is right of the battery on a shelf. Both the Teds transponder and ADSB will be in the tunnel just aft of the battery. I will use the ADSB iIN for weather and TCAS
 
My suggestion: drop the ADSB antenna. Plan to go with a mode S-ES transponder instead. Since you're starting from scratch the cost difference between mode SES and UAT will be minimal. Your GTN can serve as the ADSB nav source for several Garmin and (less expensive) Trig transponders. ADSB-in depends on what panel equipment you want to display it on, but on my GRT HX I use a $550, factory refurbished Sky radar with two small antennas mounted internally, on the door posts. Works fine, brings in lots of ground stations as well as airborn aircraft. You did not say where you are going out of the country, but remember in Europe there is no UAT, just tramsponders with mode S-ES.

I'd also suggest that just one nav receiver is enough, but again that is for the US.
 
With regard to the archer wingtip antenna. I mounted one in each wingtip. For two years it worked great until one day I was shooting an ils getting vectors to final and lost the GS signal for a few seconds. After doing several flights to determine the issue, I mapped a radiation pattern with my spec analyzer which showed significant notch due to the wing and fuse. I confirmed it in both wing tip antennas..

I now have a cat whisker installed on the belly as far aft as I could. Problem solved.

The other issue I noticed using the wingtip location was a 15 foot offset from the centerline of the runway when shooting the loc/ils.

If you are going to use a panel mount X ponder, I would mount its antenna under the pilots seat to avoide such a long run. If you are going to mount a remote transponder rear of the baggage bulkhead then the position you have selected would work fine.

Leave room for an adsb blade antenna on the belly further forward, or rearward depending on which xponder location u end up using.

I have a com antenna on the top and belly. Mainly driven by ground communications at larger airports.
 
Thanks for all the replies.

Replies by topic (split due to message size limit):

NAV:

With regard to the archer wingtip antenna. I mounted one in each wingtip. For two years it worked great until one day I was shooting an ils getting vectors to final and lost the GS signal for a few seconds. After doing several flights to determine the issue, I mapped a radiation pattern with my spec analyzer which showed significant notch due to the wing and fuse. I confirmed it in both wing tip antennas..

I now have a cat whisker installed on the belly as far aft as I could. Problem solved.

The other issue I noticed using the wingtip location was a 15 foot offset from the centerline of the runway when shooting the loc/ils.

15' should be OK, it's not like I'm shooting CAT III - even if I break out at 200' minimums I should still have time to correct (not that I routinely shoot approaches to minimums either).

My original thought was to use a whisker-type antenna at the top of the VS, so I did leave a conduit in there, I can easily switch, but a lot of people have had a lot of success with those wingtip antennas. Speaking to Don from Delta Pop at Oshkosh I got the impression that as long as they're installed strictly according to instructions, they should work properly - did you have any deviation from instructions, or did you get the reception gap even with the correct install?

I'd also suggest that just one nav receiver is enough, but again that is for the US.

That's easy to change later, given that a single antenna can be used for multiple receivers. I may as well start with a single NAV.

Transponder and ADS-B:

If you are going to mount the XPDR antenna that far aft then recommend selecting a transponder that does not mount in the panel. That way you can mount the transponder itself closer to the antenna.

...

For the ADS-B receiver antenna, mount it anywhere you want but here again mount the ADS-B receiver with it. Note that there will be minimum spacing requirements between the ADS-B antenna and transponder antenna. There is no concern with proximity to the battery(s). Delta Pop also makes a nice inexpensive ADS-B receive antenna.

If you are going to use a panel mount X ponder, I would mount its antenna under the pilots seat to avoide such a long run. If you are going to mount a remote transponder rear of the baggage bulkhead then the position you have selected would work fine.

Leave room for an adsb blade antenna on the belly further forward, or rearward depending on which xponder location u end up using.

I would put the transponder just aft of the firewall in the center tunnel. Then you can put the ADS-B antenna just aft of the rear baggage wall for spacing (wouldn't hurt to mount the ads-b receiver there as well. For now, I wouldn't worry almost at all about antenna placement. That can all be done later when you make your final decisions on the avionics. You actually make end up with a transponder/ads-b receiver in one that may not require 2 antennas, since they are virtually the same antenna anyway.

The issue with the belly below the seats is the MotoPOD - I tried to position all essential antennas away from it (I can't really give up the XPDR if I'm flying with the pod). Maybe if I draw it as its actual shape rather than the box I used, there'll be enough room near the corners for those antennas (but I still worry about them being obfuscated by the pod).

I do plan to use a remote-mount transponder and ADS-B, so I could position those far in the back. Does anyone have experience with having a long run of antenna coax vs having a long run of data cables? Do either of those work well?

For now, I would get at least 2 of Van's ELT mount kits for the tailcone side walls to be able to mount an ADS-B receiver and possibly other equipment when you get there.

Do you mean this one? Looking at the plan PDFs, OP-41.pdf mentions F-10112-L, which doesn't match the above part number (also unclear if that mount comes as part of one of the kits, or has to be ordered separately). Also, I don't see dimensions for either of those anywhere - can anyone share those so I can see what would really fit in them?

My suggestion: drop the ADSB antenna. Plan to go with a mode S-ES transponder instead. Since you're starting from scratch the cost difference between mode SES and UAT will be minimal. Your GTN can serve as the ADSB nav source for several Garmin and (less expensive) Trig transponders. ADSB-in depends on what panel equipment you want to display it on, but on my GRT HX I use a $550, factory refurbished Sky radar with two small antennas mounted internally, on the door posts. Works fine, brings in lots of ground stations as well as airborn aircraft. You did not say where you are going out of the country, but remember in Europe there is no UAT, just tramsponders with mode S-ES.

Europe and South America would be my primary long-distance trips (though I admit I dream of joining EarthRounders some day :D ).
I'm only counting on ADS-B working in the US (hence the TAS).

The reason I'm hesitant to move the ADS-B antennas inside is that I've played with the stratux recently in busy airspace in an aircraft that also had built-in ADS-B, and while I got a lot of the same nearby traffic as the built-in system did, I didn't get it all - there were multiple missing targets (and interesting, a couple that only showed up in the stratux). If I do get the TAS, maybe that won't matter much anyway.

ELT:

For the ELT, mount it horizontally under the empennage fairing, the base of the antenna attached to the bulkhead just forward of the fairing.

The ELT antenna under the tail one fairing is my recommdation as well. You can mount the ELT there also.

I like the idea, but I've heard several people recommending against that. The arguments were not enough ground plane, putting your antenna inside a Faraday cage (the fuselage), mounting the antenna horizontally gives the wrong polarization for satellites to pick up the 406MHz signal and increasing the chances of the fuselage bulkhead itself breaking the antenna during a crash.
Thoughts on this? Has anyone tested this beyond the 121.5 signal being picked up by a nearby receiver? I'm interested in any measurements or arguments that indicate the 406MHz would still radiate properly and get you rescue.
 
COM:

The cabin top is problematic for comm antenna mounting. if you go this way, recommend moving it aft to perhaps center the two bulkheads aft of the canopy top. If you leave it on the cabin top, add a ground plane (several small wires glassed into the bottom of the top or a thin aluminum plate). Note also the overhead area aft of the baggage a common area to mount the EFIS ADHARS modules so keep in mind any spacing requirements.

I have never mounted a COM antenna under the wing, but I don't see any rof Aston why that wouldn't work, except possible the yaw effect of the drag created. A low drag antenna would certainly be the best there. My main concern here would be the chance of hitting something with the antenna when taxiing in tight spaces. Under the fuse has to be clean because of the wheels.

I have a com antenna on the top and belly. Mainly driven by ground communications at larger airports.

I've seen a few people mount COMs under the wing, always relatively close to the fuselage, never far out where it might hit something. I do worry about ground reception, which is why I added the second COM to the top.
This is the antenna I'm looking at for the under-wing mounting (the "lower mount" one), but I'm open to any recommendations on low-drag COM antennas. Notice I put mine on the right wing, while I'm assuming NAV and pitot tube will be on the left wing - the thought was to minimize interference and balance the drag somewhat.

Thanks for the hint on the ADHARS - I'll definitely look into the guidelines for that (one more consideration).

I'm aware of the ground plane issues with the cabin top, I just didn't find any other places to put it (given that Stormscope is supposed to be far back and not close to the battery ground current path). If I change the ELT to be inside the empennage fairing I can certainly move both further back, but probably still not all the way onto metal (have to check).

Motopod:

Recommend you consider the mission and decide if you really need the pod or not. As is the RV-10 can hold an amazing amount of stuff - enough to easily hit gross weight limits.

You may be right - I was thinking of the pod more in terms of added volume than weight (e.g. if I go camping, there's all this equipment which is not that heavy but is big). Can anyone who decided in favor of or against using the pod share their decision/rationale?

I guess I'll also run the numbers on a few loading scenarios (with some reasonable assumption of empty weight) and see if the pod really makes sense. If it doesn't, then antenna placement becomes a whole lot easier.

GPS/etc:

I installed the cat wisker VOR/ILS antenna on the tail, under the horizontal. My GPS antennas, GTN 650 and one G3X same spot as yours, with a third dash mount for the second screen.

Did you have a particular reason to have 3 GPS antennas? afaik the screens will communicate GPS data to one another anyway - the odds of two of them failing and a third still working are very slim, so I'm assuming there's an advantage other than reliability?

For thought, with the G3X you will have the magnetometer. I believe most are mounting that in the tail. Mine will be locatied at the top of the second station aft of the baggage bulk head. You will need to be concernered with magnetic interference.

Good point. Yet another consideration.
(still wishing that FOG gyros or carrier-phase-diff GPS were cheap enough to get rid of magnetic sensors altogether in GA)

I'm wondering if anyone has tried mounting it out on the wing, and if that reduces magnetic interference at all (intuitively, it sounds like a much "cleaner" place compared to inside the fuselage close to a lot of antennas).
 
On the ELT antenna mount. Read any ELT install manual and they tell you to mount it vertically. For 121.5mhz this makes sense IF YOU ASSUME when you need the ELT the plane is sitting upright. For me I figured the odds were I'd be upside down so the ELT antenna on top would be broken off or in the mud. Mounting the antenna horizontally under the eppenage fairing with the base on the last bull bulkead provides more than adequate ground plane, and the structure of the tail will protect the antenna when the plane flips over. Add to that the real benefit is the 406 mhz signal hitting a satellite that has little regard for polarization, the vertical mounting is, in my opinion, a secondary consideration.

On the wingtip, not sure how an antenna in each wing could be phased to use on one VOR receiver. I do know that if one does not carefully install and tune these antennas performace can be questionable. I built my wingtip VOR antenna out of scrap aluminum and used a standard MFJ antenna analyzer to make adjustments. I adjusted the dimensions to take full advantage of the wingtip width. The antenna has performed well for both VOR and ILS reception.
 
On the ELT antenna mount. Read any ELT install manual and they tell you to mount it vertically. For 121.5mhz this makes sense IF YOU ASSUME when you need the ELT the plane is sitting upright. For me I figured the odds were I'd be upside down so the ELT antenna on top would be broken off or in the mud. Mounting the antenna horizontally under the eppenage fairing with the base on the last bull bulkead provides more than adequate ground plane, and the structure of the tail will protect the antenna when the plane flips over. Add to that the real benefit is the 406 mhz signal hitting a satellite that has little regard for polarization, the vertical mounting is, in my opinion, a secondary consideration.

I understand that the antenna is more protected that way, but will the signal transmitted from it be useful?
From threads like this one it seems that it's a neverending debate (like the primar wars :) ), so I was hoping anyone would had applied a more scientific method to measuring the antenna patterns when mounted inside.
I'm not an RF engineer, but I'd assume that the vertical mounting recommendation is based on the antenna's gain pattern.

On the wingtip, not sure how an antenna in each wing could be phased to use on one VOR receiver. I do know that if one does not carefully install and tune these antennas performace can be questionable. I built my wingtip VOR antenna out of scrap aluminum and used a standard MFJ antenna analyzer to make adjustments. I adjusted the dimensions to take full advantage of the wingtip width. The antenna has performed well for both VOR and ILS reception.

Can you share what your process for tuning it was?
 
Bill, please clarify your statement that you had an Archer in each wing tip and saw a null at certain angle(s): Were you running both antennas to one receiver? That absolutely will produce nulls.
In all honesty I don't watch the GS while being vectored, but I never have noticed a loss of the localizer while being vectored.
I don't think anyone claims the Archer to be better than an external antenna. But a lot of people, including me, think it's good enough. And I don't worry about poking my eye out! -:)
 
Bob, I have two nav radios one was connected to each antenna. The problem with loosing the GS is the auto pilot disconnects when it looses the signal. It only occurred at a specific bank angle and radial to the localizer. It really should not be much of a surprise that the wing rib adds some directivity. When I originally installed them I did not think that the radiation pattern would matter that close in, but it did. I am still using one of the archers for the second nav radio. I was bummed I had to add that drag under the belly!
 
This sounds like a software fix is in order. I cannot think of a reason for the autopilot to disconnect due to loss of GS prior to localizer intercept. I use a Trio Pro driven by the GRT HX; it doesn't engage the approach mode until I push a soft key ("execute"), and I never push it until established on final course.
 
I engage the approach mode before I am established. I would rather let the a/p do the intercept. It auto switches from heading mode to loc when your within around two dots of the course.
The archer antenna is tuned for the loc/vor frequencies, the glide slope signal uses the 329-335 MHz band. I have no complaints with the archer for vor reception.
 
Not sure how far apart your GPS antennas are in real distance, but check the install manuals. Many TSO'd installs require significant distance between two GPS antennas as they can interfere with one another.

--Ian
 
Not sure how far apart your GPS antennas are in real distance, but check the install manuals. Many TSO'd installs require significant distance between two GPS antennas as they can interfere with one another.

--Ian

Thanks. Here's what the GTN install manual says:

"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."

(it also recommends 2ft distance from COM antennas)

So you're right - my GPS antennas were too close to each other.
 
I engage the approach mode before I am established. I would rather let the a/p do the intercept. It auto switches from heading mode to loc when your within around two dots of the course.
.

The GRT logic is similar, except that the autopilot remains in altitude hold mode until the "execute" soft key is pushed. e.g., once ILS arm is selected, the autopilot will track a vor or gps or heading until intercepting the localizer, which it then tracks. But it does not look for, or track, the GS until the "execute" button is pushed. I only push that button when established on final. This prevents exactly the problem you described.
 
Rodrigo,
My layout is similar to what you proposed but all of my antennas are mounted externally:

- #1 COM antenna (RAMI AV-10) is topside mounted right behind the seam of the cabin top and tailcone on the tailcone top skin. I did extend a ground-plane onto the cabin top using 3 strips of copper foil.

- #2 COM antenna (RAMI AV-17 bent whip) is mounted on the bottom of the right wing behind the spar in the bay created by the 1st and 2nd inboard ribs.

- GPS antennas (GTN 650 and one of my G3X's -- the other G3X antenna sits on my glare shield) are mounted externally on top of the cabin between the doors. They are about a foot apart with the forward one to the right of centerline and the rear one left of centerline.

- Transponder antenna (DeltaPop) is immediately behind the baggage bulkhead on the bottom of the tailcone to the right of centerline. My transponder (a GTX 23ES) is back there too on a shelf that Van's sells as an AHRS mount to the left of the battery box. It's configured for ADS-B out it and FAA flight tested without issue.

- ELT (ACK E-04) antenna is mounted on top of the tailcone between the VS and #1 COM antenna.

- VOR/LOC/GS antenna (RAMI AV-12L cat whisker) is mounted to the bottom tailcone under the HS. I have a splitter that sends the feed to my SL 30 and GTN 650.
 
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Rodrigo,
My layout is similar to what you proposed but all of my antennas are mounted externally:

Thanks for the reply, it's great to hear that someone has a similar, successful setup :)

- #1 COM antenna (RAMI AV-10) is topside mounted right behind the seam of the cabin top and tailcone on the tailcone top skin. I did extend a ground-plane onto the cabin top using 3 strips of copper foil.

Dumb question: I thought aluminum was preferred for the ground plane (on aluminum planes) to minimize corrosion? How does one decide which material to use?

- GPS antennas (GTN 650 and one of my G3X's -- the other G3X antenna sits on my glare shield) are mounted externally on top of the cabin between the doors. They are about a foot apart with the forward one to the right of centerline and the rear one left of centerline.

Is there a reason not to put them both on the centerline?
I'm also curious about why many people use 3 GPS antennas if the systems can communicate GPS data to one another.

- Transponder antenna (DeltaPop) is immediately behind the baggage bulkhead on the bottom of the tailcone to the right of centerline. My transponder (a GTX 23ES) is back there too on a shelf that Van's sells as an AHRS mount to the left of the battery box. It's configured for ADS-B out it and FAA flight tested without issue.

Thanks. Looking at another thread you replied to, I see the picture - looks like a good way to mount it.
 
My #1 Com antenna is only about 3 inches behind the cabin top. So just over half of the ground plane is the tailcone upper skin. However to extend the ground plane onto the fiberglass of the cabin top, the foil tape made an easy solution (Lancair and Glasair guys use this technique a lot, or so I'm led to believe).

As for the GPS antennas, the Garmin install manuals direct you to offset the antennas in this manner.

I have 3 GPS antennas because I have 3 GPS receivers. Each G3X display has a built-in VFR GPS receiver and of course the IFR 650 needs it own. Also the systems don't share GPS data per se. For example, if I left the G3X's GPS antennas off , yes, they would display flight plan data from the 650 but the G3X's internal GPS's would be blind. Should I lose the 650 in this scenario, I'd lose all GPS navigation.
 
Oh and the reason my Com #1 is so close to the cabin is to create separation for it, the ELT antenna and the G3X 's magnetometer which I mounted in the tailcone.
 
Oh and the reason my Com #1 is so close to the cabin is to create separation for it, the ELT antenna and the G3X 's magnetometer which I mounted in the tailcone.

I was thinking about separation for ELT antennas earlier today. Since the ELT isn't likely to be used when the other antennas are in use, does it really need separation?
 
I was thinking about separation for ELT antennas earlier today. Since the ELT isn't likely to be used when the other antennas are in use, does it really need separation?

I dunno. All of the installation instructions specify separation from other antennas. I took a conservative approach in the hopes that my layout would work the first time and not require any monkeying around with moving antennas. So far so good.
 
Supposedly (I haven't confirmed) strong com transmissions (close to the ELT antenna) get picked up by the ELT antenna, travel down the coax to the ELT which generates lots of harmonics, which travel back up and out the antenna, and can interfere with gps reception. I know some gps installations require 'traps' on the ELT coax to supress these signals.
 
I was thinking about separation for ELT antennas earlier today. Since the ELT isn't likely to be used when the other antennas are in use, does it really need separation?

Antenna theory is pretty complicated. Ask any amateur radio operator about a 'Yagi' beam antenna. The 'director' and/or 'reflector' are just pieces of metal tubing hanging out in space - no wires or coax at all. But they have a real effect.
 
Supposedly (I haven't confirmed) strong com transmissions (close to the ELT antenna) get picked up by the ELT antenna, travel down the coax to the ELT which generates lots of harmonics, which travel back up and out the antenna, and can interfere with gps reception. I know some gps installations require 'traps' on the ELT coax to supress these signals.

There is a known potential for interaction between a Com transmission, ELT and GPS. The post installation checks in the Garmin manuals lists the specific frequencies to check. It is a harmonic issue that ends up in the GPS band and can cause signinicant reduction in the SNR's for the GPS signal. I have actually witnessed this and had to move antennas.
 
The GTN-650 has a known problem with RFI created by its own comm transmission being picked up by other aircraft components and end up wiping out the GTN-650 GPS reception.

I had this exact problem. When transmitting on the GTN-650 the GTN-650 GPS reception was totally blocked. Doing some digging with a respectable avionics shop I found that most common mode for this issue is the ELT antenna and connections. For me however that was not the problem (ELT mounted near the battery and ELT antenna mounted on the last full bulkhead - antenna extending under the empennage fairing). I tracked it down to COMM #2 providing a means for the GTN-650 generated RFI to get feed back to the GTN-650 GPS antenna. Rearranging COMM #2 wiring and such I solved the problem. Note - the GTN-650 GPS had no inference when transmitting on COMM #2, just when the GTN-650 itself was transmitting.

Bottom line - such interference is hard to predict so don't expend too much time chasing your tail on it. Follow the instructions as best you can but understand you will need to make some compromises. When you do your panel checks call up the GTN-650 GPS satellite page and monitor received GPS signals as you go through various transmission modes on all radios as well as strobes and LED nav lights (some are a major RFI source). Have a headset on with the radio(s) squelch open to listen for background noise changes. Do these checks with the airplane out and away from the hangar and no external chargers or power supplies connected. Hangar florescent lights tend to be very RF noisy and unless you have a nice regulated and filtered power supply assume it will create RF noise as well.

Carl
 
My #1 Com antenna is only about 3 inches behind the cabin top. So just over half of the ground plane is the tailcone upper skin. However to extend the ground plane onto the fiberglass of the cabin top, the foil tape made an easy solution (Lancair and Glasair guys use this technique a lot, or so I'm led to believe).

Yup, that's my understanding too - the question was: why copper instead of aluminum, and how does one pick between the two? (I've heard a lot of people using aluminum tape, this is the first I'm hearing of using copper)

I have 3 GPS antennas because I have 3 GPS receivers. Each G3X display has a built-in VFR GPS receiver and of course the IFR 650 needs it own. Also the systems don't share GPS data per se. For example, if I left the G3X's GPS antennas off , yes, they would display flight plan data from the 650 but the G3X's internal GPS's would be blind. Should I lose the 650 in this scenario, I'd lose all GPS navigation.

Interesting - so you can't set the G3X to use the GTN as its position source? From reading the manual it sounded like it supported that, and sounded to me like in an IFR setup, you'd only ever use the internal GPS as a backup:

"FAILURE OF THE EXTERNAL GPS NAVIGATION SOURCE
If the external GPS navigation source fails, the system reverts to the internal GPS
navigation source and ?REV? is shown in yellow in the lower left quadrant of the HSI."
 
One more question on interference and moving antennas - given that you need to drill holes in your fuselage to install each antenna, it's not like you just keep moving it by an inch at a time until you find the right position - is there a good way to test it before you drill, or do you just need to have a very good guess on which direction to move which antenna?
 
Yup, that's my understanding too - the question was: why copper instead of aluminum, and how does one pick between the two? (I've heard a lot of people using aluminum tape, this is the first I'm hearing of using copper)

I used copper because that's what the plastic aircraft guys use. I simply went with what I knew would work: http://www.aircraftspruce.com/catalog/avpages/copperTape.php?clickkey=54647

Interesting - so you can't set the G3X to use the GTN as its position source? From reading the manual it sounded like it supported that, and sounded to me like in an IFR setup, you'd only ever use the internal GPS as a backup:

"FAILURE OF THE EXTERNAL GPS NAVIGATION SOURCE
If the external GPS navigation source fails, the system reverts to the internal GPS
navigation source and ‘REV’ is shown in yellow in the lower left quadrant of the HSI."

Yes the G3X displays NAV and flight plan data from external navigators, a GTN 650 in may case for primary GPS and the 650 and SL 30 for VOR/LOC/GS. On cross countries I fly IFR almost 100% of the time so I load the flight plan into the 650 and it displays on the G3X. But any changes to the 650's active flight plan, switching the 650's CDI output from GPS to VOR and vice versa, and loading of approaches, departures, and arrivals has to be done on the 650 and not the G3X. IOW words the data flow from an external navigator to the G3X is a one-way street. The G3X also controls my autopilot (a TruTrak GX pilot). The question I was trying to answer was why have multiple GPS antennas/sources. For me it's redundancy if the 650 were to tank. So if the 650 goes down and the G3X reverts to internal GPS (remember each display has its own VFR GPS) but you didn't install the additional antennas, you'd be SOL because the internal GPS's would be blind.
 
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One more question on interference and moving antennas - given that you need to drill holes in your fuselage to install each antenna, it's not like you just keep moving it by an inch at a time until you find the right position - is there a good way to test it before you drill, or do you just need to have a very good guess on which direction to move which antenna?

I think you are danger close to overthinking this. As long as you pay attention to the min separation distances spelled out in the install manuals, you'll be fine. Once you have settled on a location, a couple of things to think through is how will the antenna feeds run to that location and does a doubler need to be installed. Installing conduit, pass throughs, and doublers is way easier before things get closed up.

Having said that, even the best laid plans require adjustment. Originally I had planned to have both of my COM antennas on the belly under the rear pax seats, and my transponder antenna under the tunnel somewhere in the forward fuse. Went as far as drilling holes, and fabricating & mounting doublers for the COM antennas. Sometime after that, I decided to add the provision for a MotoPod and installed the hardpoint kit and simultaneously trashed my carefully thought out antenna plan. What I ended up with is the result of the pod decision.
 
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GPS antenna to magnetometer spacing

It seems most Garmin installations are putting the magnetometer either in the wing or on the second bulkhead behind the baggage area, using Stein's bracket or similar. My question is whether this second location is a problem if the GPS antenna is mounted between the baggage bulkhead and the second aft bulkhead, i.e. within a foot or two of the GPS antenna.
 
I have magnetometer in the fuse at that location, just forward of the second bulkhead, i also have the dynon magnetometer just in front of it. I have a COMM antenna just behind the baggage bulkhead on top. Both my GPS antennas are on the cabin cover between the doors. The GPS antenna do not need a ground plane to operate properly. Also, it makes it easier to run coax up to the antennas through the front support
 
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