Garmin GPS antenna placement/orientation

In the RV8, the GPS antenna locations has been a battle of compromises. I'm told that under the cowl works well, despite Garmin's objections to putting them there. I was told by my avionics guru (S) that I should separate the G3X antenna from the GTN650's by 6+ inches, so here is what I came up with.





I have two questions for those a bit more experienced than I am with these things. First, under the cowl is fine right? Second, these antennas have a forward arrow on the drawings and to save space, I blew that suggestion off and turned them sideways. Can someone explain how a GPS antenna would need one end pointing forward? Of course I was completely certain it wouldn't matter...until it was finished...now I'm questioning my brilliance. Any thoughts?

The arrow is for external placement for minimal drag. Does not matter if out mounted under the cowl.

FWIW:

It looks to me like the GTN antenna is mounted off the the side and tilted. If it were my airplane, I would put this antenna in the prime location (and level). At the end of the day the GTN is the one that keeps you out of the dirt.

Also by tilting it, I believe you are also slightly impairing its performance. It is designed to receive signals down to an angle near the horizon, but not from below this angle. By tilting the antenna's "horizon" you are creating a blind spot on the high side and degrading the signal with noise from the noisy earth on the low side. Mounting it below the lip of the cowling also creates another blind spot where the sky is shaded by the firewall. I think there is information in the GTN series installation manual about how closely the antenna can be placed to vertical obstructions. That might tell you if your spacing is okay or not.

David

For VFR operations, the under cowl placement should work ok. I suggest aligning the antenna for flight direction. You will limit the antenna view of the full available GPS constellation. The antenna exposure to engine heat may be a factor over the long term.

From flight testing we have done at work, antenna placement is critical, especially for GPS/WAAS approach operations. We note elevated WAAS vertical protection levels as the antenna is masked to higher angles. This will affect your GPS/WAAS approach availability. If you are planning an IFR installation, Garmin instructions are best followed.

I have dual Garmin WAAS avionics, installed IAW Garmin instructions. I have had no GPS/WAAS reception issues over the last three years and I do operate IFR.

FWIW I mounted my G3X antenna's and the GDL 39 external antenna on a shelf under the cowling but elected to install the GTN 650 antenna per Garmin instruction manual due to the critical nature of the unit.

I don't know if this might be a possibility, but I am planning to mount my GTN-650 antenna right behind the passenger's head as a part of a head rest. It will hold the antenna up a few inches, provide separation from the Dynon GPS antenna (which will be mounted under the cowling), and also provide the cable length required from the GTN itself. I haven't built the head rest mount yet, but I seem to remember seeing photos of this being done by others.

Mine is centered and level under the cowl sitting on an 8" square ground plane just in front of the firewall.

It works great in this area for GPS based approaches including WAAS LPV, I have never had it downgrade me, nice solid signals.

I'd agree the arrow (orientation) is not going to matter, but that seemingly 45 degree angle is sub optimal. I looked at several locations on the -8 when I pulled them out from under the cowl (I don't like under the cowl), and considered a spot right behind the passenger's head on the top of the fuselage. IIRC, it would clear the canopy when sliding open, yet provide a clear view of the sky when closed. You get the zero drag benefit, but it does not live under the paint, filler and fiberglass of the cowl.

On the Rocket, my 650 antenna is mounted on the canopy side rail, but will go to the top of the roll bar eventually.