ArTex 345 ELT - Ground Plane Question

[Rodney]

Need some help here on what to do about the Ground Plane for this ELT.

I want to mount their included 24" antenna inside the Fox behind the baggage compartment.

What do I do for a ground plane???

Artex instructions call for a 48" diameter ground plane for the antenna.

Called Artex Tech Support and he said he could not answer any questions regarding the ground plane for the antenna.

I'm removing my old 121.5 transponder. It did not have an external antenna, so there is no existing antenna mount.

Need the help of our electronics experts here on the forum.

Thanks

Regards
Rodney

[efwd]

From my reading about the antennas, the airframe itself may be the ground plane. As I reported on my newly energized comm radio, my airframe is behaving like an effective ground plane to the best of my knowledge. Not flight tested of course. The ground plane can be simple wires run out from the center or it can be a disk. I contemplated installing a few radials of copper tape but so many people say that their antenna works fine having it mounted to the airframe itself.

[Rodney]

Thanks EFWD for the reply.

I guess that is the only real option here. I'm certainly not an electrical type so I sure need some help here.

All the ground plane calculators I can find indicate that the disk size needed is about 48 inches. Well, that isn't going to happen in any of our airplanes.

I did find a 406 mhz dipole antenna, but it's another hundred bucks. Ouch - especially after buying the new elt.

Some folks say you can ground it to the frame and others say you can't.

I've searched this forum, but have not come up with a definitive answer yet.

I know others here have installed the antenna inside the airframe to protect it in case of an accident. Just haven't found an answer yet I'm comfortable with.

Anyone else ground the antenna to the frame???

(Edit) So, I also checked AC 43.13-2B.310

" Most antennas require a ground plane size
of approximately 24" by 24". While the rule of
thumb is to provide a minimum of 1/4 wavelength
of the operation frequency, larger is better and
ground plane symmetry is critical. Gaps in antenna
coverage or performance may occur if a ground
plane is not symmetrical. "

OK - if I'm reading this right than I can construct a 1/4 wave ground plane
disk - which would be about 14 inches in diameter and install the antenna on it.

Is that the thing to do, or will grounding it to the airframe be sufficient??

Rodney

[Esser]

My 406 antenna is attached to a piece of aluminum the size of a deck of cards that is adel clamped to the frame. I then have a grounding wire attached to that plate. Seems to work fine.

[jrevens]

Josh's installation sounds about the same as mine, and also the same as a good friend who has a Mod. IV Speedster. FWIW, my friend had his accidentally triggered while his airplane was sitting on the ground and he didn't notice it until about 10 minutes later when the monitoring agency actually called him, so it worked just fine. I believe that there are quite a few installed that way in Kitfoxes. You definitely need to have a good airframe ground to the mounting surface and hence to the base of the antenna.


This is what mine looks like, except I lowered the mounting plate position so that the tip of the antenna is below the surface of the cargo bay sack now.

[Av8r_Sed]

I would take the approach outlined in the ArTex manual and use the adhesive strips under the fabric.

[Rodney]

Thanks everyone for the help.

Especially appreciate the picture John.

Now I can fabricate my bracket and get on with the installation.

Rodney

[efwd]

It must have been John that I copied when I installed mine. :D

[aviator79]

Just for additional comfort, the frames from the factory now ship with an ELT and antenna bracket welded to the frame. You can see it in the background of this picture. I assume this is how it's done on the SLSAs, and there is basically no ground plane other than the frame.

https://lh3.googleusercontent.com/Qs3QLxZbWlkDYIMbCJGaXSTuC--DyuABxP-hApZ5_xCKOI13YHgGO0QiswqYRjYN84FdcfizBOEExf65E9hii Z8hDFJm6JmDv-Mczewyl9XnxADVvcIHh-ZjY4L-kgt43lX_qhcUHzw=w600

[SilverFox5]

John
With the elt and elt antenna mounted in this position I’m curious where you mounted your comm antenna?
Im building a KF5 and the plate for the comm antenna is where you mounted the elt mounting plate. And the kit we picked up came with a Comant CI-121 comm antenna that we would need to mount there. But I see the artex 345 manual reguires the elt antenna to be at least 32 inches away from the comm antenna.

Joe Leeman
KF5 912 uls
Freedom, WI

[jrevens]

Hi Joe,

The mounting plate for my ELT is on the right edge of the fuselage, forward under the turtle deck cover. Are you sure that your comm antenna mount is there? My comm antenna is centered on the fuselage aft of the rear edge of the turtle deck cover. I welded the mounting plate on at that position. It’s a little aft of where the factory installs them now. I wanted to get a little more clearance from the rear of the turtle deck cover. The factory wasn’t installing a mounting plate up there at that time. I’m not sure what the distance is between the 2 antennas, but everything works fine.

[SilverFox5]

Hi John,
I see what you mean now. I thought your elt mount was in the centerline of the fuselage. Now it makes sense to me. I like like that mounting spot.

Joe