ELT Antenna: vertical or horizontal?

[Daveembry]

Chuck. Since I had problems before ….my last build which I just finished….

1. Tested on the bench with cable connectors preinstalled on both ends. No ground. Passed.
2. Installed in fuse using CC bracket on the back of the extended baggage wall. Installed exactly as it goes with nothing to change before flying. Passed.
3. Covered fuse. Retested……passed.
4. Painted fuse, installed avionics, engine, etc. retested. Passed.
5. Finished airplane and installed for last time before first flights. Failed. Absolutely nothing done to the antenna, ground, cable or Elt unit. Nothing.
6. Removed antenna from bracket and installed on separate ground plane piece of aluminum……Passed. The antenna can be held up in any position …vertical or horizontal…..touching powder coated fuse tubes or not. Passed. Passed. Passed.

So don’t let your guard down. Something ain’t right. I thought I had this one nailed.

Dave raised significant concerns about the Kannad and Rami combination when the antenna was mounted in a horizontal position. Here are my observations.

I had no problems with the horizontal orientation of the antenna on the Cub I built six years ago. The Kannad and Rami passed the function tests when checked at every annual condition inspection.

After reading this thread I left the office early and headed to the shop to hook up the ELT on the current build and see if it would work. The antenna feed wire is not ‘tuned’ to a specific length but simply cut so the connectors would fit neatly on each end. RG-142 coax is used, which is a 50-ohm cable suited to high frequency applications, rather than the more common RG-58A/U, which is less suited to VHF and UHF.

Using an ohm meter confirmed that the cable was not shorted after the connectors were installed. With the coax connected to the antenna the opposite connector showed resistance, which is expected with a base loaded antenna.

The Kannad function test passed with the signal broadcast on 121.5 clearly audible and the steady red LED at the end of the test. All is well.

When the bracket was installed, the contact surfaces were rubbed with a Scotch Brite pad to assure good ground connections. I am also a bit anal about using dielectric grease on most ground and other connections, including the BNC connectors.

Attachment 11777

So, PBY, I would say use the Kannad and the Rami per Cub Crafter’s recommendations, but follow Dave’s suggestion to confirm proper function before covering the plane, which from your post appears to be your plan. Let us know what happens.

[PBY Catalina]

...

5. Finished airplane and installed for last time before first flights. Failed. Absolutely nothing done to the antenna, ground, cable or Elt unit. Nothing.

6. Removed antenna from bracket and installed on separate ground plane piece of aluminum……Passed. The antenna can be held up in any position …vertical or horizontal…..touching powder coated fuse tubes or not. Passed. Passed. Passed.

So don’t let your guard down. Something ain’t right. I thought I had this one nailed.

Oups... and how did you installed the new ground plate, after the covering ? ... Must be hard ?

[ceslaw]

That is a real mystery.

Couple questions.

1. Could you try a different brand of ELT using existing antenna cable?

2. Could you try running a jumper antenna coax from the antenna out the bottom access hole to the ELT bypassing the existing antenna coax to test the antenna installation?

3. Also wondering if one could check the SWR of the malfunctioning set up, perhaps with a handheld transmitting around 121 MHz?

How did you resolve the issue in a fully finished plane? Is CC looking into the issue?

[Daveembry]

1. The first one I had problems with, replaced the Kannad with another one and it worked. Tried that in 2nd one and didn’t work? Didn’t try a different brand.

2. did the bypass with a different cable, didn’t work.

3. ?? Maybe? I just don’t know.

I hinged 2 5” pieces of aluminum about 16” long so it would fit up through the inspection hole.

Strange thing is I’ve never heard of anyone else with this problem? Is it only me (5 out of 6 installs) or are others just not paying any attention to the self test when they arm it? Someone at CC said they were going to check with their avionics guys there and see if it’s a problem there.

I don’t think I’ll be using a Kannad again.

That is a real mystery.

Couple questions.

1. Could you try a different brand of ELT using existing antenna cable?

2. Could you try running a jumper antenna coax from the antenna out the bottom access hole to the ELT bypassing the existing antenna coax to test the antenna installation?

3. Also wondering if one could check the SWR of the malfunctioning set up, perhaps with a handheld transmitting around 121 MHz?

How did you resolve the issue in a fully finished plane? Is CC looking into the issue?

[Andy]

Is it only me (5 out of 6 installs) or are others just not paying any attention to the self test when they arm it?

I tested the Kannad in my FX-3 and it passed. Pre-cover photos of my fuselage appear to show a completely standard CC antenna installation.

Self test fail with 3+5 is specified as a VSWR fault so my reaction to that would be to measure the VSWR at the ELT end of the antenna coax. Unfortunately this requires specialized test equipment that a homebuilder may not have.

I find nothing in the installation manual that indicates there is any test to see if a valid GPS position is being reported by the internal GPS.

Edit - The only GPS test is described as "3+6 INTERNAL GPS SERIAL LINK (ELT internal check)" but I don't know what it tests.

The manual does caution that incorrectly positioning the securing strap may prevent the internal GPS from working. It does not offer an opinion as to whether putting a carbon seat pan and the pilots body between the GPS antenna and the sky is an acceptable installation.

[ceslaw]

Self test fail with 3+5 is specified as a VSWR fault so my reaction to that would be to measure the VSWR at the ELT end of the antenna coax. Unfortunately this requires specialized test equipment that a homebuilder may not have.

Edit - The only GPS test is described as "3+6 INTERNAL GPS SERIAL LINK (ELT internal check)" but I don't know what it tests.

Good info. Was wondering if VSWR fault would be indicated and that answers it. Still wondering if the internal GPS works surrounded by grounded carbon fiber, but not sure we will ever know.

[Daveembry]

FYI. I asked kitsupport for some clarification and they have jumped on this and promise to reply soon. I haven’t heard yet if this has been a problem at the factory or not but…….One thing I will note is that all of my installs have NOT been done according to the CC manual but I’ve always moved the mounting bracket down lower onto the back of the extended baggage rear panel than called for like Chuck did so it’s more accessible from the inspection hole at the bottom. Could this be the cause? The ONLY other thing is I have not retested with the Elt removed from the mount and strap. in previous cases where it always passed …throughout construction but failed in the end, the previous tests were done without the unit being in its “compact mounting tray” with the strap installed around it. I know lots of units have been installed down lower like Chuck describes and have worked fine? Anxiously awaiting. CC said they’d post here in this thread. C04CBED8-7B13-49AC-8E03-29A7AA1F40C6.jpgHere is the manual location for the antenna. C04CBED8-7B13-49AC-8E03-29A7AA1F40C6.jpgThis is how we have been doing it mounting it low.

Good info. Was wondering if VSWR fault would be indicated and that answers it. Still wondering if the internal GPS works surrounded by grounded carbon fiber, but not sure we will ever know.

[Andy]

One thing I will note is that all of my installs have NOT been done according to the CC manual but I’ve always moved the mounting bracket down lower onto the back of the extended baggage rear panel than called for like Chuck did so it’s more accessible from the inspection hole at the bottom. Could this be the cause?

Any change of the position of the antenna on the ground plane, or any change in the position of the antenna with respect to surrounding steel fuselage tubes, will change the characteristics of the antenna and the SWR seen by the ELT. No way to know how much without measuring both installations.

Edited to add - The antenna is designed to be mounted on the ground plane with the coax feed on the opposite side of the ground plane from the antenna. The nominal SWR of this antenna is 2.0:1 at 121.5 and it could be a lot worse if not mounted as specified. It's probably worth noting that SWR is not an indicator of how well an antenna will radiate. Perfect 1:1 SWR is easily achieved with a 50 ohm dummy load but they don't radiate very well.

[PBY Catalina]

This is a picture coming from CC Dropbox:

It's an other installation !
But I guess this one is quite an old picture, and they modified the build manual since.

IMG_0303.jpg

[PBY Catalina]

FYI. I asked kit support for some clarification and they have jumped on this and promise to reply soon. ... CC said they’d post here in this thread.

Thanks Dave !
I’m waiting for their answer here and don’t bother them with an extra email then.
(I already install the bracket lower as you suggested... but no big deal to change that... )

Yves C.