ELT Antenna: vertical or horizontal?

[ceslaw]

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.

Good point. I am tempted to get a decent SWR meter (I don't think my 50 year old one is up to the task) and check it at 121 MHz with a hand held radio.

[Daveembry]

Well now that is interesting because CC has the antenna mounted on the bracket and the bracket is mounted onto the ground plane, so it is NOT on the opposite side. I had actually thought about this but could not figure a good way to mount the antenna directly into the rear baggage because the coax/antenna base and nut would then all be inside the baggage compartment.

When I have mounted the antenna to a separate piece of aluminum used as a ground plane, I have always mounted it like that, with the antenna and coax/nut on the opposite side. I could then take that ground plane and put it in ANY POSITION inside the fuse and it would work.

The last comment from CC (Jon) on this after I got it to work using a separate ground plane was that it may be a problem with the mounting bracket.

I suppose the coax could be run into the back of the baggage area and a small cover made to go around the attach point of the coax.

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.

[dayoung107]

Hello, I just wanted to help with some information from our production line.


I helped with the design of the ELT system in X-Cub. We had to prove the horizontal mounting orientation of the antenna, so I coordinated with SARSAT and ran a live test. The horizontal orientation worked acceptably. That leads me to believe there is not a problem mounting horizontally vs. vertically.


I also support the production line and help them with issues as they come up. We did have problems with the same self-test error code pertaining to SWR. As we went back and forth between the installation and the ELT itself, we found out that changing the length of the coax seemed to the quickest and easiest way to move from the failing the self-test to passing. In talking to our supplier, I believe that I was told Kannad was working on relaxing the tolerances of that self-test. Unfortunately I cannot find that correspondence, it must have been over the phone. Therefore, the only hard evidence I can offer is from our production line installers that have not had this issue quite some time.

Kit aircraft are issued the same parts that we use on the production line, so, the only difference I can come up with is the location of the antenna itself.

In general, we do try to save weight and complexity which lead to places where performance could be improved. This appears like it may be a candidate for improvement. But, our current design is currently working for the production line, and so we don't have current plans to change it.

[ceslaw]

An antenna analyzer enables one to determine the SWR and impedance of an antenna at different frequencies without transmitting a signal. It is a handy gadget and just what is needed to sort out the ELT issues. (And also tune the VHF antenna as well).

With the ELT antenna in its “stock” location perpendicular to the extended baggage mounted on a bracket that sets it three inches away from the ground plane the SWR was over three on the analog scale and 4.5 on the digital readout. Not good. Indeed, changing the frequency up or down did not lead to any better readings.

IMG_1861.jpg

An aluminum welding rod was cut to a quarter wave length for 121.5 MHz, approximately 23 inches, and a loop made at one end. This was then mounted directly under the base of the antenna. (Unfortunately it is difficult to see in this picture.)

IMG_1860.jpg

The improvement in the SWR was dramatic. Both on the analog scale and the digital scale it was virtually perfect once the radial was trimmed a bit.

IMG_1859.jpg

This explains Dave’s discovery that the ELT worked properly when directly on an aluminum plate but had issues when mounted on the bracket. It was not the perpendicular location but rather the bracket setting it three inches above the extended baggage ground plane that caused the issue. I suspect the SWR was right at the limit of what the Kannad ELT will accept and just the slightest change caused it to go from acceptable to not acceptable, accounting for the problem occurring sporadically . Andy was spot on noting that the bracket may be a factor.

The solution is so simple. A proper radial is all we need. Placing the antenna in the lower spot (not the mid-point used by Cubcrafters) leaves just enough vertical space for the 23 inch radial.

I plan to fabricate a more proper radial out of material an inch or two wide with a slight angle at the end to hopefully provide a flat SWR over a wider frequency. Trimming the thin test radial even a quarter inch can make a difference in SWR so something more substantial should help. I will report further with precise dimensions shortly.

[Daveembry]

wow. Great info Chuck. Actually that was the final thing Jon at CubCrafters said he thought might be the problem when we were testing with Rami…..that the bracket was the problem.

An antenna analyzer enables one to determine the SWR and impedance of an antenna at different frequencies without transmitting a signal. It is a handy gadget and just what is needed to sort out the ELT issues. (And also tune the VHF antenna as well).

With the ELT antenna in its “stock” location perpendicular to the extended baggage mounted on a bracket that sets it three inches away from the ground plane the SWR was over three on the analog scale and 4.5 on the digital readout. Not good. Indeed, changing the frequency up or down did not lead to any better readings.

IMG_1861.jpg

An aluminum welding rod was cut to a quarter wave length for 121.5 MHz, approximately 23 inches, and a loop made at one end. This was then mounted directly under the base of the antenna. (Unfortunately it is difficult to see in this picture.)

IMG_1860.jpg

The improvement in the SWR was dramatic. Both on the analog scale and the digital scale it was virtually perfect once the radial was trimmed a bit.

IMG_1859.jpg

This explains Dave’s discovery that the ELT worked properly when directly on an aluminum plate but had issues when mounted on the bracket. It was not the perpendicular location but rather the bracket setting it three inches above the extended baggage ground plane that caused the issue. I suspect the SWR was right at the limit of what the Kannad ELT will accept and just the slightest change caused it to go from acceptable to not acceptable, accounting for the problem occurring sporadically . Andy was spot on noting that the bracket may be a factor.

The solution is so simple. A proper radial is all we need. Placing the antenna in the lower spot (not the mid-point used by Cubcrafters) leaves just enough vertical space for the 23 inch radial.

I plan to fabricate a more proper radial out of material an inch or two wide with a slight angle at the end to hopefully provide a flat SWR over a wider frequency. Trimming the thin test radial even a quarter inch can make a difference in SWR so something more substantial should help. I will report further with precise dimensions shortly.

[Andy]

An antenna analyzer enables one to determine the SWR and impedance of an antenna at different frequencies without transmitting a signal. It is a handy gadget and just what is needed to sort out the ELT issues. (And also tune the VHF antenna as well).

I have not measured my ELT antenna yet so yours is the best information we have so far, thanks.

Just a few comments:

Antenna analyzers transmit a signal. It's a low power signal but there are anecdotal reports of MFJ antenna analyzers being heard several states away on the HF bands. It is true that the analyzer signal does not have the characteristics of an ELT signal and should not initiate any search effort.

Your chosen MFJ analyzer does not cover 406 MHz and I think it is important to understand how the antenna is matched at 406 MHz before deciding that any modification is an improvement at that frequency. I'd far rather have good COSPAS/SARSAT location than hope CAP will find my remains a week after the accident.

A single radial that gives a good match at 121.5 MHz is not likely to also produce a good match at 406 MHz. However, a circular or rectangular shaped ground plane that results in a good match at 121.5 will be quite likely to also produce a good match at 406 MHz. A large area ground plane does not need to be resonant.

A ground plane does not need to be structural. It may be ok to mount a large area, light weight, ground plane at the antenna base and to support it from the rear baggage wall.

Unfortunately optimizing the antenna installation won't help the GPS reception problem. I like the ELT location so the best solution is probably to hardwire an RS-232 GPS data source.

edit to add - Your MFJ image with radial added shows R=70, X=0, SWR=1.0. That is an inconstant data set for a 50 ohm system. Was the analyzer configured for 75 ohm measurements? It's a long time since I used an MFJ analyzer and I don't remember if it had the ability to switch between 75 and 50 ohm measurements.

[Dan L]

There is some interesting information in this thread. The question that comes to my mind though is why keep the elt antenna inside of the fuselage? Protection during a crash?

02C97C89-4EDE-44C3-8989-C75B3D59EE00.jpg

This isn’t a great picture of my antenna position but is the first one I found that shows it. I put an aluminum ground plane sheet under the turtle deck and it is attached and bonded directly to the fuselage tubes. The comm antenna works fine as does the elt. In fact I inadvertently did a live test with the elt a few years ago and got a call on my cell phone a minute later from a nice Airforce officer in Florida.

My elt is an Artex 406.

[Andy]

I followed up with Orolia by asking these questions:

"What ELT mounting orientation(s) must be used for the internal GPS antenna to have a clear view of the sky? The only related reference I can find in the installation manual is the caution regarding the securing strap.

Would you expect the internal GPS to achieve GPS position lock if the ELT was mounted, tray down, under the pilot seat which has a shell made of carbon fibre?"

They replied:

"The optimal installation is main label up, under an RF transparent windows. But it is difficult to predict the position of the aircraft in case of crash, and the behavior of the GPS receiver depending on its orientation and the condition of surrounding structures.

The installation you are describing is not optimal, although it is acceptable.

The internal GPS is an optional feature that comes in addition to the basic ELT. In other words the ELT signal will be received by the satellites thanks to the external TX antenna. With or without GPS , the identification of the aircraft in distress will be immediate.

Without GPS the ELT will be localized within 1 NM within 10 to 30 minutes.

If the GPS works, the ELT will be localized within 120 metres within 1 to 5 minutes."

[Daveembry]

It seems to me that a separate ground plane like the one I did that worked perfectly (according to Kannad self test) is the solution. It seems to me that the coax attach point needs to be on the opposite side of the ground plane as specified by the Installation Instructions from Kannad and the CC bracket does not allow that. Since it’s impractical to attach the antenna directly to (through) the rear baggage panel with the coax connector sticking into the baggage area (and coax), mounting it on a separate ground plane seems best.

This is how I did it. In the future just cut the aluminum to the correct size (what would be the correct size??) but so that it will fit on top of the fuse tubes and just methycrylate the panel directly to the tubes for a more permanent mount. Move the inspection cover back so it’s directly under the connection.

C2C9CC8F-2869-45EF-90B3-E070DA8C870B.jpg

I’m up in bighorn hunting camp with little internet access so can someone figure what size ground plane should be used according to a Kannad instructions?

wow. Great info Chuck. Actually that was the final thing Jon at CubCrafters said he thought might be the problem when we were testing with Rami…..that the bracket was the problem.

[Andy]

There is some interesting information in this thread. The question that comes to my mind though is why keep the elt antenna inside of the fuselage? Protection during a crash?

If your aircraft ends up on its back your ELT antenna will be broken off. There are lots of reasons ELT have not been useful for crash location. There include ELT not armed, ELT broke free of mounting and fractured antenna coax, ELT antenna was broken off.

Here are a couple of related reports but I expect there are many others:

https://ntrs.nasa.gov/api/citations/...9910001651.pdf

https://commons.erau.edu/cgi/viewcon...83&context=edt