ELT Antenna: vertical or horizontal?

[ceslaw]

My main amateur radio transmitting antenna presents a DC short to the transmitter. It works as intended with over 500 W RF input.

What's your call sign?

Chuck
WA9ZEH

[Andy]

Ok, but why is Kannad saying “ELT antennas should be vertically polarized when the aircraft is in the normal flight attitude”, then ?

The polarization may be significant when the ELT signal is being received at relatively short range by search aircraft with vertically polarized antennas. If you think this is important then you should ensure that your fuselage always ends up in the correct orientation when you crash.

ELT have a hopeless record of surviving crashes and emitting a signal that locates the aircraft. Turn it on before you crash and hope your signal is located before the ELT installation is compromised.

I don't know if the factory installation is good or not. I don't know how it was tested. I have so little confidence in any ELT that worrying about the installation has been very low on my priority list.

Lots of references to signal polarization in this document -

https://www.gocivilairpatrol.com/med...12F9969C5A.pdf

[Daveembry]

i thought I explained it was a ground problem? (It was.)

I was told that Ramis response was “if it’s a Kannad, they are very sensitive to grounds and to cable length. Those are the only units we have problems with”.

Their first suggestion was to do what I did. Install the antenna on a separate ground plane. It worked right off the bat.

As I said before, I have gotten them to work by cutting off the connector and installing another one just a short distance down the cable and then they worked. (Connectors were checked before removing and they were good, so it wasn’t that)

Jon at CC thought perhaps it was the mounting bracket not getting a good ground. I have no idea.

Apparently these are known factors from Rami. I’ll be either using another antenna or ELT from here on out.

The polarization may be significant when the ELT signal is being received at relatively short range by search aircraft with vertically polarized antennas. If you think this is important then you should ensure that your fuselage always ends up in the correct orientation when you crash.

ELT have a hopeless record of surviving crashes and emitting a signal that locates the aircraft. Turn it on before you crash and hope your signal is located before the ELT installation is compromised.

I don't know if the factory installation is good or not. I don't know how it was tested. I have so little confidence in any ELT that worrying about the installation has been very low on my priority list.

Lots of references to signal polarization in this document -

https://www.gocivilairpatrol.com/med...12F9969C5A.pdf

[Andy]

i thought I explained it was a ground problem? (It was.)

Ok, but everything else you say it this same post points to an antenna matching problem rather than a DC current path problem. Isolating the ground plane from the airframe will change antenna matching. Changing the length of the coax will change antenna matching. Changing the position of the antenna relative to adjacent frame tubes will change antenna matching. In this context antenna matching means the complex impedance presented to the ELT by the antenna system, where the antenna system is the antenna, the ground plane, any proximate conducting material, and the coax itself.

If Rami only reports problems with Kannad ELT then maybe the Kannad monitor is too sensitive. Perhaps the monitor sensitivity was reduced later in production.

[Daveembry]

Im an airplane builder Andy, not an engineer. I can report what I learn and pass on to others what the experts I have to rely on….tell me. Feel free to super analyze all you want.

The antenna manufacturer says it’s a grounding problem, I do what they say and it works.

Ok, but everything else you say it this same post points to an antenna matching problem rather than a DC current path problem. Isolating the ground plane from the airframe will change antenna matching. Changing the length of the coax will change antenna matching. Changing the position of the antenna relative to adjacent frame tubes will change antenna matching. In this context antenna matching means the complex impedance presented to the ELT by the antenna system, where the antenna system is the antenna, the ground plane, any proximate conducting material, and the coax itself.

If Rami only reports problems with Kannad ELT then maybe the Kannad monitor is too sensitive. Perhaps the monitor sensitivity was reduced later in production.

[ceslaw]

PBY’s original question was whether mounting the ELT antenna horizontally per the manual is appropriate. There is some good discussion and information in this thread. There now appear to be two issues. First is whether the ELT antenna can be mounted horizontally. Second concerns issues with the compatibility of the Kannad ELT and Rami antenna. Here are my two cents worth on the first issue.

Let’s start with three concepts. First is wavelength. The Kannad ELT and Rami antenna operate at VHF and UHF frequencies which means the wavelength is very short. (The formula for the half wave length expressed in feet is 468/Frequency in MHz). We are talking a quarter wave length vertical antenna of less than two feet.

Ever notice how driving through a steel girder bridge the FM radio continues to work fine (100 MHz range) but the AM (one MHz range) radio drops out? That is because the wavelength of AM is much longer than the spacing of the girders but the FM is much shorter. The FM signal easily passes through the girders.

The ELT will transmit both a VHF and UHF signal which will pass through the structure of the Carbon Cub since the spacing is greater than the wave length of the signal. There is no problem with its location, per se, especially as to the UHF satellite transmission.

Second, let’s look at the ground plane. A vertical antenna (even one mounted horizontally) needs a ground plane, typically at least a half wave length in diameter. The extended baggage is a large ground plane relative to the wavelength of the signal of the ELT. However, the antenna needs to be perpendicular to the ground plane, hence it is mounted in a horizontal position.

That brings us to the third concept: polarization. Vertical antennas are typically considered to be ‘vertically polarized’ meaning when in a vertical position the signal radiates in a direction perpendicular to the antenna, or a horizontal plane in relation to the earth. That is a good thing. It maximizes the range it radiates equally in all directions. That is why the installation directions typically recommend mounting a vertical antenna vertically.

But this nice horizontal doughnut radiating pattern is unique to a pure quarter wave antenna. In the frequency range we are talking about this would be a foot or less. But the RAMI dual frequency antenna is about a quarter wave length long at the 121.5 but several wave lengths long at the UHF frequency which requires a compensating circuit. It is base loaded, meaning there is a tuning network to compensate for the different wave lengths of the two-band function. The result of the base loaded antenna is that it no longer radiates in a nice doughnut shape. The radiation pattern will undoubtedly be skewed and less directional. So, the usual rule of mounting a vertical antenna vertically loses a bit of efficacy with base tuned antennas.

So here is the take away from all this gibberish:

1. The ELT antenna will radiate past the Carbon Cub frame, particular the UHF satellite signal.

2. The ELT will radiate in an irregular pattern, not the usual doughnut shape, because it is base loaded and the effect of the large extended baggage ground plane.

3. The ELT will only transmit when something bad happens, and one will not know what position the plane will be in if and when that happens. Imagine if one flips the plane on landing. Locating it inside the fuselage where it is protected would actually be the best possible location. If the plane noses over, the antenna may well be in an optimal vertical position. How reassuring!

So why does the manual recommend locating it in a vertical position? Because as a general rule quarter wave transmitting and receiving antennas communicate better when oriented in the same orientation. Obviously, a VHF radio is typically mounted nearly vertical because it is used frequently, but note that even those antennas may be in a horizontal position when mounted on the belly of the plane or swept back at a 45^ angle when mounted on top.

Accordingly, I see nothing wrong with the ELT antenna being located perpendicular to the extended baggage in a horizontal position inside the frame per Cub Crafter's manual.

[PBY Catalina]

Thank you for this very interesting discussion gentlemen!

It is a great opportunity for me to be able to count on your wise advice.
And thank you for these very interesting theoretical reminders about this very complex science that is the propagation of electromagnetic waves.

Here is my decision:
- First I’m going to mount the antenna horizontally as recommended by Cubcrafters and do the tests before covering as recommended by Dave.
- If the tests are negative, I will build a flat ground and place the antenna vertically as indicated by Dave.

I will keep you informed of my results.

Good building to all!

[Daveembry]

Good info, thanks. I plan to get with Pete before the next one and see if we can figure out what’s going on.

PBY’s original question was whether mounting the ELT antenna horizontally per the manual is appropriate. There is some good discussion and information in this thread. There now appear to be two issues. First is whether the ELT antenna can be mounted horizontally. Second concerns issues with the compatibility of the Kannad ELT and Rami antenna. Here are my two cents worth on the first issue.

Let’s start with three concepts. First is wavelength. The Kannad ELT and Rami antenna operate at VHF and UHF frequencies which means the wavelength is very short. (The formula for the half wave length expressed in feet is 468/Frequency in MHz). We are talking a quarter wave length vertical antenna of less than two feet.

Ever notice how driving through a steel girder bridge the FM radio continues to work fine (100 MHz range) but the AM (one MHz range) radio drops out? That is because the wavelength of AM is much longer than the spacing of the girders but the FM is much shorter. The FM signal easily passes through the girders.

The ELT will transmit both a VHF and UHF signal which will pass through the structure of the Carbon Cub since the spacing is greater than the wave length of the signal. There is no problem with its location, per se, especially as to the UHF satellite transmission.

Second, let’s look at the ground plane. A vertical antenna (even one mounted horizontally) needs a ground plane, typically at least a half wave length in diameter. The extended baggage is a large ground plane relative to the wavelength of the signal of the ELT. However, the antenna needs to be perpendicular to the ground plane, hence it is mounted in a horizontal position.

That brings us to the third concept: polarization. Vertical antennas are typically considered to be ‘vertically polarized’ meaning when in a vertical position the signal radiates in a direction perpendicular to the antenna, or a horizontal plane in relation to the earth. That is a good thing. It maximizes the range it radiates equally in all directions. That is why the installation directions typically recommend mounting a vertical antenna vertically.

But this nice horizontal doughnut radiating pattern is unique to a pure quarter wave antenna. In the frequency range we are talking about this would be a foot or less. But the RAMI dual frequency antenna is about a quarter wave length long at the 121.5 but several wave lengths long at the UHF frequency which requires a compensating circuit. It is base loaded, meaning there is a tuning network to compensate for the different wave lengths of the two-band function. The result of the base loaded antenna is that it no longer radiates in a nice doughnut shape. The radiation pattern will undoubtedly be skewed and less directional. So, the usual rule of mounting a vertical antenna vertically loses a bit of efficacy with base tuned antennas.

So here is the take away from all this gibberish:

1. The ELT antenna will radiate past the Carbon Cub frame, particular the UHF satellite signal.

2. The ELT will radiate in an irregular pattern, not the usual doughnut shape, because it is base loaded and the effect of the large extended baggage ground plane.

3. The ELT will only transmit when something bad happens, and one will not know what position the plane will be in if and when that happens. Imagine if one flips the plane on landing. Locating it inside the fuselage where it is protected would actually be the best possible location. If the plane noses over, the antenna may well be in an optimal vertical position. How reassuring!

So why does the manual recommend locating it in a vertical position? Because as a general rule quarter wave transmitting and receiving antennas communicate better when oriented in the same orientation. Obviously, a VHF radio is typically mounted nearly vertical because it is used frequently, but note that even those antennas may be in a horizontal position when mounted on the belly of the plane or swept back at a 45^ angle when mounted on top.

Accordingly, I see nothing wrong with the ELT antenna being located perpendicular to the extended baggage in a horizontal position inside the frame per Cub Crafter's manual.

[ceslaw]

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.

IMG_1802.jpg

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.

[ceslaw]

Good info, thanks. I plan to get with Pete before the next one and see if we can figure out what’s going on.

Keep us posted on what you learn. This is an interesting issue.