Where is your ELT antenna mounted?

[larryb]

Hi All:

     I'm taking a survey, where is your ELT antenna mouted?

     My ELT antenna is located right next to my com2 antenna. According to the logs, the ELT was installed 10 years ago. I'm currently experiencing sub-par range on my com2 radio, and I'm thinking it may be due to interference from the elt antenna. According to the install manual, the antenna should be 3' from the nearest vertically polarized antenna. In my case, it's more like 2". 

Thanks, Larry

[m20exec]

From your picture your ELT antenna looks closer than 2ft. What points were you using to measure this distance? While it's entirely possible your COM 2 issue might be other factors, whoever mounted that ELT antenna either didn't have a clue or didn't care. As it was installed 10 years ago we can assume it was a 121.5 mhz elt. The reason a minimum of 3 ft is recommended is to reduce any interaction between any two antennas. Having two antennas of a near or harmonically related frequency in close proximity can detune each other and distort the pattern. I had this type on my previous AC and I seem to recall being able to remove it by simply unscrewing it. If you can do this with yours I'd remove it for a few test flights and see if you're com issue improves.

[larryb]

Hi Crag:

     My antenna's are 2 inches apart, at the base, not 2 feet.  I am sure this is a problem, whether it is the only problem or not remains to be seen. I'll try to un-screw the whip. I was thinking I would have to remove the whole ELT antenna and fly it.

     I'm a ham radio operator, so I'm familiar with the effects of antenna's being too close. I have had the plane about 3 months and recently noticed that com2 performance is not up to snuff. It works, but not as well as it should. Distant stations have trouble hearing me, and I have trouble hearing them. It was very apparent flying over the Nevada desert this week. Amazing that it has likely been like this for 10 years!

     I'm interested in where other people have their ELT antenna's mounted for 2 reasons:

1)  Am I the only one with a problem like this?

2)  Where is a good position to move the ELT antenna to.

     My 3-part plan is to have the ELT antenna moved to a better location. Next I want a bent-whip comm antenna mounted to the bottom, for com2. Finally, I'd use the existing com2 antenna for my backup hand-held radio. 

Thanks, Larry

[Piloto]

Typically the ELT antenna is installed next to the vertical fin on the fuselage. Belly mounted antennas works very well and provide better isolation between COM 1 and COM 2 when listening on one and transmitting on the other.

José 

[N601RX]

Check for corrosion between the skin and antenna base.  You may have lost your ground plane for the antenna. Also check the coax connector on the back of the antenna for corrosion.  As Jose mentioned vertical seperation will give better isolation than horizontal seperation.  

I recently replaced all my original RG 58 Coax with RG400 and moved the 2nd com antenna to the bottom.  The old stuff was in pretty bad shape and had several kinks in it. I also seperated the coaxes that ran to the back of the plane rather than tie it all up together in a bundle.  I ran the Com up high, the navs along the middle and the transponder and dme up under the belly.  

The reason for not putting the elt this close to the com is that some elt's will reradiate the com signal when the com transmitts.  If your are having this problem a directional coupler on the elt coax might fix it.  I have a new 406mhz unit and it is about 30" away.  I did not want to cut a new hole.

[larryb]

Hi Mike:

     I have visually inspected the mounting bolts, no corrosion in the area. I have measured the resistance to gnd of the disconnected antenna and found it good. When I get home from my trip, I'll measure SWR of the antenna with and without the ELT antenna present and see what the difference is. But that won't be for another week. 

     Did you notice a difference in range with com2 on the bottom vs. on top? I realize you made other changes at the same time.

     At some point in the near future, I'll be doing a new stack, and I will have the coax replaced at the same time. 

Thanks, Larry

[N601RX]

There may still be corrosion between the skin and antenna.  It is very common and was present under all of my original antenna's.  RF can do funny things.  It is possible to have a low DC resistance and still have a very high impedance at 120mhz.  

The RG400 is a better coax.  It is double shieled vs single shielded and has lower loss, but but not that much lower loss in the short lengths and frequency of your com radio.  It is time consumming to replace and it is a perfect time to do it while the plane in opened up for a annual. My main reason for doing it was because the old stuff was in bad shape and been bent over double and cable tied. 

What radio do you have?  Some are 5 watts, some 10 and others 16.

[Bennett]

I mounted my 406 Mhz ELT antenna on top, near the tail fin. This unit is made by Emerging Lifesaving Technologies. This antenna housing also holds a GPS antenna that is part of their 406 Mhz unit. I would have preferred mounting the unit inside of the tail fin, but I had the paint done last year, and I didn't want remove it, re-install, and re-paint. It seems to be a constant battle of removing antennas (DME, ADF) and adding new ones - two GPS and the ELT blade with GPS. Adding ADS-B later this year will add another antenna on the top of the fuselage, and another on the belly. At least the GPS antennas are flat and fairly small, and the ADS-B antennas are also small, but collectively, a whole bunch of drag.

[larryb]

Mike:

     It's a KX155, 1984 vintage, same as the plane. I have sub-par RX as well, so I don't think TX power is making much of a difference. Long-term, I will not be keeping that radio. It can make the gps drop out when transmitting on certain frequencies. I hear that's a common problem with the older KX155's. I will re-do that GPS interference test after fixing the ELT antenna location problem, but I'm not hopeful. I'm not going to spend any time and $'s trying to make the old radio play nice with the GPS.

Larry

[N601RX]

They make a notch filter for the gps that will fix the interference issue.  That check should have been part of the flight testing when the gps was installed.  Check the install manual for your gps. It actually goes in the com coax.

[larryb]

Mike:

     GPS today is a 496. GTN650 or GTN750 is coming in the near future. From what I read, even the filter may not cure the interference. I may be willling to try cheap fixes, but I'm leaning towards replacing everything in the stack. 

Larry

[m20exec]

Sorry Larry. I should have re-read your post. As a HRO your obviously aware of the rf coupling issue. At 2" your ELT might act more like a secondary of a RF transformer.

Jose's suggestion of mounting the ELT antenna on the bottom of the AC would minimize the coupling, but you might be trading one issue for another in the event of an emergency landing. Assuming you had to make an off airport landing, didn't end up inverted and your ELT now on the bottom wasn't sheared off,

[N601RX]

A couple of other failure points I found when installing the new coax was the connectors that King uses in the back of their racks.  They have some fingers on the inside of the rack that break off. I also had one that the coax shield had broken off the back side of the connector.  The shield is soldered to the housing of the connector and there is a stress point where the solder stops and wire starts.  It wasn't supported well and over time it broke all the way around.  The center conductor was still connected and it wasn't obvious that the shield had seperated.  Here is what the connector looks like. Medium wall glue lined heatshrink is a good idea for support on coax connectors.

http://www.lane-pilot.com/servlet/Detail?no=24

http://www.jamesaircraft.com/'>

[1964-M20E]

I have been experiencing interference from radio towers located near an airport I frequently fly to. I questioned the avionics shop and he said the problem was the Ameri-King 450 ELT and resonance with the signal from the ground based commercial transmitters. I am wondering if anyone makes some type of band pass filter that I could put on the ETL antenna that would allow 121.5 and 243 to be transmitted but would attenuate most all other frequencies.

My ELT antenna is 30”+ from my com antenna which is in accordance with the Ameri-King installation instructions.

[larryb]

I have been experiencing interference from radio towers located near an airport I frequently fly to. I questioned the avionics shop and he said the problem was the Ameri-King 450 ELT and resonance with the signal from the ground based commercial transmitters. I am wondering if anyone makes some type of band pass filter that I could put on the ETL antenna that would allow 121.5 and 243 to be transmitted but would attenuate most all other frequencies.

My ELT antenna is 30”+ from my com antenna which is in accordance with the Ameri-King installation instructions.

If your problem is repeatable, I would temporarily disconnect the ELT antenna and fly near the tower and check for interference. That would prove the ELT as the source. At that point you could consider a fix.

Larry

[1964-M20E]

Thanks for the suggestion.

I did that and the interfenece did go away. So I'm quite certain it is the ELT cusing the problem. One option would be to go with the newer 451.

[larryb]

Thanks for the suggestion.

I did that and the interfenece did go away. So I'm quite certain it is the ELT cusing the problem. One option would be to go with the newer 451.

That's great, always good to know for sure what the problem is. Rather than pour money into old technology, I personally would take this opportunity to upgrade to a 406 ELT. In fact, that's what I did myself when faced with the antenna location problem that started this thread. I now have an ACK E-04 ELT. The new antenna has been moved rearward, away from the com2 antenna. The old antenna hole has been patched. The new ELT was $550, I sold the old one for $100.

Larry

[1964-M20E]

Yes the upgrade is being considered as well. I was looking for something hot fast and cheap to fix it now and then do the upgrade.

It appears that the ACK is much cheaper that Ameri-King with the GPS option and I can use my currently installed annunciator switch.

[N601RX]

Many of the old ELT 's will re-radiate strong signal that the recieve on their antenna, even though they don't contain a reciever. In addition to the above, make the 2 coaxes are not bundled together. Although not on aircraft, I've used directional couplers at work before to solve similiar problems. If the ACK 406 doesn't exhibit the problem, at least you would be getting something new for your money. I put one in earlier this year.

[Charles McDowell]

I am trying to install a new ELT in my Exp. Kitfox. On belly just to the rear of the left seat is the Transponder antenna. 2ft.back and on top of the fuselage is the comm antenna, 1 ft back from there and on the belly is the ADS-B antenna. I’m trying to install the new ELT antenna but cannot get it to the 3ft. separation from the ADS-B antenna. Can I go closer? Say 24”? Need help. -Charles

Edited August 9, 2020 by Charles McDowell

[MB65E]

Hi Charles. 
 Use 43.13 guidance. Look up some YouTube videos of where kitfox ELTs are. If your kitfox has the aft baggage with the glass doghouse you could mount it inside the rear turtle deck as long as it was not blocked by metal. Any fairing on the top side of the airplane could be used for the ELT mount. 
-Matt

[chriscalandro]

Whoever said to mount it on the belly.... don’t do that. That’s a sub-good idea. 
 

consider that antenna has been there 10 years. Has it been a problem for 10 years and around the time that antenna was installed? I’d bet not. 
 

your problem is either a radio problem or a antenna problem (ground plane, wiring, etc)

 

It’s wishful thinking to think you can just move an antenna and solve a problem, but I think you will find the real problem will cost much more than that unfortunately. 

[jaylw314]

Also, consider the ELT antenna needs to be in a place that it will not be sheared off in a survivable crash.  That means the belly is a terrible idea.  A crash that destroys an antenna on the upper surface of the rear fuselage is less likely to be survivable.  

[GeeBee]

On my PA-18 it was mounted on the aft inboard end of the right fuel tank cover on the wing. The fuel tank cover acted as the ground plane for the antenna and since it was near the wing attach point it was highly survivable. Do you belong to an owners or builders group? What are they doing?

[PT20J]

The ELT antenna should be mounted on the top of the aircraft for crash survivability and improved reception by the satellites. 

The issue is that placing it in close proximity to another vertically polarized antenna of similar length can change the radiation characteristics of the other antenna even though the ELT is not transmitting. The antennas you want to stay away from are the VHF comm antennas. That said, it is almost impossible to mount all the antennas for all the stuff we cram into small single engine airplanes in a way that meets all the manufacturer’s distance requirements - there just isn’t enough real estate. Do the best you can. Radio waves follow the inverse square law, so even a few inches can make a difference. You might talk to a couple of avionics shops about their experiences. @Baker Avionics has been really helpful. 

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