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ACK-04 in an RV-8

cdeerinck

Well Known Member
I reached out to Vans to get ideas on where to mount an ELT antenna in the RV-8, given the severe lack of choices. They replied that many owners put it under the tail fiberglass cowling.

I understand that it can't be within 30 degrees of vertical in that location, as called out in its manual, but I can't for the life of me figure out how to mount it or give it any ground plane for the antenna.

I have seen posts of people putting it in the cockpit, and that won't work for me, but I'm open to other ideas.

Can anyone share a picture, URL, or description on how they mounted theirs in that location?
 
ELT antenna location in RV-8

Here is what I did:

http://www.rv8.ch/elt-antenna-placement/

20050226171858929_1_original-1024x768.jpg
 
This has worked just fine and complies with the ACK installation manual and uses the ACK antenna.



I added a doubler plate and restricted the aft travel of the canopy by about 3 inches. Maybe not as pretty but works.
 
This is insanity

The specs as called out in the manual are crazy. They want a minimum 24" diameter grounding plane, with 48" being optimal. The book also says you can't substitute the antenna unless it complies with this:
VSWR @ 121.5 MHz <= 2.5:1
VSWR @ 406 MHz <= 1.5:1

Does anyone know of any antennas that meet that requirement, hopefully in a short whip, or a blade?

I see builders who are no doubt desperate to get their plane finished pushing it to the limits of where it won't throw an error code but probably will never reach the satellite. Many have grounding planes of a few inches. Some have installed them in the cockpit and those are not only inconvenient but don't have a grounding plane either. The closest was the last post (which the poster admits is not pretty), but it is still between the rudder and elevator, so will block 3/4 directionality. Please note, I am not criticizing any of them.

One person who swapped with another antenna in a parallel thread said "It broadcasts on Comm frequencies... ...so it should be ok", which a 406mhz is not.

ACK makes a blade, but it costs $1k (seriously!?!), but you are prohibited from putting it on the bottom.

My biggest concern is going upside down in a tail dragger, so why is the bottom bad? If I am crashing in the sticks, and end up not being able to walk away or call on the radio, or a cell, isn't it much more likely that I will be upside down?

The Search & Rescue and Civil Air Patrol guys always say to follow the book, and not to put it near the tail feathers, as it blocks transmission. And the mothership says to do what everyone else says not to do.

I understand this needs to reach a satellite (in space obviously), so I can't see a weak signal doing that, and almost everyone tests only the 121.5 frequency (even the book says not to test the satellite part).

<Ok, End RANT>

So are we all just closing our eyes and hoping for the best? I can see the accident reports saying "ELT installed not in accordance with recommended practices".

I thought maybe on top of the rudder, but there is no viable ground plane, and that will be the first thing to go if the plane flips.

Has anyone tried a mod to the tail fiberglass, to locate the antenna right in front of the rudder by an inch of so?
 
Why is it so restrictive? Liability? Does a personal ELT have such an antenna with a big ground plane, no. Mine is on the GIB's armrest, curved around the seat back, like many others I believe.
My plan is not to crash...
 
Ditto for Mickie

We went through this a long time ago and yes, what the book sets out and what you have to do in this case are hard to reconcile if not imposable.
Just like is posted above with Mickey's antenna we found that a mount on that rear former pointed to the back so the antenna can be place under the intersection cove is the best compromise. We have tested it a couple of times to see how it does putting out a good signal from that location and have found it does just fine.
If it makes you feel better we had FSDO out and asked about this when inspecting the aircraft. The answer was something like, yah that's where we have seen most of them. We too try to meet the rule every time we get it.
Some times you don't get what you are supposed to.
Hope this helps, Yours, R.E.A. III # 80888
 
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Couple more data points....

In conversations with an engineer at one of the popular ELT manufacturers we have learned the following.....

The transmit power of the 406 Mhz ELT's is 50X higher than it was for the traditional 121.5 ELT's, and their transmit signal has been repeatedly detected by COSPAS when no antenna was connected to the unit (usually on an uninstalled unit inside a building, etc., that was inadvertently activated).

There was some crash simulation testing done in conjunction with NASA and in two of the 3 test cases, the aircraft ended up upside down. In some of the tests the ant. was torn off the aircraft or the cable became disconnected, but in all test cases the ELT activated and was detected by COSPAS.

It was mentioned that as expected, the highest power transmission is with an intact externally mounted antenna.

If you look at enough crash wreckage, it quickly becomes apparent that there is no one ant. location that will work for all possible crash scenarios, when you consider the potential for it being ripped off, buried under wreckage, etc.
So it comes down to a personal decision in my mind. The ant. on my RV-6A is under the emp. fairing, because I have a high level of confidence that in any survivable crash scenario it will remain connected to the elt., and in good condition for transmitting. I accept the trade-off of potentially some level of shielding caused by the emp.
 
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