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Photo request: the "Redline" ELT antenna mount

N546RV

Well Known Member
I've been pondering where to stick my ELT antenna. I've seen numerous mentions of the Redline approach, where the antenna is mounted to the right rear armrest just ahead of the F-807 bulkhead, with the whip secured so it follows the curve of the bulkhead above the canopy rails and stays clear of the canopy.

However, I was trying to work this out yesterday and having trouble figuring out how to make it work. Since the bulkhead meets the armrest at an obtuse angle (viewed from above), even if I mount the antenna as close to the bulkhead as I can, it'll be a couple inches away from the bulkhead at the point where the rigid base terminates. I suppose I could fabricate some looong standoffs but they seem like they'd be prone to getting hit by the backseater.

An additional problem is the canopy rail - the rigid antenna base stops just short of the rail, but even attempting to position it as far inboard as possible I'm not sure the whip will clear the inside edge of the rail.

Long story short, I'm having trouble seeing how this method can work well, and this seems like one of those times where a picture = 1000 words.
 
I flew one RV-8 with the antenna mounted as you suggest. It alway got in the way.

Mount it under the empennage fairing and avoid the pain. Three RVs done like this, as will the next one. One accidental ELT test (inadvertence activation) got an immediate call from the FAA so this demonstrated it works.

Combine that with knowing this mount is the most survivable place on your airplane makes this an easy decision.

Picture will have to wait until Condition Inspection.

Carl
 
I flew one RV-8 with the antenna mounted as you suggest. It alway got in the way.

Mount it under the empennage fairing and avoid the pain. Three RVs done like this, as will the next one. One accidental ELT test (inadvertence activation) got an immediate call from the FAA so this demonstrated it works.

Combine that with knowing this mount is the most survivable place on your airplane makes this an easy decision.

Picture will have to wait until Condition Inspection.

Carl

I really like the idea of putting it under the fairing, but the 406 antenna is too long for that, at least assuming I mount the antenna base to the F-810 bulkhead:

1CAAXy7l.jpg


I suppose I could fabricate something that would bridge the longerons a few inches forward to make it work, maybe have the whip just pass through a snap bushing in the bulkhead. I don't think the fab work would be that bad, but getting a good ground plane might be interesting, not to mention the performance implications of having maybe 15% of the antenna length masked by the turtledeck.

In the interest of my own picture = 1000 words comment, here's my situation trying to mock up the armrest antenna placement. Looking across the fuselage, we see that there ends up being a sizeable gap between the whip and the bulkhead. We also see how close the top of the antenna base is to the cockpit rail (though it'd of course be lower when actually mounted):

2VvnWMll.jpg


And looking from above, we see that even with the antenna base positioned pretty far inboard, the whip is really close to the rail:

zv0E92ml.jpg


Maybe the folks doing the "Redline" mount have a different, angled antenna or something? This is the antenna that came with my Artex ELT-345.
 
I really like the idea of putting it under the fairing, but the 406 antenna is too long for that, at least assuming I mount the antenna base to the F-810 bulkhead:

1CAAXy7l.jpg


SNIP

Yep - but there are work arounds.

For the ACK ELT (antenna is not a long as you have) you can do a bend near the end that sends the antenna wire under the VS (same deal as bending the tip in your rear seat location). Just make a stand off out of scrap fiberglass to hold the end of the antenna in place. Think of your belly mounted bent whip antenna. It still works because the first third (the high current section) does 90% of the RF radiation.

As yours seems longer you may need to make a bracket that mounts the base a couple of inches further forward of the 801 bulkhead, and do the bend on the aft end as well. I don’t think there will be any practical degrade in performance - and I suggest less degrade than the shielding the hull would present in the rear seat location.

Carl
 
I really like the idea of putting it under the fairing, but the 406 antenna is too long for that, at least assuming I mount the antenna base to the F-810 bulkhead:

1CAAXy7l.jpg


I suppose I could fabricate something that would bridge the longerons a few inches forward to make it work, maybe have the whip just pass through a snap bushing in the bulkhead. I don't think the fab work would be that bad, but getting a good ground plane might be interesting, not to mention the performance implications of having maybe 15% of the antenna length masked by the turtledeck.

I agree that the turtle deck is the best place. Would there be an issue in trimming the antenna wire a few inches?
 
FWIW

Philip,

Here's what I did:
Short antenna from Artex.
Made a small angle riveted to the right side, between the 2 tubes of the mid fuselage brace.

It does not interfere with the canopy in any way.

Don't forget that Artex specifies to mount the antenna vertically:
Place the antenna in a location where the antenna can be installed vertically

My only concern is with the ground plane, though the wing could reflect some waves...
 

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Philip,

Here's what I did:
Short antenna from Artex.
Made a small angle riveted to the right side, between the 2 tubes of the mid fuselage brace.

It does not interfere with the canopy in any way.

Don't forget that Artex specifies to mount the antenna vertically:
Place the antenna in a location where the antenna can be installed vertically

My only concern is with the ground plane, though the wing could reflect some waves...

This is where I'm mounting my ELT antenna also, I'm working on it now.
 
Here's how I did it although mine is an ACK antenna and the base appears to be at more of an angle than yours.

The whip is about 3/32" from the rail.




 
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I made up a .050” plate for mine, and attached it with AN470-4-4 (I think) rivets on the right side mid cabin.
 

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I like seeing the different ELT mounts shown in this thread. My question is what kind of VSWR are people getting with these installations? I know the ACK 406 ELT does a VSWR check on power up. Are people just ignoring the high VSWR?

I have my ACK 406 ELT antenna inside under the sliding canopy with no VSWR power up errors.

Would love to do something like any of the photos above on my RV-8 as long as there is not a high VSWR problem.
 
How do you determine the grounding plane for one of these RV-8 solutions? Has anyone tested their setup to see what performance is available with a less then optimal 2x2' grounding plane?
 
In my -8, I currently have the old Ameri-King AK450 121.5 ELT. The antenna is installed next to left side of the rear seat and curved over the top of the seat back, matching the curve of the fuselage under the canopy. It's held in place there with a plastic-grommet bracket. It's very similar to the photos LKITSON posted above, but on the left side and with the longer 121.5 antenna. This seems to work fine for the 121.5 frequency, although I haven't tested reception at a distance. I'll post a photo if anyone wants it.

I'm planning to replace this with a 406 ELT this year, and wondering if a similar antenna installation would work for that frequency. If not, it sounds like the required installation is on top of the fuselage, but with the super slider there's very little space for it. Any other suggestions?
 
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Don't forget that one of the possible situations when you need an ELT is if the aircraft is flipped over or in another "non-standard" position.
 
My $0.02 view on ELTs is to find a used old 121.5 D-cell ELT and install it to satisfy the regulatory requirement. Put the antenna under the tail fairing. Batteries for the 406 ELTs are ridiculously expensive. IMO ELTs should not be required for ADS-B equipped airplanes.
 
I'm at exactly this place too. RV8 with the Showplanes Fastback Mod. I was looking at the spot between the two seats standing vertical up through the mid cabin roll bar. It's a bit of a stretch to run the cable and not go through any bulkheads to get there. I'm not sure the supplied (60" I think) cable will reach. The aft position under the fairing looks sweet, but apparently does not comply with the installation instructions from the manufacturer.

I'm surprised that such precise installation instructions are provided and required for an item that won't function until something really bad has happened. How on earth will that 30 degrees from vertical limitation operate then? Am I forbidden from crashing more than 30 degrees from level?

Seems to me they should be recommending any antenna orientation as long as you place it in the aft quarter of the aircraft, but what do I know?
 
30 deg from vertical ELT antenna

I think the 30 deg from vertical is bull. See post #3 below. Apparently placing the antenna under the tail fairing isn’t a good idea because it attenuates the antenna however if the elt goes off and the Air Force calls you then it must work. Unfortunately you aren’t allowed to test the install with an activation. Metal close to the antenna attenuates the standing wave ratio (the amount of power reflected back towards the unit). If you have an ARTEX unit they do have a subscription for $40 you can use to test the installation and verify the signal is reaching the satellite. The subscription lasts for about six months. It will email you and text you if the signal was received. I wanted to protect the antenna so initially I mounted it behind the cargo panel of my RV3. I tested it but it did not work so the picture that you see in the post below will show you how I mounted my antenna. Using a small portable network analyzer it showed about 33% of the power was being reflected back however, the signal was reaching the satellite

https://vansairforce.net/community/showthread.php?t=215374 See post #3
 
What part of the antenna is the ANTENNA?

Okay. If I go back to mounting the antenna, and I have the shorter 15" one, vertically between seats on an -8, how much of the 15" needs to be exposed above the canopy rail? All of it? Just the final metal portion? This question reflects my desire to keep it relatively out of sight. As a completely amateur electrician, I'd suspect that putting the antenna between the legs of the central roll bar must also reduce the effectiveness of the antenna, or am I wrong?

The ELT came with an included cable. Am I able to roll my own cable to the length I need? if I have to string it from the tail to mid cabin without going through any bulkheads, the cable supplied isn't going to cut it.
 
I'm curious as to how many people that want to bury a ELT antenna have steps mounted on their plane. It seems a ELT antenna drag is negligible in comparison to the step's drag.
 
I'm curious as to how many people that want to bury a ELT antenna have steps mounted on their plane. It seems a ELT antenna drag is negligible in comparison to the step's drag.

I could be wrong, but on the RV8 with a slider the challenge is finding a place where the antenna will be effective if the aircraft ends up either on its belly or its back. Most aircraft allow it to be installed just before the vertical, which would provide some protection to the antenna if the aircraft flips. On the RV8, the slider goes all the way back to the VS. No room for an antenna.

Thankfully where I fly I can use a PLB in place of an ELT, so I didn't have to "solve" this problem for my RV8, but if I were required to put one in, I'd put it under the VS, and accept the lower antenna performance, or do like many have done, and put the antenna in an arc over the rear seat back.

Each builder/owner has to decide if they are installing an ELT for compliance or actual SAR.
 
An ELT antenna in is a monopole antenna and when oriented vertically, it has a radiation pattern that is horizontally polarized and is the same as a vertical dipole. Think of the radiation pattern as a donut laying flat on a plate.

When the antenna is put in a horizontal position, the radiation pattern can be thought of as standing the donut on its side. You will have null areas on the axis of the hole. This is why the instructions state to mount vertically.

For those that want to mount in the tail but the antenna is too long, you can make the antenna a bent whip to get it to fit. Think of a COM antenna on the underside of your plane. Granted it does add some lobes to the radiation pattern but at least you didn’t shorten it.

I’m ditching the antenna that came with my ELT and will be installing a vertical rubber duck that is matched for 406 MHZ on the rear cabin bulkhead and has a clear view through the canopy. Something like the pic below.

Another advantage is the coax won’t be routed through a bulkhead since the mount will be on the bulkhead.

73’s
AD4AC
 

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