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ELT Antenna Mount

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skywagon

Joined: 11 Feb 2006
Posts: 184

Posted: Sun Jun 10, 2012 6:25 pm Post subject: ELT Antenna Mount

Jeff,

  

 I viewed this late.  Your are right, way too much  signal blocking structure, but, maybe even worse it is the antenna is positioned  horizontally.  The polarization of the signal path is all wrong.  It  must mounted vertically...

 I am sure that you figured all this out by  now....

 Dave

  

   

 [quote]   ---

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Kellym

Joined: 10 Jan 2006
Posts: 1705
Location: Sun Lakes AZ

Posted: Sun Jun 10, 2012 6:35 pm Post subject: ELT Antenna Mount

How do you know what the antenna orientation will be after a crash? Only 

then will orientation matter.

Even then, orientation to the satellite matters more than orientation to 

terrestrial receivers.

Any  mounting that prevents the antenna from being broken during the 

crash will be better than one that lets the antenna be snapped off by 

flipping the plane.



On 6/10/2012 7:17 PM, David Lloyd wrote:

 	  Quote: 	
		   Jeff,

 I viewed this late.  Your are right, way too much signal blocking 

 structure, but, maybe even worse it is the antenna is positioned 

 horizontally.  The polarization of the signal path is all wrong.  It 

 must mounted vertically...

 I am sure that you figured all this out by now....

 Dave

 ------------------------------------------------------------------------




	




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_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
KCHD

skywagon

Joined: 11 Feb 2006
Posts: 184

Posted: Sun Jun 10, 2012 9:25 pm Post subject: ELT Antenna Mount

Kelly,

I think the majority of crashes usually wind up in horizontal orientation. 

If the antenna is mounted vertically in front of the vertical stabilizer in 

most crashes that part of the airframe has a good chance of staying fairly 

intact even upside down.

About the ELT... the older 121.5 broadcasting on the guard band is usually 

not heard by a satellite but, overflying aircraft.  The newer higher band 

units should reach an overhead satellite.  It would be interesting to see 

how well the contact goes from an antenna that is oriented horizontally and 

close the to earth, i.e. crash site.

I wonder if someone on the List would respond that knows the details about 

the new ELT's performance under adverse conditions.

D



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Kellym

Joined: 10 Jan 2006
Posts: 1705
Location: Sun Lakes AZ

Posted: Mon Jun 11, 2012 5:04 am Post subject: ELT Antenna Mount

The point is we simply don't know orientation of a crash. If on flat 

ground it may very well be horizontal, although I have seen a lawn dart 

orientation as well.  In mountains, vertical is just as likely.  121.5 

is no longer monitored by satellite, so only over flying aircraft are 

likely to detect unless crash is near ATC facility. It would be 

interesting to know what orientation works best with satellites.



On 6/10/2012 10:23 PM, David Lloyd wrote:

[quote] 

 <skywagon(at)charter.net>



 Kelly,

 I think the majority of crashes usually wind up in horizontal 

 orientation. If the antenna is mounted vertically in front of the 

 vertical stabilizer in most crashes that part of the airframe has a 

 good chance of staying fairly intact even upside down.

 About the ELT... the older 121.5 broadcasting on the guard band is 

 usually not heard by a satellite but, overflying aircraft.  The newer 

 higher band units should reach an overhead satellite.  It would be 

 interesting to see how well the contact goes from an antenna that is 

 oriented horizontally and close the to earth, i.e. crash site.

 I wonder if someone on the List would respond that knows the details 

 about the new ELT's performance under adverse conditions.

 D



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_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
KCHD

grosseair(at)comcast.net
Guest

Posted: Mon Jun 11, 2012 5:51 am Post subject: ELT Antenna Mount

Whether or not the ELT works in whatever orientation you install it 

really only matters to me if I have a survivable crash.

If my plane "lawn darts" into the ground I'm probably not gonna make it 

no matter what my ELT does or doesn't do. On the other hand, if my plane 

winds up more or less horizontal there's a good chance of my having 

survived and I want that thing working. The ELT manufacturers have 

already done the testing and the design. "Use our antenna and install 

according to our directions and it will work." ELT antennas are meant to 

be installed vertically because that's the orientation in which they work.



John Grosse



OBTW Haven't we gone around on this topic before?



Kelly McMullen wrote:

 	  Quote: 	
		   

 <kellym(at)aviating.com>



 The point is we simply don't know orientation of a crash. If on flat 

 ground it may very well be horizontal, although I have seen a lawn 

 dart orientation as well.  In mountains, vertical is just as likely.

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stuart(at)stuarthutchison
Guest

Posted: Mon Jun 11, 2012 6:24 am Post subject: ELT Antenna Mount

G'day all,



Kelly is right, 121.5MHz satellite monitoring was turned off in early 2009,

but many airliner radios are configured to monitor GUARD 121.5 as a matter

of routine.  406MHz (actually 406.025) is monitored by both a special

payload on a range of geostationary (GEO) satellites and low earth orbit

(LEO) satellites intended primarily for other things.  The main advantage of

406 upgrade is the that the transmission is digital. Hence, your digital

databurst (0.44 to 0.54 seconds long, randomised at around 50 second

intervals to avoid repeated over-transmissions and TX at about 5W - much

higher than the old analogue beacons) contains a bunch of useful information

that the Rescue Coordination Centre can use immediately to differentiate one

beacon from another (i.e you).  They can then match a bunch of info to the

distress, such as your flight plan.  An in-built constant 121.5 swept tone

generator is still required as a homing localisation signal for rescuers.

406 can be homed to, but requires specialised equipment to track the short

databursts.



The GEO satellites orbit the Earth, but at a rate consistent with the Earths

rotation.  Hence, they remain in an essentially constant position relative

to the Earth, roughly overhead (about 36km up I think). A GEO satellite will

generally receive your digital databurst and relay it to the RCC

immediately, but can't work out where you are unless your ELT happens to

transmit the GPS position as well (either in-built GPS or fed from the

aircraft NAV system).  The SAR situation may then be resolved immediately

with a telephone call to you ... for example if the beacon is accidentally

switched on.



LEO satellites orbit the Earth and may also take the GPS position from your

ELT, but otherwise use Doppler analysis to fix the position.  However, a fix

is calculated on both sides of the satellite track.  A second satellite pass

on a different track is required to resolve the ambiguity (i.e. there will

be matching fixes to one side of the original track, but they won't match on

the other side).  The trouble with this is that the passes are sometimes as

long as 2 hours apart, so it may take quite some time to resolve the

ambiguity and determine where the ELT is transmitting from.  The reason 406

is substantially more accurate to Doppler fix than the older analogue 121.5

beacons is that the standard for frequency stability is much higher.  Hence,

the annulus for older beacons was about 20km and for new about 5km.  I

recommend ELTs or Personal Locator Beacons (PLBs) with in-built GPS as they

dramatically reduce the time to first accurate fix, which may matter if it's

getting dark.  Exposure to the elements is often the greater threat, so

there's no advantage in waiting longer than you have to for rescue.



Keep in mind that a great many ELTs don't work at all after a crash because

the COAX gets damaged, so keep that cable run short if possible.  On the up

side, no matter whether you're attempting to transmit straight up to the GEO

or sideways to one of the many LEO satellites, your relatively high-power

databurst is likely to be detected quickly, which provides the RCC a LOT of

information about you.  They don't necessarily need to fix you by satellite

to work out where you disappeared off radar ... but it is a huge advantage

to the RCC knowing which ELT is actually transmitting !!!



I have a fast two-seat tandem tailwheel with a slider canopy.  There isn't

much room between the canopy track and the VS, so my antenna will be bottom

mounted close to the ELT inside.  The antenna will be raked back at 30

degrees with a custom-made fairing around the 3/4" x 6" base. In my POV, the

antenna is likely to be on top after crash landing rough enough for me to

depend on it, with the 'donut' radiating towards the GEO satellite.  If not,

the LEOs are likely to see in due course.  I also carry a GPS PLB.



Hope this helps.



Stu

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sprocket(at)vx-aviation.c
Guest

Posted: Mon Jun 11, 2012 8:22 am Post subject: ELT Antenna Mount

Great technical comparison.

One thing about 406 (and other VHF/UHF frequencies) is that radio waves tend 

to bounce around off of metal/water/rock rather than be absorbed.  I'm not 

an RF guy, but this effect is seen when using a cellphone in an elevator 

(effectively a Faraday cage with small openings for radio waves to leak 

out).  Is there anything about 406 MHz that makes it better in the SAR 

application due to this effect?



What I'm getting at, is does it really matter what polarization is, or 

adjacency to metal objects is in a real-world application.... the signal 

might just bounce around until it 'leaks out' to the satellite?



Vern



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kuffel(at)cyberport.net
Guest

Posted: Mon Jun 11, 2012 8:26 am Post subject: ELT Antenna Mount

It has been almost 50 years since I worked for an antenna company but I do 

have substantial real-world experience with ELTs.



Antenna orientation doesn't matter in this application.  You have on the 

order of over 100 db of signal loss before communication with a satellite 

becomes difficult.  Antenna orientation will cause at most 3 db of signal 

loss.  A much more important factor is antenna/cable integrity.  In a 

relatively minor crash almost any location will work.  At worst you can 

remove the ELT and jury rig a workable antenna.  In a major crash (you're 

trapped, won't survive long, etc) there is a high likelihood *any* external 

ELT antenna will be disconnected.



This is why I advocate ELT antennas be mounted internally.  Bad radio 

location but a lot better than non-existent.  For fiberglass structure a 

good location is in the forward part of the tail cone.  For carbon fiber and 

metal planes you should mount it somewhere in the cabin with a view outside. 

For bubble canopies this is easy but even high-wing and tube structures will 

work from inside.  The 406 MHz signal will pass through any gap larger than 

5 inches.  Even the non-essential 121.5 MHz signal will be detectable from 

short distances no matter what pile of metal randomly surrounds the antenna.



Tom Kuffel

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skywagon

Joined: 11 Feb 2006
Posts: 184

Posted: Mon Jun 11, 2012 1:35 pm Post subject: ELT Antenna Mount

Tom,

Great review and what I was hoping would show up on the List......

Thanks, D



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