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msmeredith

Joined: 11 Nov 2013
Posts: 6
Location: United States

Posted: Tue Jan 14, 2014 7:03 pm Post subject: ELT Antenna

Hi - I'm installing a Kannad Integra 406AF ELT with built in GPS.  The  cheapest antenna for this ELT is the whip, which is what I bought with it.  The blade/vane for this unit are hundreds/thousands more expensive. But now I'm trying to figure out where to put the whip!  It's a monstrosity.



Aircraft is a Super Chipmunk which I'm totally rebuilding. (Google "Super Chipmunk Restoration".) I successfully completed a new electrical system using the Z-11 architecture and knowledge gained from the Aeroelectric Connection and this list.  (Thanks!!)  The ELT is the last thing on the to-do list before I secure all the wire bundles.



This is an unusual Chippy - it's single seat open cockpit, and I installed an 8 foot long turtleback/headrest to conceal a rollover bar behind the rear seat.  It's .025 alum, about a foot high/wide at the cockpit then tapering to about 4 inches at the fin.  I'd like to mount the antenna 2 feet back on the top of the main fuselage structure, so the rubber part is inside the turtleback, then poke the steel whip out the top through a small hole/grommet.  That approach would be unobtrusive, avoid an ugly 2 foot tall antenna with fat black base, and avoid beefing up the turtleback top to support it.  My question:  would the ELT still be heard with the bottom 8 inches or so of base hidden under .025 alum?  



Thanks!

_________________
Mark Meredith
Super Chipmunk N7DW

kuffel(at)cyberport.net
Guest

Posted: Tue Jan 14, 2014 7:55 pm Post subject: ELT Antenna

Mark,



<<  would the ELT still be heard with the bottom 8 inches or so of base 

hidden under .025 alum?  >>



Yes.



You are much more likely to not be heard by having the external part of the 

whip sheared off in the crash.



The longer answer involves not only the shielding effect caused by the 

aluminum but also the feed mismatch, both of which will reduce signal 

output.  But the application is so direct there is no doubt you could be 

heard by a satellite.  If you wish to be sure, build a mock up with foil 

over cardboard and scrap tubing.  See how it works with and without the 

foil.  Don't forget the rules about when and how to test ELTs.



Tom Kuffel

stuart(at)stuarthutchison
Guest

Posted: Tue Jan 14, 2014 8:30 pm Post subject: ELT Antenna

The Kannad whip antenna is base-loaded, so the ugly first 6" black base is

part of the radiating element that needs to be exposed.  I'll be making an

angled mount with fibreglass fairing like a COM antenna for the underside of

my Rocket, since I also have a manual 406 GPS PLB. I figure any automatic

activation of the ELT will be because of a sudden stop, so I'll probably be

upside down about then.  My life may depend on the automatic system pointing

skywards from the underbelly.



Kind regards, Stu



F1 Rocket    VH-FLY       http://www.mykitlog.com/RockFLY

www.teamrocketaircraft.com



--

msmeredith

Joined: 11 Nov 2013
Posts: 6
Location: United States

Posted: Wed Jan 15, 2014 5:10 am Post subject: Re: ELT Antenna

The longer answer involves not only the shielding effect caused by the 

aluminum but also the feed mismatch, both of which will reduce signal 

output.  But the application is so direct there is no doubt you could be 

heard by a satellite.  If you wish to be sure, build a mock up with foil 

over cardboard and scrap tubing.  See how it works with and without the 

foil.  Don't forget the rules about when and how to test ELTs.



Tom Kuffel[/quote]



Tom,

Good idea.  What do I need to measure the output with this mockup?

_________________
Mark Meredith
Super Chipmunk N7DW

msmeredith

Joined: 11 Nov 2013
Posts: 6
Location: United States

Posted: Wed Jan 15, 2014 5:31 am Post subject: Re: ELT Antenna

 	  stuart(at)stuarthutchison wrote: 	
		  I'll be making an

angled mount with fibreglass fairing like a COM antenna for the underside of

my Rocket, since I also have a manual 406 GPS PLB. 

Kind regards, Stu



F1 Rocket    VH-FLY       http://www.mykitlog.com/RockFLY

www.teamrocketaircraft.com

-- 	




Stu, 

I've thought about the "impact" of a rollover which is why I added the rollbar.  But think I'll stick with an assumption the airplane is still airborne or upright in big pieces if the ELT is ever activated!  

 

I have a very small cockpit and don't plan to carry a PLB.  There are very few places to put even a portable GPS for trips, so I went with the installed ELT (best I can tell, aircraft has never had one since it was built in 1951).  Kannad has an internal antenna also so works without the whip in a handheld mode, though I'd need a screwdriver and a good hand to access it...

_________________
Mark Meredith
Super Chipmunk N7DW

kuffel(at)cyberport.net
Guest

Posted: Wed Jan 15, 2014 7:09 am Post subject: ELT Antenna

Mark,

  

 <<   works with and without the  foil.  .... Good idea.  What do I need to measure the output with this  mockup?  >>

  

 The ELT still transmits on 121.5 so any aircraft  band hand held will do the job.  Most hand helds have a signal strength  indicator but a better way (although tedious) is to see how far away (over  flat ground) you can hear it without the foil/cardboard.  Then try it with  the shield.  No guarantee it will be just as good at 406MHz but the odds  are likely.

  

 Or contact a local amateur radio club (find at  arrl.org).  Many ham UHF hand helds can receive 406MHz.  You will  easily find a ham who would enjoy helping with your experiment.

  

 <<  But think I'll stick with an  assumption the airplane is still airborne or upright in big pieces if the ELT is  ever activated!  >>

  

 My experience suggests this is not a valid  assumption.  When this is true, the people are ambulatory, can remove  the ELT from its mount, attach the portable antenna, or just talk on the  aircraft radio.  But in most crashes I have seen, if the occupants are  badly injured (and so really need an ELT) then most external  antennas are no longer working.  Carrying a PRB in your pocket might be a  good Plan B.

  

 If you do run the test, let us know the  results.

  

 Tom Kuffel

   [quote][b]

msmeredith

Joined: 11 Nov 2013
Posts: 6
Location: United States

Posted: Wed Jan 15, 2014 10:17 am Post subject: Re: ELT Antenna

[quote="kuffel(at)cyberport.net"]Mark,

  

 <<works>>

  

 The ELT still transmits on 121.5 so any aircraft  band hand held will do the job.  Most hand helds have a signal strength  indicator but a better way (although tedious) is to see how far away (over  flat ground) you can hear it without the foil/cardboard.  Then try it with  the shield.  No guarantee it will be just as good at 406MHz but the odds  are likely.

  

 Or contact a local amateur radio club (find at  arrl.org).  Many ham UHF hand helds can receive 406MHz.  You will  easily find a ham who would enjoy helping with your experiment.

  

 <<  But think I'll stick with an  assumption the airplane is still airborne or upright in big pieces if the ELT is  ever activated!  >>

  

 My experience suggests this is not a valid  assumption.  When this is true, the people are ambulatory, can remove  the ELT from its mount, attach the portable antenna, or just talk on the  aircraft radio.  But in most crashes I have seen, if the occupants are  badly injured (and so really need an ELT) then most external  antennas are no longer working.  Carrying a PRB in your pocket might be a  good Plan B.

  

 If you do run the test, let us know the  results.

  

 Tom Kuffel

    	  Quote: 	
		  [b] 	




Tom, 

Thanks.  I'll run the test and let you know.  I have some extra .025 alum so will use that for the mockup.

_________________
Mark Meredith
Super Chipmunk N7DW

msmeredith

Joined: 11 Nov 2013
Posts: 6
Location: United States

Posted: Wed Jan 15, 2014 10:50 am Post subject: Re: ELT Antenna

 	  kuffel(at)cyberport.net wrote: 	
		  Mark,

  

 <<works>>

  

 The ELT still transmits on 121.5 so any aircraft  band hand held will do the job.  Most hand helds have a signal strength  indicator but a better way (although tedious) is to see how far away (over  flat ground) you can hear it without the foil/cardboard.  Then try it with  the shield.  No guarantee it will be just as good at 406MHz but the odds  are likely.

  

 Or contact a local amateur radio club (find at  arrl.org).  Many ham UHF hand helds can receive 406MHz.  You will  easily find a ham who would enjoy helping with your experiment.

  

 <<  But think I'll stick with an  assumption the airplane is still airborne or upright in big pieces if the ELT is  ever activated!  >>

  

 My experience suggests this is not a valid  assumption.  When this is true, the people are ambulatory, can remove  the ELT from its mount, attach the portable antenna, or just talk on the  aircraft radio.  But in most crashes I have seen, if the occupants are  badly injured (and so really need an ELT) then most external  antennas are no longer working.  Carrying a PRB in your pocket might be a  good Plan B.

  

 If you do run the test, let us know the  results.

  

 Tom Kuffel 	




Tom and other listers,

Second thoughts.  My bent comm and blade transponder antennas are under the belly amidships behind the wing, close to the ELT transmitter's position above them.  If I mount the ELT whip antenna underneath also, can I bend the steel rod beyond the black base so it's shaped similar to my comm antenna?  Would I need a large bend radius to prevent it from breaking, and would this affect the transmit functionality?   I could move it forward a foot or so (just aft the flaps) to separate it from the comm antennas.  I just need to be careful not to tune my radio to 121.5 when testing the ELT.



Of course that round antenna base will be a big source of drag, but mounting it underneath would solve the rollover and aesthetic issues.  Though now the assumption is the landing gear stay with the airplane!

_________________
Mark Meredith
Super Chipmunk N7DW

nuckolls.bob(at)aeroelect
Guest

Posted: Wed Jan 15, 2014 2:53 pm Post subject: ELT Antenna

At 09:04 PM 1/14/2014, you wrote:




Hi - I'm installing a Kannad Integra 406AF ELT with built in 

GPS.  The  cheapest antenna for this ELT is the whip, which is what I 

bought with it.  The blade/vane for this unit are hundreds/thousands 

more expensive. But now I'm trying to figure out where to put the 

whip!  It's a monstrosity.



   I presume you're talking about the AV-200 described in

   Kannad's manual . . . .



http://tinyurl.com/olaccxt

This is an unusual Chippy - it's single seat open cockpit, and I 

installed an 8 foot long turtleback/headrest to conceal a rollover 

bar behind the rear seat.  It's .025 alum, about a foot high/wide at 

the cockpit then tapering to about 4 inches at the fin.  I'd like to 

mount the antenna 2 feet back on the top of the main fuselage 

structure, so the rubber part is inside the turtleback, then poke the 

steel whip out the top through a small hole/grommet.  That approach 

would be unobtrusive, avoid an ugly 2 foot tall antenna with fat 

black base, and avoid beefing up the turtleback top to support 

it.  My question:  would the ELT still be heard with the bottom 8 

inches or so of base hidden under .025 alum?



   Probably not. That "bottom 8" is probably the 406 mHz

   part of the antenna.  Further, given that this product

   doesn't have a separate GPS antenna, I'm assuming that

   the single antenna assembly serves the GPS receiver

   as well.



   I think you should take pains to install per the

   manufacturer's recommendations.

   Bob . . .

kuffel(at)cyberport.net
Guest

Posted: Wed Jan 15, 2014 5:32 pm Post subject: ELT Antenna

Mark,



<<   If I mount the ELT whip antenna underneath also  >>



This is probably the worst location for an ELT antenna.  Any kind of brush 

at an off-airport landing will wipe it off the fuselage.



<< can I bend the steel rod beyond the black base so it's shaped similar to 

my comm antenna?  >>



Don't know why not.



<<  Would I need a large bend radius to prevent it from breaking, >>



No idea.  If the antenna there is just a steel rod as you say, then bending 

it around a 1" pipe or so should work.  But once again, this is a very bad 

location.



<< would this affect the transmit functionality?   I could move it forward a 

foot or so (just aft the flaps) to separate it from the comm antennas.  >>



The nearness of other VHF antennas will affect performance more than a bend. 

The above fuselage location has two ground planes of isolation from your Com 

antenna.



Is it time yet to consider making the cover out of fiberglass instead of 

aluminum?



Tom Kuffel

stuart(at)stuarthutchison
Guest

Posted: Wed Jan 15, 2014 6:46 pm Post subject: ELT Antenna

The ELT antenna is almost never radiating and when it is activated, COM

radio performance is unlikely to be relevant.  Bob may be able to confirm,

but I believe the ELT antenna proximity to VHF COM antennas can be

problematic because radio transmissions have been known to activate the ELT.



I acknowledge Mark's comment about the possibility of an underbelly antenna

being wiped off, which is why the base of mine will be firmly mounted

(midships) angled aft at about 45 deg (rather than bend the whip element)

and the fat part faired with fibreglass to minimise/absorb antenna impact

damage and or carve a path through the brush/grass.  Perhaps different with

a tricycle undercarriage, but I can expect the taildragger to be upside down

if I land in scrub (with the main wheels so far forward and a heavy engine),

so my priority is to have the antenna less shielded by the aircraft when

inverted.  If it were mounted on top, then it may also wiped off as the

aircraft comes to rest inverted or be pinned flat against the ally skin.

Preserving the coax link between the transmitter and antenna is a known

problem in major crashes, so keep the coax run short, protected and flexible

too if possible.



Kind regards, Stu



F1 Rocket    VH-FLY       http://www.mykitlog.com/RockFLY

www.teamrocketaircraft.com



--

Kellym

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

Posted: Wed Jan 15, 2014 7:17 pm Post subject: ELT Antenna

Almost all ELTs, especially the older ones do in fact radiate anytime 

they are near high powered VHF transmitters in the FM, TV, etc bands.

That radiation will in fact severely affect Com radio reception.

The antenna for ELT is integral to its TSO so no home brew antennas 

fullfill the TSO. While I think most DAR's are somewhat tolerant as to 

antenna placement, most install instructions do call for installation on 

top of the fuselage, and the install instructions are also part of the 

TSO approval.

So while you may have valid points about final resting of a crash being 

upside down, during the touchdown, prior to rollout odds are very high 

that any belly antenna will be wiped off the airframe. IMHO, with about 

equal experience between tail wheel(mostly off pavement in Alaska) and 

nose dragging aircraft, in off field landing situations of soft surface, 

nose wheel is far more likely to cause flipping than tail wheel.

Non-compliance with TSO is between you and your DAR, but most discussed 

here will be at least technical non-compliance.

Kelly

On 1/15/2014 7:47 PM, Stuart Hutchison wrote:

[quote] 



 The ELT antenna is almost never radiating and when it is activated, COM

 radio performance is unlikely to be relevant.  Bob may be able to confirm,

 but I believe the ELT antenna proximity to VHF COM antennas can be

 problematic because radio transmissions have been known to activate the ELT.



 I acknowledge Mark's comment about the possibility of an underbelly antenna

 being wiped off, which is why the base of mine will be firmly mounted

 (midships) angled aft at about 45 deg (rather than bend the whip element)

 and the fat part faired with fibreglass to minimise/absorb antenna impact

 damage and or carve a path through the brush/grass.  Perhaps different with

 a tricycle undercarriage, but I can expect the taildragger to be upside down

 if I land in scrub (with the main wheels so far forward and a heavy engine),

 so my priority is to have the antenna less shielded by the aircraft when

 inverted.  If it were mounted on top, then it may also wiped off as the

 aircraft comes to rest inverted or be pinned flat against the ally skin.

 Preserving the coax link between the transmitter and antenna is a known

 problem in major crashes, so keep the coax run short, protected and flexible

 too if possible.



 Kind regards, Stu



 F1 Rocket    VH-FLY       http://www.mykitlog.com/RockFLY

 www.teamrocketaircraft.com



 --

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

tim2542(at)sbcglobal.net
Guest

Posted: Wed Jan 15, 2014 7:47 pm Post subject: ELT Antenna

DAR approval not needed as the ELT is not even required unless there are 2 or more seats, or for phase 1 testing. Not saying its wise or will work, but it will not be required to be present for airworthiness. 

Tim

[quote] On Jan 15, 2014, at 7:16 PM, Kelly McMullen <kellym(at)aviating.com> wrote:

 Almost all ELTs, especially the older ones do in fact radiate anytime they are near high powered VHF transmitters in the FM, TV, etc bands.

 That radiation will in fact severely affect Com radio reception.

 The antenna for ELT is integral to its TSO so no home brew antennas fullfill the TSO. While I think most DAR's are somewhat tolerant as to antenna placement, most install instructions do call for installation on top of the fuselage, and the install instructions are also part of the TSO approval.

 So while you may have valid points about final resting of a crash being upside down, during the touchdown, prior to rollout odds are very high that any belly antenna will be wiped off the airframe. IMHO, with about equal experience between tail wheel(mostly off pavement in Alaska) and nose dragging aircraft, in off field landing situations of soft surface, nose wheel is far more likely to cause flipping than tail wheel.

 Non-compliance with TSO is between you and your DAR, but most discussed here will be at least technical non-compliance.

 Kelly

> On 1/15/2014 7:47 PM, Stuart Hutchison wrote:

> 

> 

> The ELT antenna is almost never radiating and when it is activated, COM

> radio performance is unlikely to be relevant.  Bob may be able to confirm,

> but I believe the ELT antenna proximity to VHF COM antennas can be

> problematic because radio transmissions have been known to activate the ELT.

> 

> I acknowledge Mark's comment about the possibility of an underbelly antenna

> being wiped off, which is why the base of mine will be firmly mounted

> (midships) angled aft at about 45 deg (rather than bend the whip element)

> and the fat part faired with fibreglass to minimise/absorb antenna impact

> damage and or carve a path through the brush/grass.  Perhaps different with

> a tricycle undercarriage, but I can expect the taildragger to be upside down

> if I land in scrub (with the main wheels so far forward and a heavy engine),

> so my priority is to have the antenna less shielded by the aircraft when

> inverted.  If it were mounted on top, then it may also wiped off as the

> aircraft comes to rest inverted or be pinned flat against the ally skin.

> Preserving the coax link between the transmitter and antenna is a known

> problem in major crashes, so keep the coax run short, protected and flexible

> too if possible.

> 

> Kind regards, Stu

> 

> F1 Rocket    VH-FLY       http://www.mykitlog.com/RockFLY

> www.teamrocketaircraft.com

> 

> 

> 

> --

Kellym

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

Posted: Wed Jan 15, 2014 7:55 pm Post subject: ELT Antenna

Agreed if a single seater is all that is involved.



On 1/15/2014 8:47 PM, Tim Andres wrote:

[quote] 



 DAR approval not needed as the ELT is not even required unless there are 2 or more seats, or for phase 1 testing. Not saying its wise or will work, but it will not be required to be present for airworthiness.



 Tim



> On Jan 15, 2014, at 7:16 PM, Kelly McMullen <kellym(at)aviating.com> wrote:

>

> 

>

> Almost all ELTs, especially the older ones do in fact radiate anytime they are near high powered VHF transmitters in the FM, TV, etc bands.

> That radiation will in fact severely affect Com radio reception.

> The antenna for ELT is integral to its TSO so no home brew antennas fullfill the TSO. While I think most DAR's are somewhat tolerant as to antenna placement, most install instructions do call for installation on top of the fuselage, and the install instructions are also part of the TSO approval.

> So while you may have valid points about final resting of a crash being upside down, during the touchdown, prior to rollout odds are very high that any belly antenna will be wiped off the airframe. IMHO, with about equal experience between tail wheel(mostly off pavement in Alaska) and nose dragging aircraft, in off field landing situations of soft surface, nose wheel is far more likely to cause flipping than tail wheel.

> Non-compliance with TSO is between you and your DAR, but most discussed here will be at least technical non-compliance.

> Kelly

>> On 1/15/2014 7:47 PM, Stuart Hutchison wrote:

>> 

>>

>> The ELT antenna is almost never radiating and when it is activated, COM

>> radio performance is unlikely to be relevant.  Bob may be able to confirm,

>> but I believe the ELT antenna proximity to VHF COM antennas can be

>> problematic because radio transmissions have been known to activate the ELT.

>>

>> I acknowledge Mark's comment about the possibility of an underbelly antenna

>> being wiped off, which is why the base of mine will be firmly mounted

>> (midships) angled aft at about 45 deg (rather than bend the whip element)

>> and the fat part faired with fibreglass to minimise/absorb antenna impact

>> damage and or carve a path through the brush/grass.  Perhaps different with

>> a tricycle undercarriage, but I can expect the taildragger to be upside down

>> if I land in scrub (with the main wheels so far forward and a heavy engine),

>> so my priority is to have the antenna less shielded by the aircraft when

>> inverted.  If it were mounted on top, then it may also wiped off as the

>> aircraft comes to rest inverted or be pinned flat against the ally skin.

>> Preserving the coax link between the transmitter and antenna is a known

>> problem in major crashes, so keep the coax run short, protected and flexible

>> too if possible.

>>

>> Kind regards, Stu

>>

>> F1 Rocket    VH-FLY       http://www.mykitlog.com/RockFLY

>> www.teamrocketaircraft.com

>>

>>

>>

>> --

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

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Jan 16, 2014 5:02 am Post subject: ELT Antenna

At 09:16 PM 1/15/2014, you wrote:

 	  Quote: 	
		  



Almost all ELTs, especially the older ones do in fact radiate 

anytime they are near high powered VHF transmitters in the FM, TV, etc bands.


	


   This phenomenon IS demonstrable . . . and rare. The

   ELT isn't actually producing any output power unique

   to its function. Very strong sources of local radiation

   (typically FM stations and the old analog TV stations)

   would find its way backwards into the output stage of

   the ELT's transmitter (constantly hooked to the antenna -

   receiver circuits are not so prone to exhibit this

   behavior). If there are two or more strong sources,

   then they can MIX or heterodyne against each other

   and produce a variety of spurious signals on lots

   of frequencies.



   Analog TV is gone and about the only source of such

   coherent energy would be an FM transmitter but you are unlikely

   to spend much time flying around in the FM station's

   radiation pattern and in close proximity . . . within

   a mile or two.



   One might hypothesize that the modern ELT's with

   TWO transmitters attached to the antenna are even

   more likely to exhibit the behavior . . . but you

   still need to be poking around the vicinity of the

   strong transmitter at altitudes generally 1000'

   AGL or less.



   I've heard the rumor that some ELT's were triggered

   by local radiation from a comm transmitter . . . but

   I've not seen any documentation or FMEA that supported

   the assertion. Certainly, modern ELTs are subjected

   to the standard DO-160 radiated susceptibility testing

   and perhaps even HIRF (high intensity RF) testing.

   None of this testing will concern itself with the

   intermodulation phenomenon cited above but it would

   watch for damage to the ELT and/or false triggering.



   Bob . . .

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Jan 16, 2014 5:09 am Post subject: ELT Antenna

 	  Quote: 	
		  

So while you may have valid points about final resting of a crash 

being upside down, during the touchdown, prior to rollout odds are 

very high that any belly antenna will be wiped off the airframe. 

IMHO, with about equal experience between tail wheel(mostly off 

pavement in Alaska) and nose dragging aircraft, in off field landing 

situations of soft surface, nose wheel is far more likely to cause 

flipping than tail wheel.


	


   I don't recall the source now . . . it was many

   moons ago . . . but the common wisdom circulated

   on the TC side of the house was that on top of

   the fuselage, just ahead of the vertical fin, was

   the best location for ELT antennas.



   If the wreckage did flip over, this location offered

   the highest probability for protection by vertical

   fin structure.



   The 406 mHz signal will be heard nicely even if

   the airplane is upside down . . . as long as the

   antenna isn't broken off.



   Given the magic multi-frequency design of modern

   ELTs and their antennas, I suggest the installer

   would do well to bite the bullet and strive for

   as pristine an installation as possible per the

   manufacturer's recommendations.



   Hence, bottom side location or bending of the antenna

   is discouraged.



   Bob . . .

gerry.vandyk(at)shaw.ca
Guest

Posted: Thu Jan 16, 2014 5:44 am Post subject: ELT Antenna

<nuckolls.bob(at)aeroelectric.com>



At 09:16 PM 1/15/2014, you wrote:

 	  Quote: 	
		  

<kellym(at)aviating.com>
	


 	  Quote: 	
		  

Almost all ELTs, especially the older ones do in fact radiate 

anytime they are near high powered VHF transmitters in the FM, TV, etc

bands.
	




   Analog TV is gone and about the only source of such

   coherent energy would be an FM transmitter but you are unlikely

   to spend much time flying around in the FM station's

   radiation pattern and in close proximity . . . within

   a mile or two.



   Bob . . . 



Just to throw out a thought...

Yesterday the Canadian government opened the bidding to the 700 Mhz spectrum

to cell phone companies.  The departure of analog television to digital

freed up the 700 Mhz band.  At some point in the future at least in Canada a

smart phone in a pilot's pocket will be communicating on the former analog

TV band.  Is this something we should be worrying about this side of the

border?  Any thoughts about mitigating such risks?



Gerry van Dyk

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Jan 16, 2014 6:36 am Post subject: ELT Antenna

Just to throw out a thought...



Yesterday the Canadian government opened the bidding to the 700 Mhz spectrum

to cell phone companies.  The departure of analog television to digital

freed up the 700 Mhz band.  At some point in the future at least in Canada a

smart phone in a pilot's pocket will be communicating on the former analog

TV band.  Is this something we should be worrying about this side of the

border?  Any thoughts about mitigating such risks?



   Good question . . . and far outside my personal

   experience history. But then, that's what all this

   DO-160/254/178 etc. etc is suppose to explore, identify

   and fix. That doesn't mean the occasional 'gotcha' won't

   surface in spite of best efforts . . . recent lithium

   battery issues are a good example. But that's part

   and parcel of any evolving technology.



   I'm working on an article for Kitplanes that will attempt

   to put some perspective on the lithium battery market as

   it relates to our airplanes. I'm not going to suggest

   that everybody pry the lead out of their airplanes

   and plunk lithium down in the hole . . . but lithium

   IS a rising star on the horizon . . . and there are

   ways to minimize risks while exploring POTENTIALLY

   attractive returns on investment.



   There was a similarly bumpy start up for ni-cad batteries

   on airplanes too . . . Wwwaaaayyyy back when. They offered

   some spectacular performance . . . but we set a few airplanes

   on fire with those batteries too. Today, the ni-cad is

   still attractive for some operators but it has not displaced

   the lead-acid products. Perhaps lithium will evolve to the

   same kind of market position . . . perhaps not . . .



   I'd say that the risks from the new phones are VERY low.

   Consider the actual radiate power of any hand held device,

   particularly a telephone. Digital communications technologies

   work very hard to maximize performance while minimizing

   energy consumption. This tends to make radiators less

   hazardous and receivers more resistant to co-located

   interference.



   Bob . . .

Kellym

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

Posted: Thu Jan 16, 2014 7:11 am Post subject: ELT Antenna

Hmm, I don't know why the switch to digital TV will make much 

difference. They are still broadcasting in my area on VHF RF channels 7 

and 8 through 13 in my local area. I used to have an antenna 

configuration on my Mooney that consisted of VHF com in front of 

windshield, top of fuselage behind baggage compartment, Loran about 18 

in. behind that, followed by ELT antenna right in front of vertical 

stab. When flying VFR transition over Phoenix Sky Harbor I would get bad 

squelch break anywhere within 10 nm of the antenna farm located on South 

Mountain, approx 8 mi south of Sky Harbor. Made hearing controllers very 

difficult.

I isolated it to the ELT by doing a flight with external ELT antenna 

disconnected, which completely eliminated problem even within a mile of 

the transmitters. This was a 1st generation ELT. Moving 1st com antenna 

from in front of windshield to the belly virtually eliminated the 

interference. Removing Loran antenna (custom version of Comant CI121) 

helped as well.

I understand later versions of ELTs have somewhat better isolation of 

the transmit oscillator, but doesn't completely eliminate. Good 

separation between ELT and com antennas is probably best defense.

Kelly

On 1/16/2014 6:01 AM, Robert L. Nuckolls, III wrote:

 	  Quote: 	

 <nuckolls.bob(at)aeroelectric.com>

 At 09:16 PM 1/15/2014, you wrote:

> 

> <kellym(at)aviating.com>

>

> Almost all ELTs, especially the older ones do in fact radiate anytime 

> they are near high powered VHF transmitters in the FM, TV, etc bands.

   This phenomenon IS demonstrable . . . and rare. The

   ELT isn't actually producing any output power unique

   to its function. Very strong sources of local radiation

   (typically FM stations and the old analog TV stations)

   would find its way backwards into the output stage of

   the ELT's transmitter (constantly hooked to the antenna -

   receiver circuits are not so prone to exhibit this

   behavior). If there are two or more strong sources,

   then they can MIX or heterodyne against each other

   and produce a variety of spurious signals on lots

   of frequencies.

   Analog TV is gone and about the only source of such

   coherent energy would be an FM transmitter but you are unlikely

   to spend much time flying around in the FM station's

   radiation pattern and in close proximity . . . within

   a mile or two.

   One might hypothesize that the modern ELT's with

   TWO transmitters attached to the antenna are even

   more likely to exhibit the behavior . . . but you

   still need to be poking around the vicinity of the

   strong transmitter at altitudes generally 1000'

   AGL or less.

   I've heard the rumor that some ELT's were triggered

   by local radiation from a comm transmitter . . . but

   I've not seen any documentation or FMEA that supported

   the assertion. Certainly, modern ELTs are subjected

   to the standard DO-160 radiated susceptibility testing

   and perhaps even HIRF (high intensity RF) testing.

   None of this testing will concern itself with the

   intermodulation phenomenon cited above but it would

   watch for damage to the ELT and/or false triggering.

   Bob . . .

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

msmeredith

Joined: 11 Nov 2013
Posts: 6
Location: United States

Posted: Thu Jan 16, 2014 4:06 pm Post subject: Re: ELT Antenna

All,

After digesting everyone's points, I decided I better pay attention to function over beauty...I'm going to mount the ELT antenna on top of the turtleback.  It'll be a foot or so forward of the fin - can't reach inside much farther than that anyway to beef it up and install hardware.  Ordered a longer coax today.  Thanks much for all your inputs and ideas!

_________________
Mark Meredith
Super Chipmunk N7DW

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