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elt antenna - Question for Bob

 
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Posts: 184

PostPosted: Fri Aug 17, 2018 7:15 am    Post subject: elt antenna - Question for Bob Reply with quote

Bob,
About "portable" 121.5/406 antennas....
I would like to construct a portable antenna that in the event of surviving a mishap, one could remove the ELT, take outside to a favorable location, connect the portable antenna and have a better chance of being found earlier.
I know this is a dual frequency design, but it has been too long since I studied and made lower frequency antennas, so I am asking if you could suggest a construction design.
I believe the 406 part should be cut for 3/4 wavelength, but the loading coil size and position for the 121.5 is vague. I am assuming about 1/3 up the whip so as to not interfere with the max. current wave point of the 406 signal.
Also, should the whip have a small circular ground plane attached to the BNC connector on the base.?
If you have ideas on this little antenna, I would very much like to view them.... Dave

-----------------------------------------From: "Stuart Hutchison"
To: aeroelectric-list(at)matronics.com
Cc:
Sent: Friday August 17 2018 12:19:53AM
Subject: Re: elt antenna

Barry,

For sure many aircraft finish upside down after a sudden stop … the 400lb engine up front of my tailwheel aircraft will almost guarantee that. However, I've installed the whip antenna just forward of the vertical stab where it should be reasonably protected from a tip over. I have reviewed numerous RV and Rocket wrecks and think my ELT installation has a pretty good chance of remaining operational, provided I mount the coax with sufficient slack and flexibility to withstand a buckling fuselage. Even an inverted whip is correctly oriented for low altitude (i.e. closer to the horizon) satellites to receive from a ‘donut’ radiation pattern. And, no matter which Local User Terminal around the world receives the signal, the position will still be re-routed to the correct RCC for SAR response in near real time. The 406MHz data burst is .45 to .55 seconds long at 5W and randomised around 100 second intervals, so it has a good chance of being received and relayed by at least one satellite.


I don’t mean to preach and there are a great many wise people on this forum, but respectfully, from a Search and Rescue Officer's point of view, I couldn't disagree more with those who think an ELT is a waste of time and money. Years of SAR missions as a P3 Orion Tactical Coordinator for the Australian RCC, plus many more years as an Operations Officer, then SAR Officer and SAR instructor have proven time and time again to me the value of ELTs, especially 406 with embedded GPS. Crashed aircraft are incredibly difficult to see from the air when moving at between 2 and 4 nautical miles a minute. Yes ADS-B is a great starting datum for ATC & SAR, but if, for example, you have an electrical issue late one afternoon that stops ADS-B output (smoke and fumes or inflight fire that requires load shedding) and becomes a crash situation, just a few minutes of cruising off flight plan can easily delay location and rescue way past your overnight survival time. I can tell you from first hand experience in the RCC that it is very distressing for rescuers to miss you by just a few hundred feet or a few hours. It is even worse for the family left behind to realise that you might have been alive long enough to rescue if we had known exactly where to look. In fact, a former Senior Naval Officer at our School of Air Navigation lost his son in a light aircraft crash. A small piece of media misinformation meant a critical piece of search intelligence was overlooked. The pilot and passenger were located some months later in a follow-on search funded by the father, only to find that his son had survived with a broken leg for what was believed to be a week after the crash before succumbing to exposure.


It’s a morbid subject, but aviation is intolerant of fools and as aviators we need to be properly trained, equipped and prepared.  The mandate is there for good reason, so we should install the ELT correctly, register and maintain it … it’s definitely not worth risking your life to neglect the ELT / PLB. And, carry a jumper Smile The priorities of survival are Protection (from the elements), Location (signalling devices, fires etc), Water, Food … in that order. We can survive for three days without water and 30 days without food, but who would want to do that? I’d rather get outta there.


Kind regards, Stu

Quote:
On 17 Aug 2018, at 03:59, FLYaDIVE <flyadive(at)gmail.com (flyadive(at)gmail.com)> wrote:
Many, if not Most crashes wind up with the plane Up-Side-Down.
So top mounted antennas become buried in the dirt or broken off.
When an aluminum plane is upside down there is even more of a barrier between the ELT antenna and the sky.
Wing Tips usually have the least amount of damage from a crash.
And, they are usually the farest away from the fuel tanks, the Horz Stab wing tips being the best location.
Personally I feel ELT's are one step above useless. They are usually in a fixed location, broken, buried,or have a dead battery.  They are an FAA Reg, not a true safety, Search & Rescue device.
Consider a Personal Around the neck, or leg mounted PLT (Personal Locating Transmitter).


Choose wisely Grasshopper,


<div class="gmail_default" sty



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PostPosted: Fri Aug 17, 2018 8:01 am    Post subject: elt antenna - Question for Bob Reply with quote

At 10:14 AM 8/17/2018, you wrote:

Quote:
Bob,

About "portable" 121.5/406 antennas....

I would like to construct a portable antenna that in the event of surviving a mishap, one could remove the ELT, take outside to a favorable location, connect the portable antenna and have a better chance of being found earlier.

I know this is a dual frequency design, but it has been too long since I studied and made lower frequency antennas, so I am asking if you could suggest a construction design.

I believe the 406 part should be cut for 3/4 wavelength, but the loading coil size and position for the 121.5 is vague. I am assuming about 1/3 up the whip so as to not interfere with the max. current wave point of the 406 signal.

[img]cid:.0[/img]

It would be interesting to get a lessons-learned dump from
an antenna expert . . . anyone else out there with additional
input?

I presume antennas like this one start with a 1/4 wave at
highest frequency of interest, add a loading/trap coil
at the top tailored to resonate at 406 with the parasitic
capacitance afforded by the radiators attached at each
end, then trim the top segment for best match at lowest
frequency of interest.

No doubt the resonance effects are interactive so it
would take some fiddling to optimize the design. Obviously,
an oversized 'hand held' is not an idealized ground plane
so should the optimizing studies be done using a rough
mock-up of the ELT's 'ground plane' signature.

I've got a new vector network analyzer that I've not
taken out of the box yet . . . too many irons in the
fire. But I'm hoping the tool will open some new windows
on my bag of tricks . . . the only thing I really miss
about my tenure at Beech was access to that RF lab!


Quote:
Also, should the whip have a small circular ground plane attached to the BNC connector on the base.?

If you have ideas on this little antenna, I would very much like to view them.... Dave

I think that given the 'dire straits' of conditions
that would prompt hand-held operation, I'd build
a 406 quarter wave on a coax connector and call it
good. As others have pointed out here, the ability
of the 406 satellites to sniff out a weak signal
is really good. If you're out on the ground with
a 406 device in your hands, the 1/4 wave whip is
going to get the performance you need.


Bob . . .


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