ELT antenna in composite planes.

ELT normally uses an externally mounted antenna. But in composite
planes it may be advantageous to use a ducky.

1. On crash the external antenna may be destroyed - brushing against
trees, whatever. An attached ducky on the ELT will likely stay on.

2. A ducky radiates nearly isotropically - i.e. near equal in all
directions. If the standard externally mounted ELT antenna survives
the crash, a half wave or a quarter wave radiation pattern is mostly
perpendicular to the antenna - in normal situations that would be
horizontal. But on the ground after the crash, horizontal directions
are often obscured by terrain - one reason why HAMs use VHF repeaters
- and the signal is weak above where the search planes are. This is
where the ducky may have an advantage and will send signals in all non-
obscured directions equally. Note that a plane crash does not
guarantee any orientation of the plane/antenna or after crash
worthiness of the antenna.

Even if Fosett's ELT worked its antenna may have radiated horizontally
against mountain obscurations or was broken off.

You can get a 121.5MHZ tuned ducky (low SWR) for only $16 bucks from
http://www.smileyantenna.com/ I don't work for them.

As you say, a ducky is nearly isotropic ... but equally poorly isotropic in
all directions. While the radiated signal from an ELT is pretty low to
begin with, you lose about 15 dB on a ducky on the average over a standard
quarter wave whip or dipole.

In a composite airplane you have the luxury of mounting a good dipole ELT
antenna internally on the biggest piece of plastic likely to survive the
incident. If you mount it on the bulkhead behind the pilot or rear
passenger, then the likelihood of both occupants and antenna surviving the
incident is nearly the same. As to the orientation of the dipole, if you
can tell me how the airplane parts are going to come to rest in the
incident, I'll tell you how to mount the antenna.

A tuned ducky for 121.5? Great. How do you radiate the 243.0 component
since the antenna will be nearly anti-resonant at that frequency.

Jim

--
"If you think you can, or think you can't, you're right."
--Henry Ford

wrote in message
ups.com...
ELT normally uses an externally mounted antenna. But in composite
planes it may be advantageous to use a ducky.

1. On crash the external antenna may be destroyed - brushing against
trees, whatever. An attached ducky on the ELT will likely stay on.



You can get a 121.5MHZ tuned ducky (low SWR) for only $16 bucks from
http://www.smileyantenna.com/ I don't work for them.

OK. I expected your reply quickly.

On Oct 31, 1:00 pm, "RST Engineering" wrote:
As you say, a ducky is nearly isotropic ... but equally poorly isotropic in all directions.

If the ducky is well tuned it will radiate very well - I measured
pretty low reflection on one I have. The radiation gain in larger
antennas comes from directionality and not from nothing - it does not
radiate more RF energy than the transmitter generates. I have a 5W
APRS (VHF) tracking unit with a ducky in my aircraft and it reaches
about 60 miles direct to my iGate. Not bad.

As to the orientation of the dipole, if you
can tell me how the airplane parts are going to come to rest in the
incident, I'll tell you how to mount the antenna.

Yeah, but that is the trick. Nobody knows how the plane will come to
rest. And don't forget even in ideal situation (vertical) most
radiation is against horizontal obstructions and not up - and neither
121.5 nor 243 will get help from repeaters. AND if the plane is
mangled your seat mounted or whatever does not likely have survival
rate as an a small attached ducky. ELT failure rate is about 25%.

A tuned ducky for 121.5? Great. How do you radiate the 243.0 component
since the antenna will be nearly anti-resonant at that frequency.

The dual freq loss problem is true of any single ELT antenna. You can
tune a ducky to 243, your choice - I understand 121.5 satellite
tracking is being abandoned.

Personally I prefer APRS tracking. You can see my today's track at
http://aprs.he.fi/ - just enter N416 and then again at right in the
box. For those who want more info about APRS see http://www.abri.com/sq2000/GPStrack.html
Its fantastic for GA aircraft tracking.

On Oct 31, 5:44 pm, wrote:
...... You can
tune a ducky to 243, your choice - I understand 121.5 satellite
tracking is being abandoned.

Whoops. Its the 121.5/243 that is being phased out and replaced with
the 406 which then make it an ideal tuned ducky candidate.

wrote in message
ups.com...
The radiation gain in larger
antennas comes from directionality and not from nothing

Yes, but in this case, we are comparing a shortened (ducky) antenna to a 1/4
wave antenna, not a gain antenna. A 1/4 wave antenna has a pretty high angle of
radiation. I raised my eyebrow at Jim's estimate of 15 db, but when you start
adding factors, (eliminate the loss of the stubby antenna, antenna in the clear
outside of airframe, elevated antenna) you could end up with more difference
than you think.

You are correct that a long antenna gets its gain from decreasing the angle of
radiation and concentrating more of the signal at (or even below) the horizon,
but I have never seen a gain antenna used for an ELT, have you?

Vaughn (WB4UHB)

Ok. I don' t actually use a short "stuby" ducky. There are some 9-12"
long duckys with pretty decent gain which is still small enough to
directly mount on the ELT. Check http://smileyantenna.com/ choices.
The disadvantage of inside mounted ducky depends where it is - the
composite material by itself does not attenuate the signal
significantly. In fact my wing mounted VHF aircraft regular antennas
are mounted "inside" composite winglets ( see http://www.abri.com/sq2000
)

A quarter wave with ground plane has a donut pattern with a hole on
top. Also, my logic tells me (gain reciprocity notwithstanding) that a
ducky radiates better than receives - there is simply not enough
antenna surface to collect signal like in a larger antenna. But for
ELT transmission is what counts.

Paul (KC0WIF)

On Oct 31, 6:19 pm, "Vaughn Simon"
wrote:
wrote in message

ups.com...

The radiation gain in larger
antennas comes from directionality and not from nothing

Yes, but in this case, we are comparing a shortened (ducky) antenna to a 1/4
wave antenna, not a gain antenna. A 1/4 wave antenna has a pretty high angle of
radiation. I raised my eyebrow at Jim's estimate of 15 db, but when you start
adding factors, (eliminate the loss of the stubby antenna, antenna in the clear
outside of airframe, elevated antenna) you could end up with more difference
than you think.

You are correct that a long antenna gets its gain from decreasing the angle of
radiation and concentrating more of the signal at (or even below) the horizon,
but I have never seen a gain antenna used for an ELT, have you?

Vaughn (WB4UHB)

wrote in message
ups.com...
OK. I expected your reply quickly.

On Oct 31, 1:00 pm, "RST Engineering" wrote:
As you say, a ducky is nearly isotropic ... but equally poorly isotropic
in all directions.

If the ducky is well tuned it will radiate very well - I measured
pretty low reflection on one I have.


Ya know, I've learned a lot by listening to people who know what they are
talking about. Some of my E&M profs were really quite good at drilling the
fundamentals of electromagnetic propagation into my head. Some of my ham
friends were really quite good at grounding (no pun intended) me with the
practicalities of antennas. VHF antenna design has been one of my
specialities since I was a young pup.

Now, why don't you tell me how you tune a ducky? Radiation does not
necessarily come from "low reflection". A 51 ohm carbon comp resistor has a
VERY low reflection but doesn't radiate worth a damn. Some manufacturers
get their "low reflection" by helixing the radiating element around a lossy
core. Some get it by using the correct pitch and length of the helix. But
in any case, coiling the element is a lossy way of radiating and there is no
real way around it. Radiation has to do with reception at a distance, and
reflection coefficient (or VSWR, or whatever you want to call it) is one
component, and a rather minor component at that.


The radiation gain in larger
antennas comes from directionality and not from nothing

No kidding. Did Hiram Percy Maxim himself come down from the mountain and
tell you that personally?


- it does not
radiate more RF energy than the transmitter generates.

Come on. Don't insult our intelligence. Any passive device (and a metal
antenna IS a passive device) that creates more RF energy than it takes in is
a sure way for somebody to win the Nobel in Physics.



I have a 5W
APRS (VHF) tracking unit with a ducky in my aircraft and it reaches
about 60 miles direct to my iGate. Not bad.

A 5 watt transmitter with a zero gain (isotropic) antenna with a pretty poor
1 microvolt receiver with a 2.14 dB gain quarter wave whip on the other end
has a theoretical range of about 2500 miles. I'd say a 60 mile range is
pretty **** poor, wouldn't you?


Yeah, but that is the trick. Nobody knows how the plane will come to
rest. And don't forget even in ideal situation (vertical) most
radiation is against horizontal obstructions and not up - and neither
121.5 nor 243 will get help from repeaters. AND if the plane is
mangled your seat mounted or whatever does not likely have survival
rate as an a small attached ducky.

(a) in a plastic airplane, you can mount the antenna as a V pointing up
(which is where most of the folks looking for you are going to be and (b) if
the seat is that mangled, what the hell do you care if they EVER find what
is left of your mortal remains?


ELT failure rate is about 25%.

Where in the devil did you come up with THAT number?




The dual freq loss problem is true of any single ELT antenna. You can
tune a ducky to 243, your choice - I understand 121.5 satellite
tracking is being abandoned.

Sonny, I can and have tuned a dipole arrangement to be resonant at both 121
and 243 withOUT traps. It ain't rocket science and it has been written up
in Kitplanes. I'd bet a couple of thousand flying examples by now.


Jim

Horsefeathers. At 406 a quarter wave radiating element is going to be about
6 inches long. Make it out of spring steel or piano wire and you can forget
your tuned ducky.

Jim

--
"If you think you can, or think you can't, you're right."
--Henry Ford

wrote in message
ups.com...

Whoops. Its the 121.5/243 that is being phased out and replaced with
the 406 which then make it an ideal tuned ducky candidate.

"Vaughn Simon" wrote in message
...

Yes, but in this case, we are comparing a shortened (ducky) antenna to a
1/4 wave antenna, not a gain antenna. A 1/4 wave antenna has a pretty
high angle of radiation. I raised my eyebrow at Jim's estimate of 15 db,
but when you start adding factors, (eliminate the loss of the stubby
antenna, antenna in the clear outside of airframe, elevated antenna) you
could end up with more difference than you think.

Jim's wasn't an estimate. Jim got up onto the top of a mountain (not
difficult in Northern California) with a calibrated spectrum analyzer and a
lab standard ground plane antenna and did a test for the local Search &
Rescue group on 2 meters. Using the best engineering practices and
measurement techniques I could muster, I had about twenty of the S&R folks
use first their ducky and then a regular old brazing rod - SO239 mickey
mouse ground plane.

THe spectrum analyzer showed somewhere between 10 and 20 dB of difference
between the duck and the ground plane. The average was very close to a 15
dB difference between the two.

I've since repeated that same test with us both at the same level (like
across a flat meadow about four football fields long) and with THEM on the
mountain and me in the valley.

Same same.

Jim

Your reference that your 5 watt will work out 60 miles. Unfortunately the
old style ELT has only a .1 watt transmitter which is 1/50 the output of
your example. I do not have the expertise to tell how this will reduce the
distance, but I will bet it does curtail the range quite a bit.
--
Cy Galley
EAA Safety Programs Editor
Always looking for ideas and articles for EAA Sport Pilot
New address -

wrote in message
ups.com...
OK. I expected your reply quickly.

On Oct 31, 1:00 pm, "RST Engineering" wrote:
As you say, a ducky is nearly isotropic ... but equally poorly isotropic
in all directions.

If the ducky is well tuned it will radiate very well - I measured
pretty low reflection on one I have. The radiation gain in larger
antennas comes from directionality and not from nothing - it does not
radiate more RF energy than the transmitter generates. I have a 5W
APRS (VHF) tracking unit with a ducky in my aircraft and it reaches
about 60 miles direct to my iGate. Not bad.

As to the orientation of the dipole, if you
can tell me how the airplane parts are going to come to rest in the
incident, I'll tell you how to mount the antenna.

Yeah, but that is the trick. Nobody knows how the plane will come to
rest. And don't forget even in ideal situation (vertical) most
radiation is against horizontal obstructions and not up - and neither
121.5 nor 243 will get help from repeaters. AND if the plane is
mangled your seat mounted or whatever does not likely have survival
rate as an a small attached ducky. ELT failure rate is about 25%.

A tuned ducky for 121.5? Great. How do you radiate the 243.0 component
since the antenna will be nearly anti-resonant at that frequency.

The dual freq loss problem is true of any single ELT antenna. You can
tune a ducky to 243, your choice - I understand 121.5 satellite
tracking is being abandoned.

Personally I prefer APRS tracking. You can see my today's track at
http://aprs.he.fi/ - just enter N416 and then again at right in the
box. For those who want more info about APRS see
http://www.abri.com/sq2000/GPStrack.html
Its fantastic for GA aircraft tracking.