ELT antenna in composite planes.

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.

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.

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)

A quarter wave with ground plane has a donut pattern with a hole on
top.

No sir, a vertical dipole has a donut pattern with a hole on the top. A
quarter wave with a ground plane has a donut sliced longitudinally (like
slicing a bagel for cream cheese) with a hole on the top. Practically zero
radiation on the back side of the ground plane.



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.


Oh, my dear Lord. First the man cites the reciprocity property of antennas
(which in a hundred years has yet to be disproven) but HIS logic says that a
ducky has to transmit better than it hears.

Sorry, sir, I want nothing more to do with this conversation. You evidently
belong with those geniuses who sell magnets to put in the carburetor to
double the gas mileage.

Jim

On Oct 31, 7:33 pm, "RST Engineering" wrote:
A quarter wave with ground plane has a donut pattern with a hole on
top.

No sir, a vertical dipole has a donut pattern with a hole on the top. A
quarter wave with a ground plane has a donut sliced longitudinally (like
slicing a bagel for cream cheese) with a hole on the top. Practically zero
radiation on the back side of the ground plane.

Aw comon. Now we are nit picking to win an argument. My main intended
point was that it has a hole on top irregardless if its a half or full
donut.


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.

Oh, my dear Lord. First the man cites the reciprocity property of antennas
(which in a hundred years has yet to be disproven) but HIS logic says that a
ducky has to transmit better than it hears.

Please take a look at http://en.wikipedia.org/wiki/Radiation_pattern .
Reciprocity refers to radiation/reception "pattern" (geometry) being
the same and not to total radiation/reception efficiency. The second
equation says the total power actually received depends on
A(theta,phi) the "effective area or effective aperture of the antenna"
for a receiving antenna - i.e. the size of the antenna. A small tuned
antenna can send most of its power out (not necessarily directionally)
but will receive much less signal than a large antenna simply because
it has small receiving area. We are confusing directional gain with
RF power transmission efficiency.

Sorry, sir, I want nothing more to do with this conversation. You evidently
belong with those geniuses who sell magnets to put in the carburetor to
double the gas mileage.

Why do you have to use insults? If you really don't want to comment
just don't.

Because, you stupid imbecile, if you don't get answers to your posts, people
might really believe the bull**** that you are passing on as fact. And then
it takes me HUNDREDS of HOURS to tell people why your stuff isn't right.

Don't you understand that? Or pass your credentials on as a professional
antenna designer and we'll carry this discussion on at a whole different
level.


Jim


Why do you have to use insults? If you really don't want to comment
just don't.

"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