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