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#1
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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. |
#2
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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. |
#3
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ELT antenna in composite planes.
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. |
#4
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ELT antenna in composite planes.
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. |
#5
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ELT antenna in composite planes.
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) |
#6
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ELT antenna in composite planes.
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) |
#7
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ELT antenna in composite planes.
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 |
#8
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ELT antenna in composite planes.
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. |
#9
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ELT antenna in composite planes.
"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 |
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