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#11
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Transponder antenna - blade vs stub monopole
Alan wrote:
In article brianDG303 writes: Is there a downside to mounting on a ground plane inside the fuselage of a non-carbon glass glider? Since the ground plane should extend at least 23 inches in each direction around the antenna, it is probably hard to get that inside the glider. 23 inch radius for a quarter-wave antenna? Most installations I've seen use no more than a 3 inch radius ground plane... |
#12
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Transponder antenna - blade vs stub monopole
For what it is worth Schempp-hirth are mounting the transponder antenna in the fin on the Duo Discus. Right next to the
radio aerial. I presume the same applies to the Nimbus 4D that uses essentially the same fuselage. Apparently there is lots of space for a good dipole antenna, and the foam cored glass structure is not opaque to the frequencies used. Nothing external to get damaged but heaven forbid you should ever have to work on it... Presumably it is not something you could retrofit (for the same reasons you would never want to have to repair it). Bruce Tim Mara wrote: I strongly suggest you DO NOT but the blade antenna's!.... ever if there were some small aerodynamic advantage (I doubt it could be measured at any glider speeds anyway) but these are very expensive and they are easily damaged.... I offer blade antennas, the "preferred" simple and cheap stub antenna's and the composite dipole antenna's..I've had several buyers of the blade antenna's break them (they aren't typically very happy when this happens).it doesn't take much effort to damage the composite blade antennas since they are essentially a tiny wire on a thin metal plate with small (6/32) screws in a very nice looking shark fin that is mostly filler (putty) with a thin composite shell.. best regards Tim Please visit the Wings & Wheels website at www.wingsandwheels.com "5Z" wrote in message ... Any aerodynamic thoughts on the two available choices? My ASH-26E came with the stub mounted just behind the wheel. Every now and then it catches on the trailer, so I just straighten it, no big deal. An ASW-24B in our club is about to get a transponder and the factory recommends the installation to be part way up around the fuselage above the rear gear door hinge - looks like just behind the oxygen bottle. The blade looks cool, but is it really any better aerodynamically than a $20 (http://www.aircraftspruce.com/catalog/avpages/ ted_transponder.php) stub? -Tom |
#13
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Transponder antenna - blade vs stub monopole
The 23" is for a 1/4 wave antenna at the frequency for the aviation radio
band (123 MHz). The transponder operates at about 1,000 MHz, so a 1/4 wave is much smaller. You do not even need to do much calculation, since the radius of the ground plane should be equal to the height of the antenna. So, the diameter would be twice the antenna height. The shape need not be round. It can be square or rectangular. Generlaly, a slight improvement will be achieved with a larger ground plane. Also remember, that, even though the transmitter is pulsed at a very short duration, there could be some possible danger having it close to your body. If you use the ground plane to "hide" the antenna from you, there would be little danger from exposure. The jury is still out on this one. I do not think there is any credible evidence of a danger, but I would not expose myself unless there was no other choice. Mine is mounted under my seat, upside down. Upside down is better because radio waves at this frequency are "line of sight" and the ground stations interrogating you are all below you. Do a good job of coax installation. I have seen some crappy jobs. Also remember that coax attenuation at 1,000 MHz with RG-58 is very high - so keep the run short. Colin Lamb |
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Transponder antenna - blade vs stub monopole
wrong! A ground plane should have a radius of at least 1/4 wavelength of the
antenna's operating frequency, a 23" antenna ground plane then would be about right for a VHF airband communication antenna (118-136 mhz) and in gliders this is seldom possible so use what you can get.. Transponder antenna ground planes are typically recommended to be 6" diameter. These are ideal....but not always possible so use what you can, too large isn't a problem but performance can degrade from the ideal as it becomes smaller. tim Please visit the Wings & Wheels website at www.wingsandwheels.com "Alan" wrote in message ... In article brianDG303 writes: Is there a downside to mounting on a ground plane inside the fuselage of a non-carbon glass glider? Since the ground plane should extend at least 23 inches in each direction around the antenna, it is probably hard to get that inside the glider. One that extends front and back but is more limited to the sides (perhaps wrapping around inside) probably would work, but would be a bit reduced from a full ground plane. Alan |
#15
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Transponder antenna - blade vs stub monopole
I installed a blade antenna in the recommended location for my Discus
2 just behind and to one side of the gear door and an internal ground plane about 6" square. It works very well in this location (also had a short coax from the transponder, which is a consideration at these higher frequencies). Tim is absolutely right, though. It is physically a right royal pain in the ass. I don't know anyone who has one in this location who hasn't broken it as it is perfectly located to snag on the trailer ramp. I have a short quarter wave monopole as back-up. This will be my default the next time the blade breaks! I doubt that the extra drag is worth worrying about. If you can locate the blade above and behind the cockpit, this is a much safer location and I have seen a couple of these installations that also work well. Mike |
#16
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Transponder antenna - blade vs stub monopole
There is a unique antenna that works fine as a transponder antenna - with no
blade or stub to break off or impede airflow. It is called an annular slot antenna. Many antenna engineering textbooks contain information on the, but they are not common. The entire antenna is flush with the aircraft skin and they are vertically polarized. They would work with fiberglass aircraft. I did find a photograph and description of a commercial one at www.mircrowaveeng.com Look for the L-Band Annular Slot Data Sheet (8 of 10). I suspect this particular unit may be beyond the budget of the average sailplane owner, but you can see how what they look like. It would be a good candidate for a slippery sailplane that does not want stuff hanging out. Colin Lamb |
#17
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Transponder antenna - blade vs stub monopole
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#18
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Transponder antenna - blade vs stub monopole
On 15 Jun, 01:39, "COLIN LAMB" wrote:
Whoops, I misspelled the website. *It is: www.microwaveeng.com Colin Lamb The CAA made some comments about slot antennas in their study on low power SSR transponders. See page 9 of: http://www.caa.co.uk/docs/810/Study%...er%201%202.pdf John Galloway |
#19
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Transponder antenna - blade vs stub monopole
John gets a gold star. Excellent report. About the only question not
answered is the comparative aerodynamic losses for a glider with the different antennas. Colin |
#20
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Transponder antenna - blade vs stub monopole
On Sun, 15 Jun 2008 02:41:42 -0700, jpg797 wrote:
On 15 Jun, 01:39, "COLIN LAMB" wrote: Whoops, I misspelled the website. *It is: www.microwaveeng.com Colin Lamb The CAA made some comments about slot antennas in their study on low power SSR transponders. See page 9 of: http://www.caa.co.uk/docs/810/Study%...er%201%202.pdf It looks, from that, as if the optimum for those of us with glass fuselages would be a vertical dipole mounted inside the rear fuselage. A 14.5cm dipole sounds small enough to fit fairly easily. However, I have two questions for the experts, as I don't understand RF radiation patterns or coax losses. - my glider (Std Libelle) uses pushrods for the rudder and elevator linkages. IIRC these run along the bottom of the fuselage. If the dipole was mounted vertically in the upper part of the fuselage would the control rods wreck the radiation pattern? - with transponder in the panel and the antenna behind the oxygen cylinder mount this would put the antenna at the end of 3 - 3.5m or co-ax. Would that be an acceptable installation? I hope I never have to fit a transponder, but I'd like to understand this type of issue in case it becomes a requirement here or I take the glider some place where one is needed. -- martin@ | Martin Gregorie gregorie. | org | Zappa fan & glider pilot |
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