ADS-B / UAT devices using external stubby transponder antenna

[RV8R999]

Originally Posted by Brantel

Pay for the machining, materials, labor, mold for the insulator, connector, rubber gasket, lock washer and nut. $17.95 at ACS seems like a good deal.

Don's antenna can't be compared with a stub antenna. His are low drag blade style and cost more to make.

Those are nice looking ANT for sure but not likely to be lower drag by any, if any, measurable margin by any equipment owned or installed in GA aircraft. Those ANT have significantly more cross sectional area and orders of magnitude more surface area than the cheapo stub ANT do. Just because it has a pretty shape doesn't mean less drag. The coefficient of drag is likely lower but you have to multiply by the area and dynamic pressure to get drag. Any sides slip or misalignment will increase the drag as well. Psychologically I'm sure they make things go faster.

[roee]

Originally Posted by RVbySDI

Now this information is what I would term INFORMATIVE!

Sure the charts and numbers above are great for all you engineer types who want absolutes. However, the truth is our airplanes and the components that make them work DO NOT fly in a world of absolutes. After all, doesn't the old adage that "every airplane is a compromise" apply here also? If the compromise works without spending all that extra money why not?

As one of those engineering types, I'd just like to point out that Ian's post was so valuable precisely because it was laid on a foundation of absolutes (and same goes for the original post by D. Pansier) -- a deep technical understanding and quantitative knowledge of the relevant performance metrics, the requirements, and what it takes to meet them.

Yes, in airplane building there are many compromises to be made. To be made well, those compromises should come from a point of knowledge and understanding.

Originally Posted by RVbySDI

Oh, and just exactly why does a 978 MHZ antenna cost more than a 76.7 mm piece of coat hangar or welding rod soldered to a BNC connector?

A commercially available blade antenna is likely to be more robust physically and environmentally, as well as produce less aerodynamic drag, than a do-it-yourself coat hangar antenna. There is also a cost associated with the development, tooling, manufacturing, and testing of the antenna, all of which provide a better value to the consumer due to being amortized over a production volume greater than just one unit.

Most of us choose to buy a transponder antenna off-the-shelf rather than make one for the same reasons why we buy sheets of aluminum rather than go mining for bauxite. If you want to do it yourself for the experience, more power to you. That is, after all, one of the reasons why we're building our own airplanes. But when it comes to these antennas, this is a case where the off-the-shelf product really does provide a good value.

Originally Posted by RV8R999

Those are nice looking ANT for sure but not likely to be lower drag by any, if any, measurable margin by any equipment owned or installed in GA aircraft. Those ANT have significantly more cross sectional area and orders of magnitude more surface area than the cheapo stub ANT do. Just because it has a pretty shape doesn't mean less drag. The coefficient of drag is likely lower but you have to multiply by the area and dynamic pressure to get drag. Any sides slip or misalignment will increase the drag as well. Psychologically I'm sure they make things go faster.

Here's some published data from RAMI antennas:

AV-22 rod-style transponder antenna produces 0.41lb of drag at 250mph
http://www.rami.com/product-view.php?pid=164

AV-74 blade-style transponder antenna produces 0.09lb of drag at 250mph
http://www.rami.com/product-view.php?pid=24

[RVbySDI]

Originally Posted by roee

As one of those engineering types, I'd just like to point out that Ian's post was so valuable precisely because it was laid on a foundation of absolutes (and same goes for the original post by D. Pansier) -- a deep technical understanding and quantitative knowledge of the relevant performance metrics, the requirements, and what it takes to meet them.

Yes, in airplane building there are many compromises to be made. To be made well, those compromises should come from a point of knowledge and understanding.

My point was that although the hard and fast numbers are valuable information, they need context. Ian provided context. If an antenna does not exactly meet a specific specification (i.e. instead of meeting the <1.7 VSWR requirement it is 2.6 VSWR) what does that mean in the real world of using that antenna with my radio? Ian's explanation did more to enlighten me on that question than any graph of "absolute" numbers ever did. Information without context just creates more noise and provides limited help with assimilation of knowledge.

[Brantel]

Originally Posted by RV8R999

Those are nice looking ANT for sure but not likely to be lower drag by any, if any, measurable margin by any equipment owned or installed in GA aircraft. Those ANT have significantly more cross sectional area and orders of magnitude more surface area than the cheapo stub ANT do. Just because it has a pretty shape doesn't mean less drag. The coefficient of drag is likely lower but you have to multiply by the area and dynamic pressure to get drag. Any sides slip or misalignment will increase the drag as well. Psychologically I'm sure they make things go faster.

I just base my guesstimates on this video:

http://www.youtube.com/watch?v=ftq8jTQ8ANE

[grayforge]

Yeah, that's a great video... and not too far from our airspeed either. Makes me think that a round stub antenna with a ball on the end will be dragier than an airfoil shaped antenna.

[rzbill]

Originally Posted by Brantel

I just base my guesstimates on this video:

http://www.youtube.com/watch?v=ftq8jTQ8ANE

You could always take a ball peen hammer to the rod antenna and beat the heck out of it (producing golfball dimples). That would reduce the drag somewhat by reducing the wake via turbulent boundary layer attachment on the backside of the antenna.