fredoyster
Well-Known Member
I’ve been away from aviation for about 15 years getting a company on its feet, and now looking at the various options for homebuilt planes, looks like a great time to get in the air again! Nav has certainly become a lot simpler, with slick EFIS systems looking like a lot of fun. What hasn’t changed since I was flying Grummans and Mooneys is the com, and particularly the antennas.
This is part general discussion and part market research, I guess. I’m an EE with an electromagnetics background and have developed a number of advanced antennas over the years, the technical issues aren’t the question. What puzzles me is the range of, err, incomplete information that’s out there on this topic. There are exceptions – Bob Archer’s pages are correct as far as they go – but I’ve read a lot of bad advice out there, particularly if we want to follow the FAA in using the whole 118-138 MHz band for com. There are some good alternatives for embedded antennas in composite and wood aircraft, but very little for metal planes (tube or metal skin).
A bent rod antenna on the belly (and maybe another on top) work fine for a few MHz of bandwidth – if we stick to Unicom frequencies and we terminate the coax cable correctly those can be fine. But broadband and low drag antennas are silly expensive, particularly TSO’d ones. VOR/LOC antennas likewise can be greatly improved in signal performance and drag over what was common in the 1950s. The alternatives seem to be designs intended for King Airs and bigger.; fine, but expensive and often heavy. A number of advanced antenna designs have found their way into cell phones and military applications, that when built for the aviation band would be significant improvements over what’s out there. How much interest is there in this?
There seem to be two approaches – out-of-the-box solutions that people hope will work by magic, and hopeful tries based on fitting embedded/conformal antennas into existing nonconducting structure, that fail to produce desired performance because people don’t really understand what they’re doing. (Performance depends at least as much on where an antenna is installed, as on any detail of the antenna itself.) No wonder, it isn’t simple, but it isn’t impossible either. You measure airspeed precisely with three-leg GPS-timed runs, why not measure com and nav performance as carefully? (Maybe this is already done, I just haven’t found much in the av-blogosphere on it.)
Performance can even be improved for GPS antennas (bringing more satellites into view for any given flight attitude). Soon there will be a requirement for 978 MHz antennas for the new ADS-B, will this be yet another rod-and-ball on the belly?
I am thinking of making a line of very low drag com and nav antennas for metal airplanes using a design that hasn’t been common in general aviation. Nothing magic, but should produce significantly improved range and reliability over what’s out there. How much interest would there be in that? Would people want to assemble and finish their own, or would they rather pay a couple hundred bucks more for something they could just bolt in place? Actual measured performance (I don’t mean just VSWR which can be very misleading) would be important, and it’s not that difficult to do.
This is part general discussion and part market research, I guess. I’m an EE with an electromagnetics background and have developed a number of advanced antennas over the years, the technical issues aren’t the question. What puzzles me is the range of, err, incomplete information that’s out there on this topic. There are exceptions – Bob Archer’s pages are correct as far as they go – but I’ve read a lot of bad advice out there, particularly if we want to follow the FAA in using the whole 118-138 MHz band for com. There are some good alternatives for embedded antennas in composite and wood aircraft, but very little for metal planes (tube or metal skin).
A bent rod antenna on the belly (and maybe another on top) work fine for a few MHz of bandwidth – if we stick to Unicom frequencies and we terminate the coax cable correctly those can be fine. But broadband and low drag antennas are silly expensive, particularly TSO’d ones. VOR/LOC antennas likewise can be greatly improved in signal performance and drag over what was common in the 1950s. The alternatives seem to be designs intended for King Airs and bigger.; fine, but expensive and often heavy. A number of advanced antenna designs have found their way into cell phones and military applications, that when built for the aviation band would be significant improvements over what’s out there. How much interest is there in this?
There seem to be two approaches – out-of-the-box solutions that people hope will work by magic, and hopeful tries based on fitting embedded/conformal antennas into existing nonconducting structure, that fail to produce desired performance because people don’t really understand what they’re doing. (Performance depends at least as much on where an antenna is installed, as on any detail of the antenna itself.) No wonder, it isn’t simple, but it isn’t impossible either. You measure airspeed precisely with three-leg GPS-timed runs, why not measure com and nav performance as carefully? (Maybe this is already done, I just haven’t found much in the av-blogosphere on it.)
Performance can even be improved for GPS antennas (bringing more satellites into view for any given flight attitude). Soon there will be a requirement for 978 MHz antennas for the new ADS-B, will this be yet another rod-and-ball on the belly?
I am thinking of making a line of very low drag com and nav antennas for metal airplanes using a design that hasn’t been common in general aviation. Nothing magic, but should produce significantly improved range and reliability over what’s out there. How much interest would there be in that? Would people want to assemble and finish their own, or would they rather pay a couple hundred bucks more for something they could just bolt in place? Actual measured performance (I don’t mean just VSWR which can be very misleading) would be important, and it’s not that difficult to do.
