Today’s work on my Bede BD-4C airplane included two tasks: building one end of the coax for the VOR antenna and installing the transponder antenna.
Coax (coaxial cable, in which both conductors run along the same axis) is an “unbalanced” line. The VOR antenna is a balanced load. Ideally, you connect the two via a “balun” which properly converts between a BALanced load and an UNbalanced line. Without a balun, more energy gets lost in the transition and the VOR will be less sensitive to weak signals. That said, many many airplanes are winging around the skies with VORs that work perfectly well and they do not have baluns. I decided to take the easy way out, connect the coax directly to my antenna without a balun, and test things out in the air. If I do not get the performance that I want, I will add a balun later.
I pulled the coax apart, separating the shield from the center conductor, and added heat shrink tubing to insulate the shield from the metal fuselage. Then I crimped ring terminals onto the two conductors. You can see the result in the photo above. (Click that, or any other photo, to see a larger version.)
That coax will run through the tunnel from the instrument panel, past the trim lever for the horizontal stabilator, underneath the push-pull rod for the stabilator and the rudder cables, and connect to the VOR antenna. Here is a photo of the first test fit. For the real installation, it will go underneath the trim level bracket, parallel to the white brake line that you can see attached to the tunnel.
I attached the transponder antenna to the bottom of the fuselage, in front of the pilot’s rudder pedals. I read several recommendations that it should be at least three feet away from the VOR antenna and this position is just about that distance.
The connection for the transponder antenna is a BNC connector, instead of a pair of screw terminals. You can see it in this photo, next to the base of one of the brake master cylinders.
I ordered a little three blade coax trimmer off of eBay. When it arrives, I will be able to attach the BNC connectors to the transponder and VOR antennas.
Art,
I am wondering why you are equipping your aircraft with a VOR. I recently received notification that the FAA is preparing to decommission the St. Louis VOR, and I suspect this trend will continue.
Jim,
I want VOR as a backup to GPS. Though rare, we do occasionally see a NOTAM for unreliable GPS navigation. Also, the VOR antenna serves as the antenna for the ILS receiver and the glideslope receiver.
It is the Cardinal VOR (CSX) that is being decommissioned. The St. Louis VOR (STL) is not going away.
— Art Z.
I understand. I’m aware that you are no doubt building this airplane for IFR use, since your level of experience is much higher than mine. I guess I only have the simple requirements of VFR and GPS navigation. When all else fails, I still have the visual.
There are times when I seriously consider building a VFR-only airplane… times like when I am looking at the cost of the ILS receiver and the WAAS-certified GPS receiver and and and…. 😛
Hi, Art,
It has been my experience in automotive work that crimped Sta-Kon connectors are a weak link elecrically. A long time ago I got in the habit of tinning and soldering those connections after the mechanically sound crimp is completed. If needed for insulation I added heat shrink after the solder joint was completed. I did that for two reasons: To eliminate salt and brine corrosion and to eliminate high resistance connections. I’m sure that aircraft don’t have the same exposure as road vehicles do but a poorly formed crimp can be a problem.
Pete,
Take a look at this article on the AeroElectric Connection, Anatomy of a Good Solderless Terminal Connection. In particular, note the last paragraph:
— Art Z.