One of the unexpected consequences of being a very slow builder – although I prefer the term methodical, thanks – is that whole new technologies that barely existed when you started can advance to the point of being practically de rigueur before you're finish. One example is ADS-B, which everybody is now required by law to have by 2020… at least if you plan to fly anywhere useful. I'm already ADS-B Out equipped by virtue of having the proper kind of 1090ES transponder and a pair of compliant TSO C146 WAAS GPS navigators, but to benefit from the free traffic and weather data my tax dollars are already providing I need to install a separate ADS-B In receiver. Of course the obvious choice for my G3X system is the GDL 39R:
I'm seriously running out room to install stuff in the airplane, especially something the size of the thousand-page biography of Winston Churchill I've been meaning to read all year. I picked out a place on the F-7107R subpanel rib, where there was still some mounting structure left over from when I was plannig to put a Lightspeed ignition module there.
I used a piece of 0.050" aluminum to make a mounting plate, and bent flanges on two sides to give it some stiffness:
GDL mounted and wired. It's quite simple, just power, data, and an antenna. It will be possible to remove this unit for servicing later, but only if I press forward with my plan to cut access holes in the forward fuselage skin.
Close-up detail of antenna coax routing – an adel clamp keeps the cable from chafing on the sharp edge of the mounting plate:
For the antenna itself, I bought a 978 MHz blade antenna from Delta Pop. Relatively inexpensive for an airplane component, and gets good reviews.
According to the boffins at work whom I leaned on for specific installation guidance, the ADS-B antenna should be mounted at least six feet or so from the transponder antenna, and at least three feet from the comm antennas. It probably also wouldn't like being right next to the hot exhaust pipes. The only places on the bottom of the airplane that satisfy all these criteria are the very tip of the tail, and a spot in the middle of the forward fuselage floor about midway between the spar and firewall. I chose the forward location to making the antenna routing easier.
Of course the fuel pump and its associate plumbing are already located there, so things are a bit tight. I temporarily installed the pump assembly so I could mark the location for the antenna. The coax connector sticks up a bit, so it needs to be far enough aft to be underneath the fuel pump cover, but far enough forward that it doesn't run into the bottom of the fuel pump mounting plate.
Luckily there is a loop of fuel line in a convenient spot to run the coax straight up and through, shown marked by a dot here prior to drilling:
To mount the antenna to the floor I made a small doubler out of a piece of scrap 0.063. I forgot to take pictures of the process, but you should know what one looks like by now because my airplane is positively bristling with antennas.
For corrosion resistance and maximum electrical conductivity I treated the doubler with alodine:
As well as the floor, after removing an area of paint:
I enlisted the help of the lovely Dr. Mary to rivet the doubler to the floor. She did a great job even though I haven't given her much rivet practice for quite some time.
Antenna doubler installed, complete with antenna:
The coax runs along the floor and then does a loop to duck down through the fuel plumbing and connect to the antenna:
View of the underside of the airplane sporting a brand-new antenna:
I'm now equipped to both transmit and receive ADS-B. Here's hoping I get this thing finished before 2020 rolls around.