This is my version of the Stratux project suggested by By Roger Whittier and Robert Latham, of EAA chapter 538 in their article about building a Stratux ADS-B IN receiver. Please read this article to get familiar with the project.
I put together the unit suggested by the article and Dennis Huwe and Craig Spetman took it for a test flight in Dennis's 172. We are in Southwest Arizona in a little town on highway 60 called Aguila at an altitude of 2200' The FAA maps indicate that we should receive a tower at 1800' AGL which is 4000' indicated. It was found that the unit equipped with the portable antenna did not receive any towers until they reached about 5600'.
Various positions of the unit were tried with the antenna moved to the door window or in the back seat area did not improve the reception.
At this altitude several aircraft with ADS-B OUT appeared on the Wing X running on an iPad.
I came to the conclusion that the unit and antenna sitting on the instrument panel was not going to work for our area. The antenna sitting on the panel was always in the shadow of the fuselage from towers transmitting up from the ground.
The next logical step was to mount an antenna on the bottom of the aircraft. A standard post type transponder antenna is ideal because it is matched to the frequency of the ADS-B IN at 978 MHZ. (Note: Now there is an actual ADS-B In antenna available and it is part of the package.)
Dennis already had an old antenna with a single hole on the bottom of his airplane so we didn't have to do any drilling for the next test. I also put all the parts together in a prototype case to test for CPU temperature during the flight.
While taxiing to the runway, the affect of the belly antenna was immediate because we received 2 ADS-B OUT equipped aircraft, one going to Kingman Arizona climbing out of 4,000 feet and the other in approach to Phoenix descending through 9,000'.
After these results on the ground we were really excited to see what would happen after take off. When we reached 3900' indicated we received 2 towers and about 4 more aircraft in our immediate area showed on the iPad. This is 100' below FAA minimums for tower reception. As we climbed up over 4000' to 5000' the aircraft on the Stratux application showed that we had over 2 pages of airplanes, maybe about 50 airplanes showing on the screen. Mostly they were in the Phoenix area, but one was heading 303 degrees and climbing towards us. Another was going in circles about 45 miles to the northeast.
In conclusion it can be seen from the tests that there is a real advantage to using a belly mounted antenna for the Stratux. If some of you fly in more congested areas all the time the portable antenna may be adequate, but if you intend to go out into the wilds as it were, the belly antenna seems to work the best.
The temperature of the CPU in the enclosed case was getting quite warm 120°F, so the unit will need a fan to keep it cool. About 130°F is maximum for the Raspberry Pi.