Date: 201907
Local time of day: 0601-1200
Location: RDU.Airport, NC
Flight conditions: VMC
Weather elements / visibility: 10
Light: Daylight
Aircraft: Amateur/Home Built/Experimental
Operator: Personal
Operating under: Part 91
Flight plan: None
Mission: Personal
Flight phase: Cruise
ATC / advisory: CTAF ROA
Aircraft component: Transponder
Component problem: Failed
Reporter organization: Personal
Reporter function: Pilot Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Private
Human factors: Confusion
Anomaly: Aircraft Equipment Problem Less Severe; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Took Evasive Action
Contributing factors: Aircraft; Procedure
Primary problem: Aircraft

Narrative: We left ZZZ to proceed to ZZZ1. We'd recently completed Phase 1 testing and found the airplane to be ready for flight. While proceeding north bound towards ZZZ2 for our planned fuel stop; we proceeded under VFR rules just west of Raleigh airspace. Raleigh had a lot of traffic in and outbound. At our slow speed of 65 kts; we couldn't do much to avoid traffic so we relied on Raleigh to sequence jets around us. We did not contact RDU for flight following as we'd planned to go without flight following during the entire trip; allowing us the freedom to change routes and altitudes as the situation dictated. We noted an inbound airliner not only coming towards us; but also descending towards our altitude. We acquired the airplane both visually and on ADS-B. When Raleigh didn't vector the airliner away from us; we changed course and altitude to avoid the airliner. We also picked up a wing in a pretty sharp turn to show more of our aircraft to the airliner. Between our movements; and whatever else Raleigh or the airliner had going for them; they noted our position and also turned to avoid us. I estimate they never closed to closer than 1-2 miles from our position. We wondered aloud why Raleigh had been so slow to vector the airliner away from us. We proceeded uneventfully north bound till reaching ZZZ2; where we contacted them to land. We received a squawk code and then learned that our transponder had quit working somewhere during Phase 1 testing. Cutting the story short; we had a bad connection on our antennae which we fixed at another airport along our route as ZZZ2 had no ability to fix a simple connector. Raleigh had not vectored the airliner around us because they had not seen us on their radar due to the unknown failure of our transponder. Had we contacted ZZZ3 for flight following; we'd have learned our transponder wasn't working while still in sight of home base and our tools. We'd also have had positive separation while in a busy airspace. Also contributing was the fact we had a brand new airplane just out of phase 1 and we were making our first forays into controlled airspace. We'd avoided controlled airspace during phase 1 testing as both ZZZ3 and ZZZ4; our two closest facilities; were specifically prohibited to us during Phase 1. We'd requested ZZZ4 airspace be available but were told that would not be allowed. I think it would be better to have access to a controlled airspace during Phase 1 testing. Even if it is on a limited basis it would be beneficial. I'm sure we could have requested flight following of ZZZ4 during our testing; but we'd gotten the impression we were to avoid controlled airspace all together; leaving us to assume our transponder was working when it wasn't.




Synopsis: JA30 Pilot reported an airborne conflict due to a failed transponder.