GA pilot reported an NMAC with a helicopter while in the traffic pattern of C17 non-towered airport.

Narrative

Aircraft X was conducting a local flight between untowered airports. When approaching the destination; position and intent callouts were made on CTAF/UNICOM at approximately 7 and 3 miles; those callouts indicated Aircraft X was west of the airport and intended to cross at midfield to enter a left base for Runway 17. At the time another (uninvolved) aircraft was conducting operations at the destination; landing 17. At approximately 1.5 miles; Aircraft X was on a course of approximately 120M at a ground speed of 120kts and a helicopter crossed behind Aircraft X within 50 ft vertically and within 100 feet laterally. The helicopter appeared to be on a northeast course (after landing; public ADS-B data was viewed and it indicated the helicopter was on approximately a 050M course). The pilot in command (PIC) in the left seat of Aircraft X did not see the helicopter until after it passed behind. A private pilot rated and flight current passenger in the right front seat of Aircraft X observed the helicopter at approximately the 4 to 5 o'clock position immediately before it passed behind Aircraft X leaving no time for any evasive maneuvering. As Aircraft X entered the left downwind for 17; a bystander on the ground transmitted on CTAF indicating a helicopter was present or maneuvering around the approach end of 17. Aircraft X kept the helicopter traffic in sight; which appeared to continue enroute to the northeast; until turning final for 17 and completed the flight without further incident. The helicopter made no transmissions on the destination CTAF/UNICOM even though it passed within 1.5nm of the field at pattern altitude. Aircraft X did have an operational Stratux ADS-B receiver and ADS-B traffic was being observed around the time of the NMAC by the right seat occupant on an iPad. The helicopter did not appear on the ADS-B traffic layer; while other nearby traffic was populated on the map. Aircraft X was squawking VFR with altitude reporting enabled. As Aircraft X was only 1.5 miles from the destination; the pilot's focus had shifted towards identifying the position of an aircraft on final; identifying any other traffic in the pattern; and completing pretending checklists. Unannounced traffic from the right was unexpected due to the traffic pattern specified for Runway 17 at the destination. All lights on the Aircraft X were on including Nav; strobes; beacon; and left and right wing landing lights. The helicopter was over a populated area with multiple towers and obstacles. Neither occupant of Aircraft X recalls identifying any anti-collision lighting on the helicopter as it passed; however due to the nature of the incident it may have just been missed. The Aircraft X failed to identify the approaching traffic off its right wing.Based on the flight path of the helicopter; it appeared that the helicopter failed to identify the approaching traffic which should have been within their forward field of view.