A pilot reported an NMAC with a Cessna in the FDK traffic pattern. The Cessna pilot had entered a left downwind for Runway 23; while the other aircraft in the pattern were making right traffic for Runway 5.

Narrative

While approaching Frederick Muni (FDK) I checked ATIS several times beginning from 20 miles out. The wind was 030 at 4-9; favoring Runway 5. I monitored the CTAF from 20 miles out; and all aircraft were using Runway 5. Additionally; one landing aircraft called Unicom for recommended runway; and was told Runway 5. I reported ten and five miles out for a right downwind entry to Runway 5; which is the standard pattern for this runway. I then heard a Cessna report left base for Runway 23; and no other calls from this aircraft. I reported entry to the right pattern for Runway 5 and 'right midfield downwind; landing five.' At that point I heard the Cessna report left downwind for Runway 23 and immediately saw him pass from my 1:00; then off my right wingtip at less than 500 FT and at the same altitude of 1;300 FT (standard pattern altitude for FDK). I radioed the Cessna and asked him if he saw me as he passed. He stated he saw a helicopter on the other side of the field. I then radioed that he was flying opposite to the recommended winds for landing and that we had just had a near miss. He radioed 'okie-dokie.' A King Air awaiting takeoff radioed asking the Cessna 'why are you going the wrong way?' The Cessna did not reply; was not heard on the frequency again; and did not land. I landed without further incident.