Flight Instructor reported a near miss with a helicopter while entering the traffic pattern at a non-towered airport during a training flight. The Instructor took evasive action to avoid the helicopter.

Narrative

I am a flight instructor and we were completing a normal lesson and coming back in for the final landing. We entered the traffic pattern properly executing a midfield cross for the left downwind runway 23 (SMD). We have class Charlie airspace directly above the airport which prevents the tear drop entry to the traffic pattern for all runways. All runways at SMD are left hand traffic patterns. We entered the pattern properly and once we were established in the left downwind for runway 23 we noticed a medevac helicopter who had just departed the hospital north of the field fly opposite direction in the downwind. We made an evasive steep turn to the right to avoid the helicopter. We had roughly 250 feet horizontal and 250 feet or less vertical separation as stated above. This is not the first time this happened with the local medevac helicopters operating out of the hospital north of the field. They are constantly flying in the downwind of the active runway opposite direction between 1500-1700 feet MSL. Our traffic pattern altitude is 1800 feet MSL at the field. I personally have encountered them 3 separate times being too close but this one was first time we had a near miss and had to actually take action to avoid them. I do not have the type of rotorcraft or any specifics other then it is called Aircraft Y and operates the hospitals around the area.