RE: 400 feet
Ask the club officers if they have notified the operators of the local airports. There is always an owner. The airport owner might want to consider posting a map with the location of your airfield clearly posted.
In our case, our airfield is outside the 3 mile radius, but one of our club members is also a VP with the AOPA, which is headquartered at the airport. He has told us that all the regular pilots from the airport are aware of our field. They often fly circles around our field watching us fly. We also keep a close eye out for any full scale traffic.
We had a heli pilot a few months ago that was flying out from the airport, directly over our field at about 200-300 feet. We did some asking around with our contacts, and apparently he was told about the location of our field. We still see him fly past, but he now does so well clear of either end of the field.
Working with the airport owner and pilots is the best way to ensure cooperative flying and avoiding interference.
Brad