I have a buddy who would like to put up a 40meter antenna on a steel airport building. The roof area where the antenna can be placed is 29ft wide x 100's of feet long, but the vertical part of the antenna should be no more than 8ft tall, so as to keep it less than 30ft agl (22ft building height + 8ft antenna height). The first idea is a screw-driver mobile whip, using the steel roof as the ground plane. A much-larger-than-mobile capacitance hat could be added. That would allow easy qsy, and even multiple bands. Overall efficiency might be low? Another might be an inverted-L, fed against the steel roof, where the vertical part is only 8ft, but the horizontal part can be almost any length that makes sense... Likely to be more efficient, but radiate mostly high angle. A remote tuner is not out of the question... Other suggestions?