Give us more info and we can help you. How high is your tower ??. Is it a crank up or something like a Rohn fixed tower you can climb ?. If it is at least something like 30 or 40 feet, as BYU suggested, probably a dipole or inverted V with the apex near the top would be easy, and possibly better than a vertical, unless you have room for lots of radials, buried, or on the grass at least 20 feet long. If the grounded tower/beam combo is not 1/4 wave resonant near 40 meters then a vertical wire suspended out 2 to 4 feet from the tower fed against the radial field may give you OK DX performance, but not great for local or regional (within a state or two) performance where the dipole would probably be better. If the tower is say 25 feet, the beam will top load it and it may be resonant perhaps close to 40 meters. If the tower sections are electrically well bonded (not a crank up), then the tower could probably be easily loaded....they call it shunt feeding and performance should be equivalent to a stand alone vertical. I have shunt fed most of my many dozens of verticals for nearly 70 years. They should be resonant at least not much above 40 meters, but lengths longer than 1/4 wave resonance (34 feet), can still be shunt fed just fine, and may work even better. If you shunt feed a tower run ALL your RF coax cables, rotator cables, etc., inside of the tower and exit at ground level, and ideally bury them for some distance toward the shack. This alone decouples much RF from the cables, but additional choking, especially if you run high power might help. Read up on this and then we can give you some more info. If a vertical is surrounded by buildings and has a compromised radial field, the dipole higher up may often work better. Rick KL7CW