TOA Controller described a potential conflict after clearing a helicopter for takeoff at the antenna farm without observing an aircraft on Taxiway Alpha; the reporter notes restricted visibility near the antenna area.

Narrative

Helicopter called up for departure from the antenna site; a position adjacent to Taxiway Alpha; our main taxiway that Local Control occasionally uses to depart helicopters. I was working the Local position. I coordinated for a departure on Taxiway Alpha from the helicopter's position and the Ground Controller in training approved it. I scanned the taxiway and did not see anything that would be an obstruction for the helicopter; saw none; and cleared the helicopter for take off on Taxiway Alpha then continued to work other aircraft in the airspace. The CIC pointed out that there was a aircraft on Alpha that the helicopter was about to overfly. I instructed the helicopter to remain north of the aircraft and he said he would comply. Helicopter passed off the the aircraft's right and departed without further incident. Recommendation; moving the antennas off the antenna site to a different location since they partially obstruct the view of Alpha. The only reason I can think of for not noticing the aircraft on the taxiway is that the antennas camouflaged him at the same time I was scanning the area. Incidences like this do not happen often; so the effort of moving the antenna site would probably not be worth the benefit of having a completely unobstructed view of Taxiway Alpha.