In a previous life I managed the communications for a state police agency. We used 45 MHz for our main system and had forty some odd tower sites, almost all running DB212-3 antennas.
Two of the sites were on 1000+ towers and used a single DB-212 element due to the large tower face and the great height. One was a repeater using a receive antenna at 1450' and a transmit antenna at 1350'. The other was a remote base station with the single loop at about 850'. As we were an investigative agency, almost all of the mobiles were using AM/FM disguise antennas. (Yeah, I know, but we were stuck with the band that the State Division of Communications had dictated...) Despite the radiating dummy load antennas, we had excellent mobile coverage in virtually all of the state. A consideration for DB212 antennas is that lining them up on one leg can make them pretty directional. For towers that were very close to the coast, I would put all three elements on a single leg, but skew them so that only one was pointed directly off of the leg. This seemed to give me a somewhat cardioid pattern, but with a little better pattern to the back than if all three elements were in line. Another consideration is that they were designed to be used on Rohn 45/55/65 sized tower. If you put them all on one leg, a larger tower face doesn't matter much except that the rearward pattern will likely have a larger null. Mounting them on all three legs of a larger face tower will result in reduced gain and a pretty messed up pattern. I don't know if I'd worry a whole lot about adding a fourth element- the three element antenna will deliver excellent results. Doug K4AC (Running for ARRL Southeastern Division Director- please check out my website at www.k4ac.com)
