I've never trusted Comet gain specs and when compared to other antennas with known gain they seem to fall short. The last time I did a comparison was between a Comet GP-9 and DB Products DB-411-B which is a 4-element UHF dipole array, all dipoles in line for 9dBd gain in cardioid pattern. I was running a GP-9 on a 70cm amateur repeater at my house and wanted to see how the commercial dipole array compared so I disconnected the repeater and took precise receive levels off several distant repeaters on the GP-9 within the estimated pattern of where I would point the DB-411 using a spectrum analyzer with peak search and readout accurate to .1dB. I quickly took down the GP-9 and installed the DB-411 on the same mast, same height with same feedline and receive levels were around 3 to 4dB better on the DB-408.
Looking up gain specs on the GP-9 it claims 11.9dBi gain which translates to 9.76dBd gain. This means the Comet should have been slightly better than the DB-411 but it was actually around 3 to 4dB worse or around 5 to 6dBd gain range. The GP-9 was operating within its rated frequency range where the DB-411 was a 450-470MHz antenna and being operated slightly out of band but this is fairly normal for surplus commercial antennas used in amateur service. I left the DB-411 in place and its been the primary 70cm repeater antenna here for a good 4-5yrs now and repeater range has been slightly better compared to the GP-9.
Other tests in years past have shown commercial antennas with known gain have outperformed Comet and other amateur grade antennas with similar advertised gain.
the GP-9 seems for some reason to suffer on UHF, regardless of the printed specs. We did a similar comparison with a Hustler G-6 and the G-6 made the GP-9 look pretty poor on UHF. I know of at least two people that had new defective ones out of the package on UHF. A third has been replaced and still is intermittent on UHF while VHF seems pretty stable on it.
The testing was done with a calibrated IFR 1200 Super S and while not the most sensitive spectrum analyzer, it, and another smaller non calibrated SA showed the same results. Also, the G6 hustler vastly out performed the GP-9 at the repeater site for receive also. On air live testing to multiple radios in our area, yielded the same results. Given the length of the antennas, that honestly shouldn't have been the case. Obviously the G6 stayed up until the repeater was taken off the air, and is likely still floating around "somewhere".
I agree with the others though,. a dual band antenna is likely your best move. Redundancy on bands if often a good thing. Truthfully I don't know the Anytone models off the top (578's) but several of us also use them on 1.25 meters and even at only 5 watts, we have a blast with it in our area. For those that use those there the Comet CX-333 is the preferred "single antenna", although some guys triplex it out to 3 separate antennas.
