i agree with 6mtrs, the lw150/sigma4 style antenna correctly tuned will beat any groundplane or endfed out there especially when both antennas are mounted near the ground, regular endfed verticals without radials and decoupling waste most of your radiation talking to the clouds, they should be re classified as 1/2nvis antennas,
my imax mounted low is piss poor compared to the i10k, the i10k is poor compared to the 5/8 over 1/4 wave sleeve sigma style antenna.
its a pity the sirio sold by hy are of such poor quality build,
on the upside its nice to see im not alone in realising the capabilities of the sigma style antenna, if you can squeeze an antenna 30" wide in there, it will by far outperform the a99/imax,
i would only use a a99/imax mounted low if there REALLY was no alternative, if that is the case you have my condolences.