The Diamond BB2M is broadbanded by making it lossy. Quoting the Operating Intructions:
'Due to its broadband structure, in some case propagation
efficiency of the antenna may be reduced to 1/10
compared with conventional resonant type antennas."
With "conventional resonant type antennas" of this size having a radiation efficiency of perhaps 10% at 7 MHz, the BB2M is admitted to have a radiation efficiency of 1%. Dave's (that's me) Rules of Small Antennas:
Pick any two