Losses in coaxial cable are closely related to cable diameter for competitive cables. If you must have a certain minimum loss, then you will be stuck with a certain minimum diameter. For example, Heliax, which is a good quality low loss coax from Andrews has losses vs diameter as follows:
dia loss at 60 m (est)
0.,25 inches 7.5 dB
0.375 inches 6.5 dB
0.50 inches 3.5 dB
This type of cable is not very flexible since the outer conductor is corrugated solid copper. Typically the more flexibility, the greater the losses, so it probably doesn't get too much better than this.