The existing design uses a 30" insulating mast section just below the feedcone, but still has the coax going down the mast. The effect of the coax is to short out the vertical component of the lower antenna elements and that's the key radiating polarization. It may be a good idea to place ferrites (RS 273-104 7/16"x7/8" or 273-105?) at the base of the feedcone and at the joint between the metal mast and insulating mast section to block any RF currents running on the outside of the coax. The effect may show up in the impedance looking into the end of the coax or in signal strength.