Eddie,
I doubt you are guessing as much you may think,
avanti can't change the physics of how transmission-lines work,
length, tube diameters, spacing & dielectric,
since they make no mention of loading the lines with a dielectric we can assume its air spaced,
nor do they talk about using different diameter tubes or mast,
they don't talk about using a coax transformer like the astrobeam,
all you have left to achieve what they are claiming is spacing,
avanti can't change what length & spacing of the 1/2" tubes will give a good match to your 50ohm coax.