The lengths of coax from the center BNC connector must be identical. If you find it easier to mount a flange style SO-239 connector on the boom instead of a BNC, use it instead. You will then need a cable with an PL-259 at one end and a BNC connector at the other to connect the antenna to the RDF unit. Whatever you do, make the unit collapsible so it can be stored when not in use. It is very important that the diodes on the antenna be connected as shown on the schematic.
Most diodes have a band which shows which end is the cathode.
The best diodes for use in the antenna are RF PIN diodes. I used 1SS103 PIN diodes I purchased at a flea market. An equivalent diode is the MPN 3404. In a pinch, you can substitute a 1N4007 power diodes. They have a similar internal diode structure to the PIN diodes and will work almost as well.