Welcome to the world of ADS-B Grahame.
A few of us tried to import DPD antennas last year but were not successful due to the US postal requirements regarding length. Interesting that Dave has come up with a solution.
Can I make a suggestion regarding the RG213 coax? Check out LMR-400 coax first.
While LMR-400 is more expensive than RG213, the db/100 lost is about half. It may not mean much for local plots but if you are chasing distance, every little bit helps.
http://www.timesmicrowave.com/cgi-bin/calculate.pl
Cheers,
Noel