You need a 24 foot/7.35 metre RG-213 cable between the PA3 and the LC31 link coupler. This is minimum length and part of the tuning, do not shorten or substitute for other cable.
It is recommended that the cable is a little longer, as it can be a bit tricky to remove the insulation on the ends. It will leave you some extra in case you need to do it twice. Longer than 24 foot/7.35 metres is a lot better than too short.
Also, if you are using plugs and sockets instead of plugging straight into the PA3, you’ll likely need a little extra as well. See this page.
RG-213 cable can be ordered from Spektrotek or from Farnell/Newark (part #: 2425425).