He also describes a 20m / 40m version which is 39ft long and uses 47uH inductors.
I thought this would be ok for my small loft and so it was constructed as per the article.
I managed to squeeze it in with the ends folded across with the inductors and 2ft9 end sections.
A 1:1 balun was built from a PW antenna Workshop article (may 2010) by Mike Jones G3UED which uses a ferrite rod as the core.
More by luck than judgement the aerial shows low swr across 20m band and centres on 40m 7.1mhz but rises to the band edges to 2.5-3.0:1 without any trimming..
I will probably run as a doublet again with the smartuner to cover other bands but may also try adding dipole elements for 10m etc.