Monday, 20 June 2011

Roofspace Doublet / Dipole MkIII




Vince Lear G3TKN / ZL1VL described in the October 2004 Radcom (and Practical wire antennas 2) a design for an aerial which uses loading coils in place of traps to achieve two band operation.This also results in a much shorter dipole but with reduced bandwidth.The main article describes a 80m /40m dipole at 80ft long.
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.

Saturday, 18 June 2011

Roofspace Doublet MkII

Just a quick update.
I have shortened the doublet and made an inverted V which resonates in the 20M band.
As it is fed into my smartuner it also seems to work quite well on 30m too, well into Europe at least.
Still running just 5 watts QRP CW but trying a new Kent single lever paddle so if you hear me please excuse my occasional mistakes :-)