FT240-43 Torriod based on the W1JR design.
This form of choke is refered to as an Inline-Isolation Balun. Design is quite simple. 70cm of RG-142 high power coax coiled 8 times round a FT240-43 Torroid housed in a waterproof box and potted with Polyester resin. Termination is a chassis-mount SO-239 on opposite ends of the box.
Balun is tested with full UK power limit due to power restrictions but under perfect 1:1 SWR conditions the FT-240 can handle 1KW of power.
Fully Assembled Balun £50
Parts to build your own
70CM of RG-142 £6.00
Typical Sweep of FT240-43 Type 1:1 Balun Sweep is from 1 - 54Mhz
Click for large image
All measurements taken on a MiniVNA
While every effort is made to replicate the sweep above each balun does vary slightly due to the tightness of the windings ETC.