This just in from Barry Allison (VE3NA):

The original instructions for the Slim Jim 2 metre antenna gave dimensions that were in error due to calculations based on an incorrect value for the velocity factor of the 300 ohm twin lead given by the cable vendor. Folks that received and already built their kits based on the previous instructions will be given a replacement length of 300 ohm cable to correct the problem that resulted in a high SWR.

Download the (updated) project documentation here: