Measure the 3 slanting radials 69mm from edge of disc to the tip of the radials. Do not measure 69mm from center line of the antenna / whip.
While making the DIY Spider using SO239, the slanting radials are measured from the edge of the flange, not from center of whip. The additional horizontal surface provided by flange improves the performance. In MaxRad antenna a small disc does the job of flange.
TomW of Planefinder forum has tested two Spiders, one with radials measured 69mm from vertical whip, other with radials measured 69mm from the edge of flange, and found that the one in which radials are measured 69mm from edge of disc performed better than the one in which radials are measure 69mm from vertical whip (center line of antenna)
EDIT: Jan 08, 2016 - Corrected last sentence (in red color) above.
Click here: Side by side antenna testing - by TomW
TomW’s two Spiders under comparison test:
Left one has radials measured 69mm from the antenna center line /vertical whip.
Right one has radials measured 69mm from edge of the disc.
Spider measurements with SO239
Note the length of radial is measured 68mm starting from edge of flange.
