I needed a omnidirectional antenna for the 40m.
Here is the schema of the antenna :

on8im vertical 40m

The design formulas are quite simple :

Dimensions in feet

A = 230/F (feet)
B = A * Vf
C = (A - B)
S = 4"
R = A * 1.05

Dimensions in meters

A = (230/F)*0.305 (meters)
B = A * Vf
C = (A - B)
S = +/- 10cm
R = A * 1.05


The angle of the radials will be idealy between 35° and 45°

Vf (velocoty factor of used coax) depends of the type of cable. Don't trust to much the  clasic 0.66 value : the best is to mesure the Vf with a antenna analyser, results are some surprising (espacialy when you use old coax cables).

Applied as there are, those formulas gived me results close to what theoreticaly expected.

swr 40m

No re-tuning was necessary.