This is a design of an omni antenna for 24 GHz. It is a bit difficult to
make because it needs a total of 24 slots which are 0.7 mm wide.
I have added a scan of the design which was realised with help of
a friend of mine. The only thing he could not realise where the slots of
0.7 mm which were enlarged to 1 mm. In practice this works good enough.
On the side of the waveguide I added some extra waveguide to improve the
omni behaviour. The antenna is made in brass waveguide.
Also a small screw of m2 needed for improving VSWR. For finding the
correct position I always take a very small needle and look at the vswr
while I insert the needle in the slot closest to the flange. By moving
the needle you can finf the place where VSWR dips. Then I do the same at
the next slot and measure the position difference between the first slot
dip. Same again for slot 3 and again measure the difference. With these
measurements it is possible to find the place for the m2 screw just
below the first slot in the middle of the waveguide.
Gain in practice is arround 13 dB which is as expected. Be sure that all
slots are in line to each other so that you can look through the
waveguide slot from the front and the back.
You will need 12 slots on each side of the waveguide.