Planar Antenna Design

I'd like to design a planar antenna with an opening angle of about 60° (H)
and 6° (V). The frequency is 24GHz. The antenna must have a small size,
the dimension in the V must be below 20mm, so a patch-array won't work.
Is there any (unconventional?) idea?

The radiation beam width is determined by the dimensions of the array. There is no way to fool mother nature.

why do they ask you to design this antenna?

Yes, no way.

The idea of this system is to detect objects in this area. My current point
of interest is to use two receiving antennas spaced half wavelength. By
comparing the phase of the two receiving signals i can make such a small
field. The problem is unwanted reflections outside this area.
Perhaps it is possible to use endfire antennas (slots) that have a small
beam in one direction. The advantage is the thickness (20mm) could be
reached.
What do you mean?

With synthesis of antenna array you can not get so high direction in smallest dimension. I'm get wanted characteristic only with H-type of horn antenna (1x6 cm open end), but you need smaller than 6cm.
Try to do that with one gunnplexer on 24Ghz, this is resonator with gunn, pin and varactor diode (receiver, transmiter and mixer in resonator hole). On easy way you can comparing the phase. Standard WR-42(UG-595/U) flange is output open end.
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Sensitivity of gunnplexer is about -90dBm and 24Ghz is standard freq.

No way, I think.

24GH ?

What the use of it ?

The radiation is so mcuh that maybe need not a ANT at all.