phase array antennas with monopoles or dipoles

[senmeis]

Hi,

Phase array antennas with monopoles or dipoles can be found in the attachment.

Question: why do monopoles lie perpendicular to the plain but dipoles on the plain?

 

[vfone]

Compared to λ/2 dipoles, λ/4 monopoles requires a ground plane for the radio wave to form.

λ/2 dipoles don't need ground plane, actually is good to be far from the ground plane. Ideally at least 2λ from GP.
In your case the dipoles are at λ/4 distance from the GP, which in this situation behaves as a reflector.

 

[senmeis]

OK. That means the array with monopoles doesn’t have radiation in Z direction so the receiver must be somewhere other than Z direction. Is it correct?

 

[vfone]

Yes, the λ/4 monopoles will not radiate in Z direction.