Figure 1: Group of 8 PCB yagi antennas with 5 elements each
If the radar should operate in the frequency band 2.4 to 2.5 GHz (free ISM band, mainly used for WLAN), then yagi antennas are the best choice.
If you dare to build them yourself, you will usually be able to calculate their dimensions yourself. There are also enough programs on the Internet with which you can optimize the dimensions.
A further possibility is to use simply finished WLAN antennas from the trade. They don't have to be as complex to set up as shown in Figure 1. It is sufficient for the beginning to simply operate a transmitting antenna and a receiving antenna. This is possible even if these commercially available WLAN antennas are logarithmic-periodic antennas.
In the example shown, all yagis are connected to an 8-fold power divider. The cables must all have the same length so that the antennas are fed with the same phase position. It is also possible to use only two 4-way power dividers, so that one half works as the transmitting antenna and the other as the receiving antenna.