Here are some quick and dirty prototype antennas for the 5.8-6 GHz spectrum. They are simple cylindrical waveguide antennas fabricated from 1.5" copper tube (about 36mm true ID) in a simple cylindrical waveguide design 69mm long. TE11 and TM01 cutoffs are at about 4.9 and 6.4 GHz respectively. Soldering the copper plate onto the end is a fun change from soldering a bunch of 0402 capacitors.
One early prototype uses a bit of semirigid coax soldered straight into the side as the feed probe, but I'm not really happy with this and will probably go for chassis-mount SMAs soldered on there in the final ones. Construction or acquisition of other antenna designs to try may come a little later, with wide-band Vivaldis for example. Tony Long's RADARduino project, which was a significant source of inspiration for my project, uses some beautiful PCB Vivaldis made and sold by Kent Britain WA5VJB.