They are grounded with a precise length of coax (multiples of 1/4 or 1/2 wave, IIRC) to keep the VSWR ratio low for the intended frequency. In a pole you can ge

They are grounded with a precise length of coax (multiples of 1/4 or 1/2 wave, IIRC) to keep the VSWR ratio low for the intended frequency. In a pole you can get to do that by probing the signal at certain height, but the active part of a patch antenna (which I assume a manhole works as) is at the extremes of it (as such, probes are always off center for circular polarization, and at the extreme for linear), and those are grounded. Patch antennas are normally grounded at the center, as the voltage/current there should be close enough to 0 anyway unless there's an electrostatic discharge.