An ultrasonic antenna is mounted with an ultrasonic transducer on an agricultural implement and directs an ultrasonic burst directly towards the ground in front of the agricultural implement for the purpose of range finding. The transducer receives the reflected signal and, following signal processing, a computation is made of the height of the reference point on the implement above the ground. Ground tillage, seeding, and similar equipment including above ground harvesting implements can be controlled with the equipment. The antenna is characterized by a moulded body having a sensor mounting at one end and an opening at the other end with co-operating first and second concave surfaces for reflecting the ultrasonic beam between the sensor and the opening and vice versa.

The first and second reflectors are of part cylindrical shape extending around respective axes arranged substantially at right angles to each other with the distance from the second reflector, which is nearer the opening of the antenna, to the virtual source of the transducer being substantially less than the radius of the curvature of the second reflector, whereby a slightly diverging transmitted ultrasonic beam of elongated cross-sectional shape is provided.