Figure 1: A pair of elliptical gear wheels
Gear wheels are not always circular, there are also elliptical gear wheels!
The conversion of a rotational movement into a oscillating movement causes a sinusoidal change in velocity of the oscillating movement normally. This speed change is undesirable in a approach radar. The aim is as linear as possible scanning of the approaching direction.
Therefore, an elliptical gearbox is used in the PAR 80 which consists of two identical elliptic gear wheels.
This gearbox changes the gear ratio depending on the current angle of the drive wheel. Thus, it causes a speed change of the rotation opposite to the regular sinusoidal speed change of a connecting rod. A change of the moving direction of the nodding antenna can now be carried out much faster.
To ensure smooth running of the sum of the force-transmitting part radii of both gears must always be the same. In practice, both gear wheels are matched to each other individually. They can therefore be replaced in pairs only.