The principle of function is shown on the figures to the right.
The rope cage is intersected at the middle of the tower in order to feed the antenna at this point. The lower parts of the cage ropes are connected to the tower by the cross arms, the upper parts of the rope cage are connected with the output of the matching circuit. Thus the power is fed to the antenna at the intersection of the rope cage.
Upper rope cage
The tower and the rope cage can be regarded as a coaxial line, where the tower is the inner conductor and the rope cage the outer conductor. The cage and the tower form the stub L2 with its input at the feed point and the short circuit at the top cross arms. The length of this short circuited coaxial line is approximately lambda/4. Therefore the effective reactance, in parallel with the feed impedance, is nearly infinite. This system also ensures that all parts of the antenna are at zero electrostatic potential.
Lower rope cage
The lower section of the cage together with the short circuit S1 and the tower form the stub L1. The input of this coaxial line is at the bottom of the tower. The height of about 50 m of the short circuit is less than lambda/4. A coaxial line shorter than lambda/4 acts like a coil. Altering the height of the short circuit means changing the inductivity, changing the current distribution and changing the veritcal pattern as shown above.
Current distribution and vertical pattern
The height of the null of the current distribution and the elevation angle of the null of the vertical pattern depend on each other:
Increasing the height of the current null means reducing the elevation angle of the pattern null. The side lobe is growing correspondingly. This can be achieved by diminishing the reactance of the lower cage. It means that the length of the short circuit of the lower rope cage must be reduced.