Each of both dipoles has an exactly equal-length coax cable down into the bunker. The circular polarisation is achieved by feeding them with a -90 or +90 degrees phase difference, using a coax delay line of 1/4 electrical wave length. And of course, the dipoles can also be used as stand-alone.

To properly switch between all possibilities, a PCB relay board was developed. The modes are:

• Left hand rotation + external diversity Rx
• Right hand rotation + external diversity Rx
• Dipole North-South + external diversity Rx
• Dipole East-West + external diversity Rx
• Dipole North-South + diversity Rx on EW-dipole
• Dipole East-West + diversity Rx on NS dipole

 

In the circular modes the 50 Ohm antenna impedances are in parallel, resulting in a load of 25 Ohm. This is transformed back to 50 Ohm by an autotransformer of 17 turns with a tap at 12 turns (ratio is 17^2 : 12^2 = 289:144 = 2.007:1), wound on an FT240-43 core.