Pontefract and District Amateur Radio Society (PDARS) runs construction evenings on Tuesdays and, when I was recently doing some of the preparatory work with G0NQE, the next summer project was The PDARS HF Doublet. While experimenting with designs for 'full size' and 'half size' doublets and with 450 ohm and 300 ohm feeder, I discovered that matching these antennas was a whole lot easier when an impedance matching balun was used between the end of the balanced feeder line and the ATU. I was looking for a balun that would be both quick and easy to make and would be suitable for members who fell into both QRP and QRO camps. With an idea that small baluns wound on toroids are easy to cook, I went looking for a design that would do the job and handle the power...

I came across the Collins Balun.

As the layout diagram above shows, the Collins balun consists four lengths of coaxial cable, each wound into a single loop (A single turn coil, that is.)

Two cables, A and B, are wound anti-clockwise while the other two, shown as C and D in the picture, are looped in a clockwise direction. The braid of cables A and B is connected at both top and bottom; similarly, the braid of cable elements C and D is connected at the top of the loop and again at the bottom. This is shown in the wiring diagram, here.

The unbalanced input - usually a nominal fifty ohms from the radio - is connected to the balun through the centre cores of A and C together and of B and D. The balanced side of the transformer is presented at the centre cores of cable A and cable D.

The circuit of the transformer is: