Collins Balun

The Collins balun for HF is made from four pieces of RG58 coaxial cable, each 1300mm long.

Other materials required are heat shrink sleeving or waterproof tape to weatherproof the balun and some cable ties to secure the coax once the coils are formed. I used a block connector to join the balun and the twin feeder from the doublet (shown in the photo) - The coax to the ATU was soldered directly to the balun, centre core to cores A and C; braid to cores B and D.

The photograph probably gives a better idea of construction than a verbal description ever will.

Layout of the balun: Click for a larger image

Once the balun was finished I tested it using a 300 ohm resistive load. The results were very encouraging!

Freq MHz	Rs	Xs	Ztotal
1.8	7	73	80
3.5	83	62	145
7.1	109	3	112
14.2	105	0	105
21.0	83	10	93
28.0	68	7	75
50.0	0	18	18
70.0	2	4	6

Since the project was intended to provide a HF antenna, only Top Band was less than we'd hoped for. As it turns out, strapping the feeder conductors at the bottom end allows the antenna to be tuned (tolerably well) for 160, so the balun seemed to do the job.

I repeated the measurements on a commercially-made 4:1 balun that I'd bought a year or so back. The commercially-made balun is toroid wound although I can't see how it is constructed as it is potted into a small, black box.

The results are shown below:

Freq MHz	Rs	Xs	Ztotal
1.8	6	30	36
3.5	26	60	86
7.1	125	40	165
14.2	70	21	91
21.0	118	0	118
28.0	16	83	99
50.0	1	28	29
70.0	1	4	5

On 40m and 20m the Collins balun seems to out-perform the toroidal balun in terms of resistive impedance while keeping its reactive component to a minimum.

While other baluns are smaller and others, still, may have their own benefits, the Collins balun seems easy to make, durable and its performance is good. It adds another pretty effective arrow to the antenna builder's bow.