Low Loss Balun for FM Broadcast Band

These are the design notes for constructing a low loss 75 Ohm unbalanced to 300 Ohm balanced transformer suitable for the FM broadcast band (88 - 108 MHz). Properly constructed, it will typically have a loss of 0.3 dB at the center of the FM band.

The balun is wired as a current balun (center tap on balanced side is floating) as shown in the schematic.

The following photographs show the construction of the balun. Both windings are three and a half turns around the core (pass through the core 4 times). Since the winding length is long, it is important to use high impedance twisted pair wire, as specified in the parts list. The chassis is 0.010 inch thick brass shim stock, but a grounding lug for a 3/8 inch stud is a suitable substitute for soldering the ground wires to. The balanced line is 300 Ohm twinlead, but two 14 or 16 gauge solid wires spaced 3/4 inch apart should work as well. The balanced line is anchored and weather-sealed with urethane foam (aerosol can from Home Depot, etc.). Do not pot the entire balun with foam, as it will degrade performance if allowed to penetrate into the windings.

Photograph showing the balun mounted on a Radio Shack 6-element antenna. Note that cable ties are only good for about 5 years of exposure to the sun, so for a permanent installation, use a stainless steel hose clamp instead.

Parts List

Balun Core Amidon Associates, Part number BN-61-002

Amidon Associates - (714) 850-4660

Enclosure Radio Shack, Catalog number 270-1801

300 Ohm Twinlead Radio Shack, Catalog number 15-1174

F-61 Connector Radio Shack, Catalog number 278-212

Winding Wire Belden 8132, 12 inches long, jacket and shield removed.

For a kit consisting of one balun core and a length of Belden 8132 wire, send $14 cash to: