Make a wideband antenna matching transformer

Most antennas have balanced 300 ohms output. How to make a wideband impedance matching transformer (balun) for FM, VHF and UHF reception. This will convert 300 ohm balanced to 75 ohm unbalanced suitable for coaxial cable.

Most do-it-yourself TV antenna projects I could find use a readymade impedance matching transformer between antenna and cable. But what should you do if you can't find one at your local supplier? Let's attempt to make one with nothing more than some copper wire and a ferrite core. You could use some hard to find double hole pig-nose RF ferrite cores, but this transformer performed fairly good with ordinary toroidal cores.

This matching transformer can be used for VHF and UHF reception (from 50 to about 800 MHz). The input is 300 ohms balanced and the output is 75 ohms unbalanced, ready for coaxial cable connection. I found out that a device similar to what is presented here is used in commercially available antenna preamplifiers with 300 ohms input.

To build it you need a toroidal core with an outer diameter of 8 to 16 mm and a few copper wires of 0.3 to 0.5 mm diameter (you can use wires from UTP network cables). You will make 2 by 2...3 turns as described below.

Some wire and a toroidal core are all you need to build a matching transformer

Construction detail

Look at the above photo; there is an upper coil and a lower one, each of 2.5 turns. Wire endings from left represent inputs and on the right, there are the outputs. On the left, connect one wire from the upper coil to one from the lower coil. That's the input ground (GND). You can connect this to the antenna ground, you can let it floating (not connected to anything) or join it with the output ground. The remaining wire endings on the left side are 300 ohms inputs. On the right make two pairs of upper with lower wires, but make sure you don't connect wires of the same pair. Joined endings represent the 75 ohms output and the output ground (it doesn't matter which one you choose to be GND or 75 ohms). The following photo will make things simple:

Wideband balun connections

The equivalent schematic is this:

Matching transformer schematic

Note: if using this transformer, do not turn on active antenna from your TV or receiver settings. Do not use any kind of power supply through coaxial cable. This transformer is a short circuit at DC. In order to prevent this risk, you could add a 0.47...1 nF ceramic capacitor at the output of the transformer before connecting the coaxial central wire.

Besides toroidal cores, if available, you can use double hole pig-nose cores which are specifically designed for radio frequency. Here is a photo of an antenna amplifier that uses such transformers. As you can see, input ground is not connected.

Pig nose core matching transformers

The matching transformer presented here can be used for any kind of receiving antenna where 300 to 75 ohms conversion is required and can be followed by 75 ohms input antenna amplifier.

References

• А. Пахомов. Новые антенные усилители on RadioRadar.net, published on 27.07.2003 (accessed on Oct 30, 2015).
• Brian Beezley (K6STI). Baluns for 88-108 MHz (accessed on Nov 9, 2016).