Make a wideband antenna matching transformer

Published on October 30, 2015 . Updated on: -
Most do-it yourself TV antenna projects online use a ready made impedance matching transformer between antenna and cable. But what can you do if you can't find one at your local supplier? This post will show you how can you make one. You could use some hard to find double hole pig-nose RF ferrite cores, but this transformer has good performance even with ordinary toroidal cores.

This 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. A device similar to what is presented here is used in commercially available antenna preamplifiers with 300 ohms input.

Make a wideband antenna matching transformer
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.

wideband balun parts
What you need
wideband vhf-uhf 300:75 4:1 balun construction detail
Construction detail
Looking 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 ground (GND). You can connect this to the antenna ground, you can let it unconnected or connect it with the output ground. The remaining left wires are 300 ohms inputs. On the right make two pairs of upper with lower wires, but make sure you don't connect the same wires from the left. Pairs represent the 75 ohms output and the output ground (it doesn't matter which one you choose to be GND or 75 ohms). Maybe the following photo will make things simple:

Wideband balun conenctions
Transformer connections
The equivalent schematic is this:

Matching transformer schematic
Balun 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 you have available, you can use double hole pig-nose cores which are specifically designed for radio frequency. Here is a photo of a ready made antenna amplifier that uses such transformers. As you can see, they left the input ground not connected.

Pig nose core matching transformers
Pig nose core matching transformers
The type of matching transformer presented here can be used for any kind of reception antenna where 300 to 75 ohms conversion is required and can be followed by 75 ohms input antenna amplifier.


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