Build a 500 metre radio data link for under $40.

Step 3 Build a dipole antenna with a balun

At the antenna is a balun made of coax cable. A balun is needed otherwise the shield of the coax ends up becoming an antenna instead of being the earth and radiates RF down near the picaxe which defeats the purpose of the antenna. There are lots of balun designs but I chose this one because it just uses bits of coax cable. Common wavelengths are 95.24cm for 315Mhz and 69.34cm for 433Mhz. The coax lengths are 1/4 and 3/4 of the wavelength respectively. The dipole wires are 1/4 of the wavelength. So for the modules I used at 315Mhz the coax wires were 23.8cm and 71.4cm and the dipole wires were each 23.8cm.

The coax shield and core are joined together where the coax splits into two. At the dipole note the shields are also connected. If these joins are out in the weather then they need to be weatherproofed in some way - eg with paint or non conductive silicone. Antennas work best when at least 2 metres off the ground.

Acknowledgement and thanks to I0QM for this design.