DIY UHF Half Wave Dipole Antenna

Sep 29, 2017

Here is my quick notes on how to build a UHF dipole antenna for UHF operation at 433MHz.

Before you start, you should check:

How to come up with basic dimension of a dipole antenna

Calculating the each leg of the dipole antenna is pretty straight forward:

Speed of Light (m/s) / Frequency (Hz) / 4

The Jupyter notebook below calculates the antenna length to cm and inches. Each leg is a quarter of a wavelength, hence the division by 4.

Now, bear in mind that a normal dipole antenna would have a an Z impedance of 73Ω. Varying the angle from 180 degress to a lesser angle, like 120 degrees, would bring the impedance closer to 50Ω.

Design a 3D Printed adaptor for the antenna

Next, a 3D printed adaptor is designed in Fusion 360. The adaptor can be re-used for other frequencies and projects obviously.

Assembling

All you need are some copper welding rods, connector of your choice and a wooden rod.

1. Cut 2 copper welding rods with length roughly 18cm. (Yes, it is supposed to be 17.321cm, but better longer than short. You'll have to cut it shortly.) Insert it to the 3D printed mount.
2. Solder each of the legs to the outer braid and central core, of a RG-58 coaxial cable respectively.
3. Solder the RG-58 cable with the connector of your choice: SMA, N or M connector.
4. Mount the adaptor on a wooden rod, that's it.

Testing

Use an antenna analyzer or SWR meter to check the SWR. Cut each legs of the dipole to get the SWR as close to 1.0 as possible.

I happen to use Yaesu FT-857D which does have an on-board SWR meter, so that would do.

There could be more improvements, such as making a choke balun by doing a few loops at the feedpoint. But otherwise, this is the simplest antenna you can possibly make.