[Radio] Making a good Antenna for a High Altitude Balloon

Ill try to demonstrate how to make a proper antenna to use with, for instance, high altitude balloons. The antenna we will be making here is a “quarter wave antenna”. It will not surprise you that the length of this antenna will be equal to a quarter of the length of the wave.

The formula for calculating the wavelength for a frequency is as follows:
[latex]lambda = c/f [/latex]
[latex]c=3.0*10e{8} m/s[/latex]
This is the speed of the medium (light in our case) devided by the frequency. For 434MHz, this concludes a wavelength of 70cm.

The best antenna to use (from experience) for use with high altitude balloon is probably the quarter wave. This is similar to a half wave antenna.. but.. its half as long as a half wave, Hence, the quarter wave. Are you with me?
Anyway let me simplify the following quite a bit; a quarter wave antenna does need a groundplane, to reflect its mirrored counterpart. This means it mirrors the power in one direction (through the groundplane) to the same direction as the quarted wave it pointed. This way, you are not losing any power to space. Now, let’s build one.

The driven element of the antenna should be a quarter wave. So, if we are using a 434MHz antenna with a 70cm wavelength, we need 70/4~=17cm size of the driven element. The groundplane has to be at least this size. The groundplane can consist out of just a few well spaced wires, but its better to make a continuous plane, i will demonstrate this by using conducting aluminiumfoil.

In my experice, its pretty handy to have a SMA connector on your radio. So, buy a SMA cable that connects to that, then cut it in half, and use the “cut end” to make the following antenna. I guess making this is pretty self explanatory though the images:

The housing of the antenna, foam keeping everything stirdy and straight

The woven shield stripped down a RG85 wire

Spread the stripped shieldwire into four ~equal pieces

Then when the shaft is in, divide four wires and spread them out

This concludes the how-to. The result is the following. And, afterwards, do make sure that the aluminium plane is conducting with the ground of your wire, and the driven element is not conducting to your groundplane.

All done! The result with a nice big groundplane to reflect the 1/4 wave