This post describes the second of two ground antennas that we used during the Stratoballoon chase. Unlike the egg beater, which is an omnidirectional antenna, the yagi is a directional antenna. We didn’t use the yagi very much during the chase – as Katherine and I discussed earlier, we ended up relying on the egg beater for most of the tracking.
I studied a few different sites to see how a yagi works, and how to build one:
- "Directional Antennas": https://www.youtube.com/watch?v=lslHtCUSfN4
- "Controlled Impedance ‘Cheap’ Antennas": http://wa5vjb.com/yagi-pdf/cheapyagi.pdf
- "Yagi Design to go with 433MHz tracker": http://www.ausrocketry.com/forum/viewtopic.php?f=60&t=2807
- "How to Build an Antenna": http://www.skyscan.ca/Antennas.htm#Formulas
I thought building a yagi would be similar to building a 1/4 ground plane antenna – start with the target wavelength, and calculate the length of each element and how far apart they are from each other. As it turns out, it’s more complicated than that. Adjustments in length and placement of the elements will shape the signal that the antenna will be tuned for, so a lot of the articles I read talked about tweaking it until I got the desired reception.
I was looking for something a little simpler. The SkyScan.ca article above gave some rough formula for deriving the length of each element of the yagi, and how far apart to place them from each other. I decided to go with that, and build a 5-element yagi for 434 MHz. I first needed to determine the wavelength for that frequency:
Wavelength = 300 / 434 MHz = .691m
Next, I had to calculate the distances that the yagi elements would be placed along the frame so I could figure out how long the frame needed to be. For a 5-element antenna, the elements from back to front are: 1 deflector, 1 driven, and 3 directors. In short, here is how these elements work in conjunction with each other.
- The driven element, located second from the back, is where the signal comes in, and is where the lead line (that goes to your radio) is physically hooked up to. The other elements aren’t actually attached physically. Their job is to shape and focus the signal.
- The reflector is in the far back, and does just what the name suggests – it reflects the signal back to the driven element.
- The other elements sit in front of the driven element, and are known as directors. Their job is to focus the incoming signal. The more directors you add, the weaker the signal you can pick up. However, when you add more directors, it narrows the beam, which means you have to be increasingly accurate where you point the yagi.
The distances between those 5 elements for our antenna ended up being:
- Reflector-Driven = 0.125 x 0.691 = 8.6cm
- Driven-Director 1 = 0.125 x 0.691 = 8.6cm
- Director 1 – Director 2 = .250 x 0.691 = 17.3cm
- Director 2 – Director 3 = .250 x 0.691 = 17.3cm
So, I would need a frame that is at least 51.8 cm long. I had several pieces of wood trim that I thought would be sturdy enough to hold the elements in place, so I cut a piece that was (I believe) 60 cm long.
I cut the handle from the same trim, cut a notch into the frame, and glued the handle in place.
I cut notches in the top where the elements would go. Next, I measured out the wire for each element. The driven element would be the most difficult because it would be folded back on itself:
- Reflector = 0.495 x 0.691 = 34.2cm
- Driven = 0.473 x 0.691 = 32.7cm
- Director 1 = 0.440 x 0.691 = 30.4cm
- Director 2 = 0.435 x 0.691 = 30.1cm
- Director 3 = 0.430 x 0.691 = 29.7cm
I stripped off the wire from one side of the driven element,
and placed them in the notches on the frame, hot-gluing them down.
The driven element was extra challenging even on this step because I had to carve out a channel for the folded ends.
Now it was time to attach the lead line. I had to cut and strip away the two-layers of insulation again, just as with the eggbeater.
And then attach those two lines to the driven element, where the insulation was cut away. The MakeZine.com article "Listening to Satellites with a Homemade Yagi Antenna", was particularly helpful here.
I applied a good amount of solder to keep the line in place. Once I had everything in place, and the hot-glue had dried, I straightened the elements out, and got them all running in the same plane.
I added a couple of zipties to keep the lead line from pulling away from the driven element, and I was done.