I wanted to build a 2 meter Yagi antenna so I could start playing around on 2 meter SSB and CW.
Initially, I considered the tape measure Yagi.
I'd helped another ham build one this past summer and I was impressed with it's performance. There is a downside to the design though. In even a mild breeze, the tape measure elements vibrate and sometimes flap around in the wind.
Also, I live in an RV and travel a lot. My antenna will be up and down often, and packed away every time I travel. It has to be durable since it will undoubtedly get bumped around from time to time.
I also wanted a fairly wide bandwidth, providing a low SWR across the band so I could operate down in the SSB region or up in the FM area with good performance and no tuning required.
With durability and bandwidth in mind, I decided to build the structure out of PVC pipe and the elements out of window line. PVC is cheap and strong. If the antenna ever broke from an impact, it would be easy and cheap to repair. The *fat* elements created by the window line should provide for a broader bandwidth.
Initially, my plan was to use a folded dipole for the driven element. I've seen many commercial Yagi antennas using one. Folded dipoles are less noisy and a bit better at rejecting out-of-band signals, also providing a nice uniform pattern.
I spent a couple of days working on that idea, but had trouble getting it to work. A folded dipole presents around 450 ohms of impedance at it's feed point so you need a matching network to match to the 50 ohm feedline. However, in a Yagi, the parasitic elements of the reflector and directors change things. Yagi's usually have a lower impedance at the feedpoint of the driven element and I just couldn't get a good match to 50 ohms using a trombone coaxial match.
Abandoning that approach, I decided to start with the tape measure yagi design and adapt it.
Pictured here is the resulting antenna and a plot of it's SWR over the 2 meter band.
*Be sure you cut the boom pieces so the distance between the actual elements are correct!*
Also, it's a good idea to seal up the coax end with silicone or liquid tape to prevent moisture from getting into the coax. (I hadn't yet sealed it when I took this picture.)