To build a yagi antenna, you need a dipole as the driven element, a reflector and one or more directors. I've created a holder for both the dipole driven element and the reflector/director that fit on a 3/4" non-conductive tube (21.5mm).

The dipole holder is made for 1/8" material with a separation between sides of the dipole of 2mm. A cavity is provided to accommodate solder joints from the coax (RG6) connection. The top is held on with 1 inch 6-32 screws. I chose to tap the dipole holder body for these threads and it ended up being quite strong. Silicone sealant is used to fill the rest of the internal void (coax connection) as well as the surface between the top and dipole holder body. I also filled the screw holes and antenna grooves with sealant to avoid as much moisture contamination as possible.

The reflector/director holder is a simple 1/8" through hole where the required length of material is held at its center.

Both dipole and R/D holders have a countersunk hole in the bottom intended to anchor the unit to the correct position on the antenna shaft.

The driven element is two sections of 1/8" brass, and the reflector/director are 1/8" aluminum (obtained at a welding supply store).

I built this antenna for a 390 MHz, but I've also done one in the same fashion at 435MHz (UHF HAM band). I highly recomment YagiCad by VK3DIP as it was incredibly useful to determine a starting point and then to tune it exactly. I highly recommend tuning with a Vector Network Analyser.

Using these the antenna came out to 52 ohm impedance, -5 ohms reactance, with a SWR of 1.13. Gain is simulated at 7.5dBi, but I've not verified it in the field yet.