10-element yagi antenna for 70 cm
This is a short build description of a 10-element yagi for the 70 cm band, based on a design by DK7ZB [1].
Boom
The boom is a 15x15 mm aluminium square tube with a length of 2 m which is very lightweight.
Elements
The elements are 10 mm aluminium tubes, which are connected to the boom by 3d-printed element holders.
| Element | Position | Length |
|---|---|---|
| Reflector | 0 mm | 327 mm |
| Radiator | 140 mm | 315 mm |
| Director 1 | 235 mm | 300 mm |
| Director 2 | 425 mm | 287 mm |
| Director 3 | 650 mm | 281 mm |
| Director 4 | 920 mm | 272 mm |
| Director 5 | 1205 mm | 269 mm |
| Director 6 | 1495 mm | 269 mm |
| Director 7 | 1780 mm | 274 mm |
| Director 8 | 1995 mm | 269 mm |
Simulation
The whole construction was modelled in Autodesk Inventor and a 3D field simulation was performed to verify the design.
The farfield simulation results in a gain of 15.8 dBi, a 3 dB angular width of 32.5° and a front-to-back ratio of 30 dB.
Impedance Match
The feedpoint impedance of the radiating element is about 28 Ω. The simplest matching solution consists of two 75 Ω coaxial cables in parallel with a length of λ/4 [2]. The resulting line impedance of 37.5 Ω transforms the feedpoint impedance to 50 Ω. By coiling up the coax some common mode rejection can be achieved.