This is a short build description of a 10-element yagi for the 70 cm band, based on a design by DK7ZB .
The boom is a 15x15 mm aluminium square tube with a length of 2 m which is very lightweight.
The elements are 10 mm aluminium tubes, which are connected to the boom by 3d-printed element holders.
|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|
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.
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 . 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.