70cm Portable Yagi Antenna

This antenna uses 12mm copper plumbing pipe as its elements - these should be readily available at any DIY-store.

The antenna geometry was optimized in Matlab. It is very well matched to 50Ω without any additional matching device required and should give you SWR < 1.1 in the 70cm ham band and < 1.2 in the PMR446 band.

BOM:

• ⌀12mm copper pipe
• coaxial cable (I used 2 meters of off-brand LMR-195)
• coaxial connector of your choice
• copper tape with conductive glue for the sleeve balun
• 1x M3x25 screw
• 7x M3x20 screws
• 8x M3 nuts
• 10x M3 washers
• 1x ¼-20 UNC nut for the tripod mount

Tools:

• pipe cutter (these are cheap, also sold at DIY-stores)
• soldering station, with the largest tip you have because these copper pipes can sink quite a lot of heat
• drill + 3mm drill bit to drill out the holes in the boom

Use PLA or PETG. I printed mine out of PETG, but PLA might be a better option to make the boom more rigid.

Use enough perimeters to make the boom solid - i.e. 3 perimeters with a 0.6mm nozzle or 4 perimeters with a 0.4mm nozzle.

Element length:

• Reflector: 325 mm
• Driven elements: 2x147.5 mm, spaced apart by a 10mm insulator
• Director 1: 267 mm
• Director 2: 261 mm

The antenna uses a sleeve balun made out of copper tape. This is strictly speaking not necesary, but it will noticeably improve the performace. The sleeve balun length is ¼λ x 0.8 = 13.8 cm (0.8 being the velocity factor of the coaxial cable). Measure this length of the copper tape and wrap it around the end of the cable. Cut the outer jacket at the end of the sleeve further from the driven elements and solder the sleeve to the outer conductor - see pictures.

The real measured front-to-back ratio of this antenna is about 10 dB. I have yet to measure the real gain (calculated gain is about 6 dBd), but it is a very substantial improvement over a whip antenna.

If you build this antenna, please let me know about how it performs for you!

Update: that joint between the front and rear part of the boom was a weak point, so I have now uploaded an updated version of the design with a much sturdier joint.