2×5-el K6STI Yagi

Brian Beezley, K6STI designed a small 5-element Yagi for the 88-108 MHz VHF-FM band with a bent driven element which greatly improves the radiation pattern in a simple single reflector design. His antenna optimizer managed to achieve 5-7 dBd gain and F/R 20-22 dB over entire bandwidth. I built two of them in the 2011 and installed in the 2012 for receiving vertically polarized signals. They were replaced in the 2013 with 2× 9-el K6STI Yagi setup. In 2020 I installed them back again, but this time in a horizontal stack.

This antenna uses a bent driven element with its size optimized for horizontal polarization. Because of that, the ≥20 dB F/R figure applies only to that polarization. The plot on the left compares both patterns for vertical and horizontal installation of a single antenna. The second plot on the right compares it with a stack (1.7 m distance).

Construction

The antenna files are available on Brian’s website: http://ham-radio.com/k6sti/five.htm#files

Bill of materials:

  • aluminium square booms: 15×15×1.5 mm,
  • 40×5.5 mm non-conductive round PP cross-boom with non-conductive guys,
  • 10×1 mm aluminium tubes with insulated mounting brackets,
  • 8 mm fiberglass rod for reflectors reinforcement,
  • some screws, nuts and washers (stainless),
  • plastic boxes, cable glands, ferrite chokes, BNC connectors, 75Ω coaxial cable,
  • BNC T-adapter and CNT240 50Ω coaxial cable for 37.5Ω -> 75Ω matching.

All elements should be installed on insulated mounting brackets. I used RSB RAPR-110 hydraulic tube clamps. Each tube end should be closed. I filled all tubes with spray foam. The most difficult part of this construction is the driven element, which must be mounted at a specific angle.

Measurements

I measured the antenna radiation patterns using a Airspy HF+ tuner together with GNU Radio and XDR-GTK software. The multipath interference is visible as noise in the following plots.

  • 87.7 MHz: PR 1 Konin, 60 kW ERP @ 95 km
  • 88.1 MHz: Płock FM, 1 kW ERP @ 10 km
  • 92.2 MHz: PR1 Sierpc, 60 kW ERP @ 35 km
  • 93.9 MHz: PR2 Włocławek, 1 kW ERP @ 46 km
  • 94.3 MHz: RMF FM Sierpc, 60 kW ERP @ 35 km
  • 96.1 MHz: PR3 Sierpc, 60 kW ERP @ 35 km
  • 97.3 MHz: Zet Sierpc, 60 kW ERP @ 35 km
  • 98.1 MHz: PR2 Sierpc, 2.5 kW ERP @ 35 km
  • 98.9 MHz: RMF FM Konin, 30 kW ERP @ 99 km
  • 99.7 MHz: PR24 Płock, 0.2 kW ERP @ 10 km
  • 101.6 MHz: PR1 Kutno, 10 kW ERP @ 48 km
  • 101.9 MHz: RDC Sierpc, 60 kW ERP @ 48 km
  • 103.3 MHz: PR3 Konin, 30 kW ERP @ 99 km
  • 106.3 MHz: Maryja Sierpc, 60 kW ERP @ 35 km
  • 107.1 MHz: Zet Konin, 60 kW ERP @ 95 km
  • 107.8 MHz: PR1 Łódź, 30 kW ERP @ 95 km

 

More details coming soon.