If someone is looking for a Yagi useable in the 868MHz ISM range – here is my DIY construction:
Construction details from calculation:
VK5DJ's YAGI CALCULATOR Yagi design frequency =868,50 MHz Wavelength =345 mm Parasitic elements fastened to a non-metallic or separated from boom Folded dipole fully insulated from boom Director/reflector diam =4 mm Radiator diam =4 mm REFLECTOR 167 mm long at boom position = 30 mm (IT = 77,5 mm) RADIATOR Single dipole 160 mm tip to tip, spaced 69 mm from reflector at boom posn 99 mm (IT = 74,0 mm) Folded dipole 164 mm tip to tip, spaced 69 mm from reflector at boom posn 99 mm (IT = 76,0 mm) DIRECTORS Dir Length Spaced Boom position IT Gain Gain (no.) (mm) (mm) (mm) (mm) (dBd) (dBi) 1 146 26 125 67,0 3,5 5,6 2 144 62 187 66,0 5,8 7,9 3 142 74 261 65,0 7,3 9,5 4 140 86 348 64,0 8,5 10,7 5 139 97 444 63,5 9,5 11,7 COMMENTS The abbreviation "IT" means "Insert To", it is the construction distance from the element tip to the edge of the boom for through boom mounting Spacings measured centre to centre from previous element Tolerance for element lengths is +/- 1 mm Boom position is the mounting point for each element as measured from the rear of the boom and includes the 30 mm overhang.The total boom length is 474 mm including two overhangs of 30 mm The beam's estimated 3dB beamwidth is 53 deg A half wave 4:1 balun uses 0,70 velocity factor coax and is 121 mm long plus leads FOLDED DIPOLE CONSTRUCTION Measurements are taken from the inside of bends Folded dipole length measured tip to tip = 164mm Total rod length =350mm Centre of rod=175mm Distance HI=GF=69mm Distance HA=GE=87mm Distance HB=GD=105mm Distance HC=GC=175mm Gap at HG=3mm Bend diameter BI=DF=23mm If the folded dipole is considered as a flat plane (see ARRL Antenna Handbook) then its resonant frequency is less than the flat plane algorithm's range of 10:1 The cap between the dipole halfs at the feedpoint is not critical but should be less than 5mm.
The required balun is made from RG316 coax cable and calculated with a velocity factor of 0.7.
And finaly the plot of the VSWR measurment to proof the construction:
Material costs <10€.
Feel free to contact me if you have questions about the antenna.