Apr 182010
 

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.

Feedpoint detail:

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.

  2 Responses to “DIY Yagi for 868MHz”

  1. Hello. Is it possible to feed this antenna via 50 ohm cable? how to match yagi’s separated dipole (~28 Ohm) to 50? Help)

    • You can feed this antenna through a 50 Ohm cable, the dipole is already matched for 50 Ohm.
      Matching is done by a 4:1 Balun, see pictures.

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