A 2m 3ele Yagi-Uda Antenna

Original a design de DK7ZB with all elements 10mm in diameter. To save weight I substituted the parasitic 10mm elements with 6mm alu tubes. They have to be prologed to achieve the same datas.

The mechanical datas:

Elements

position

length [mm] free space

half length [mm]

R

0

1015,5

507,75

DE

480

974

487

D1

820

913

456,5

Take a look at the horizontal and vertikal diagramms by AO 6.59 De K6STI.

The electrical datas (at 145 MHz) with AO 6.59 de K6STI

  • Gain: 8,2 dBi

  • F/R: 21.4 dB

  • Impedance: 28 Ohm

  • Azimuth beamwidth: 63°

  • Elevation Beamwidth: 99°

Mechanical construction

Parasitic elements

The material is 6mm aluminium tube. You do a raw cut with the saw and use the file for the last precision. The elements were hold in place by 3 mm screws (stainless steel if posssible) in the center using the same polyamid holders as the 1.6l 2m beam.

Driven element

The driven element just consists of a straight split dipole made of 10mm aluminium tube. To match the 28 Ohm impedance to the 50 Ohm cable a lambda/4 cable balun is used . The balun consists of 2 x 75 Ohm cable in parallel. Use the best cable you can find and avoid RG-59 or similar. I choosed TVRO sat cable as recommended by the antenna designer. Another possibility may be RG-179 a 75 Ohm teflon cable but it has only a diameter of 3mm. No problem with low power sat operation.

Actually the RG-179 teflon cable can be used up to 300W PEP in SSB (2m) without problems.

Testing the cable balun

To avoid a lot of frustration you should test the cable balun before final installation. Terminate one end of the balun with a pair of 56 Om/2W metaloxid resistors andf measure the VSWR. You should have nearly 1:1.0. Especially if you use TVRO sat cable the factor you have to short the cable is sometimes unknown or uncertain. If you achive a good VSWR the only tuning that must be done is the length of the driven element, the split dipole. That is very easy compared to a folded dipole.