3 Elements
VHF Yagi antenna

FIG.1 shows a 3 elements VHF Yagi "homebrew" antenna designed
with YAGIMAX 3 and made by SV1XY and me with excellent results

FIG.1


FIG.2 shows a table for the SWR, GAIN and F/B ratio.

FIG. 2


The maximum forward GAIN is about 8 DBi (5.9 DBd) into 2 m. Band.
With almost 6 DBd gain, we have an Effective Radiation Power 4 times greater of the tranceiver output (without Coaxial-Loss), i.e if you have a VHF transceiver with output Power of 50 Watts, your ERP will be multiplied 4 times = 200 Watts ! (in the forward GAIN direction)
The antenna is compact (78 cm. Boom) with excellent F/B ratio ( 20 DB) .

The Radiation Resistance on center frequancy (145,000 MHZ) its 65 Ohms.
If you connect a 50 Ohms coaxial cable the SWR is 1.3:1

In practice the antenna needs a "matching system" for a 50 Ohms coaxial cable feeder ( H-100, RG-213 or similar) to minimize the SWR ratio to 1:1

I have used a "Hairpin" system (FIG. 3), because is very simple and effective. YAGIMAX antenna design program has a calculation-tool for the Hairpin's dimension, depending on from "data" of the table above (FIG.2)

The Drive Element is an "Open - Dipole" (two pieces of about "Lamda/4")
with overall length 0.92 m. (see FIG.1)

FIG.3 shows the Drive-element (Dipole) and the Hairpin construction on the dipole's plastic box. The space between the two screws is 2.2 cm (dimension B) and the dimension "A" is 5 cm for 1:1 SWR (on my antenna).
If you have not the optimum SWR (1:1) you can increase or decrease the "A" dimension a few millimeters, looking for the minimum SWR.

FIG. 3

The antenna has been constructed on a 15 x 15 mm aluminium boom and I have used for the elements aluminium tubular rods of 8 mm diameter. The plastic box wich I have used for the dipole it was from an old TV-antenna.

Finally, the FIG. 4, 5 shows the polar-plots of 3 Elements VHF Yagi

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FIG.4

FIG.5

Have fun !
Makis SV1BSX