3 Elements VHF Yagi antenna

3 El VHF Yagi Antenna

  FIG.1 shows a 3 elements VHF Yagi "homebrew" antenna designed with YAGIMAX 3.


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

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FIG. 2

The maximum forward GAIN is about 8,17 DBi (6 DBd) on 145 MHZ. With 6 DBd (6 DBd = 8,15 DBi) gain, this antenna offering an Effective Radiation Power 4 times greater of the transceiver 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 enough (78 cm. Boom between Reflector-director) with excellent F/B ratio ( 20 DB) .

The Impedance on center frequency (145,000 MHZ) its 65 Ohms. If you connect a 50 Ohms coaxial cable directly to dipole, 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) in order to minimise the SWR ratio to 1:1

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

The Drive Element is an "Open - Dipole" (two pieces of about "Lambda/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. Keep in mind, by using a Hairpin the dipole must be a bit-shorter than normal, in order to act as a capacitive-element (25.42-j31.62 at 145 MHz). Thus the overall length of dipole is about 92 cm.
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) we can increase (or decrease) the "A" dimension a few millimetres, looking for the minimum SWR.

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FIG. 3

The antenna is being constructed by using 15 x 15 mm aluminium boom and I have used for the elements tubular aluminium rods of 8 mm diameter. If the elements are placed directly to the boom, without a plastic piece for isolation, the dimensions must be 5mm longer (Reflector = 1025 & director = 895). The plastic box which I have used for the dipole it was from an old TV-antenna.

*NOTE: if you place the antenna with Vertical polarization, the "center-cable" of Coaxial must be connected with the upper-section of dipole and the "shield" of Coaxial with the down-part of dipole.

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


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Final View of antenna

The right picture shows my Homebrew 3 Elelements VHF Yagi Antenna

This antenna has been constructed
by SV1XY and me in several copies with excellent results.

The above picture shows the constructed antenna by Jose EB8AUU, which made the 3 Elements VHF Yagi for his father Pepe - EA8BQD.

That's all folks! Have fun !
Makis SV1BSX

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