3 Elements VHF Yagi antenna

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

 FIG 1

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

 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 1.3:1In 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.

 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.

*********

FIG 4

FIG 5

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Coaxial Cable's Specifications 50 Ohm

Loss in DB/100m (Att), Max power in Watts(P), Diameter in mm (D), Velocity factor (VF) :

Lengths (Cm) of   LAMBDA/2 (½ wavelength) lines   and  LAMBDA/4  (¼ wavelength) lines :

 

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2 m Band VHF Collinear with 2 Dipoles

This page is the second part of my VHF Collinear antenna project with matching-system for 2 dipoles model (Gain = 5.15 DBi). 

 

Fig. 1 shows the 2 dipoles with details about the coaxial cables. The space between the two dipoles is 1,5 m (0.75 wave length).

"Z1"(with BLUE colour) is the "RG58" section which I've described ïn the "Gamma match Dipole" page. On this collinear the length of Z1 is 1.36 m, 1 Lamda "electrical" (Lamda/2 x 2), because the Lamda/2 is not enough from Gamma match up to "Z-TR" (L/2 is too short).

The "Z-TR" (GREEN colour) section its a piece of 75 Ohms coaxial cable (RG59) which acts as "Z" transformer. Further details of "Z-TR" shows the FIG.2

"Z-TR" acts by transforming the 50 Ohms impedance (from each dipole) to 100 and then we have again 50 Ohms on the "T" match connection point (2 impedances of 100 Ohms in parallel = 50 Ohms)

The "Z-TR" section is an "electrical quarter wave" (Lamda/4) x velocity factor of 75 Ohms RG59 (= 0.66). The impedance of Z-TR is given by the formula:

Z-TR = SQUARERoot Z1*Z2

where Z1 is the antenna impedance(50 Ohm) and Z2 the request impedance (100 Ohm).
The "quarter-wave" length on 145.000 MHZ its 0.517 m. So, the "electrical" Lamda/4 is:

0.517 x 0.66 = 0.341 m

That means a length of about 34.1 cm of RG59 coaxial cable.

FIG.2 above shows the construction details between RG58 and RG59 coaxial cables. The "A" point must be connected with the 2nd "Z-TR" from the other Dipole and both with the RG213 (cathode coaxial to transceiver). This way, we have made a "T-match" for the 2 dipoles.

FIG.3 shows the contruction of "T-Match".

After the dipoles and coaxial cables placement into the mast, its time for the final tuning. Each dipole is pre-tuned (see the previous page about dipole). With the mast down (in parallel to the roof) and the two dipoles up (looking to the sky), the SWR ratio it must be lower of 1:2 SWR ratio. By moving the two "slide-brackets" simultaneously (for example, that means 5 mm on the upper dipole and also 5mm on the lower dipole), we looking for a low SWR ratio, near to 1:1.
If that is possible, the collinnear its OK. If no, check again carefully the dipoles, the cables etc... something is going wrong.

In case which the antenna has a good SWR we are ready to settle the mast on the final vertical position. Now its time for the final tuning. Keep in mind, the lower dipole it must be 2 m. (1 Lamda) or more above the roof or ground but for higher perfomances 2 or 3 "Lamda" its recommended .

My antenna has SWR 1:1 on 145 MHZ and 1:1,2 on 144 & 146 MHZ (the final tuning was very easy). After the final tuning don't forget to secure the two screws on both slide-brackets with a screw-driver. A plastic-tape is nessecary on the connections betweens coaxial cables for WX protection... additionally a plastic-spray on the slide-brackets, connections etc. helps for an operation without problems for many years.

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