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 8,17 DBi (6 DBd)
on 145 MHZ.
With 6 DBd (6 DBd = 8,15 DBi) 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 is being
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
*********
FIG.4
FIG.5
That's all folks! Have fun
!
Makis
SV1BSX
