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 1.3:1
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.
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
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.
(http://es.geocities.com/eb8auu/ea8bqd/index.htm)
That's all folks! Have fun !
Makis
SV1BSX
