5 Element Yagi-Uda
antenna for 250MHz to 260MHz . September 2022
Investigating a practical and easily Yagi-Uda antenna construction
technique for VHF and UHF and also keen to have a listen to the
known pirate activity on a United States Navy communication satellite a
small 5 element Yagi-Uda antenna for 255MHz was chosen.
The goal was to develop an element mounting technique that allowed easy
installation and replacement of the elements, allow for easy
adjustment of the element attachment point on the boom while
requiring no drilling of holes into the boom. Also when the antenna
is no longer required it can be easily recycled into another
antenna.
Photo 1 255MHz
Yagi mounted on repurposed tripod.
Construction
The element mounting assemble shown in Figure 1 and Photo
2 & 3 consists of a stainless steel hose clamp with 5mm stud hole drilled
in the strap and counter sunk headed set
screw mounted with the flat head against the boom as shown
in Photo 4. The hose clamp stud
mount requires no holes be drilled into the boom and allows for
infinite lateral adjustment along the boom.
The element mounting bracketshown in Photo 5 is
fabricated from 12 x 12mm aluminium channel pop riveted to the
element mounting plate. The aluminium channel is notched out so that
it mount flat against the boom and clear of the hose clamp strap.
Photo
6 shows the Completed255MHz
Yagi-Uda antenna.
Figure 1Element
to Boom mounting arrangement..
Photo 2 Element to Boom mounting
assembly.
Photo 3 Element to Boom mounting
assembly.
Photo 4 Stainless steelhose clamp and counter
sunk headed set screw.
Photo 5 Element mounting bracket.
Photo 6 CompletedYagi-Uda
antenna assembly.
Antenna
details
Frequency:255 MHz, (useful from 249.9 to 260.1 MHz)
Wavelength:1176 mm
Rod Diameter:10 mm
Boom Diameter:20 mm
Boom Length:855 mm
Elements:5
Gain:7.99 dBd
(Based Yagi antenna calculator)
Ref
Element
Length
(mm)
Position
from
Reflector (mm)
Note
R
Reflector
574
0
D
Dipole
See details
282
Distance Reflector - Dipole: 282 mm
D1
Director 1
541
371
Distance Dipole - Director 1: 88 mm
D2
Director 2
536
582
Distance Director 1 - Director 2: 212 mm
D3
Director 3
532
835
Distance Director 2 - Director 3: 253 mm
Balanced Dipole
The dipole was modelled withYagi
Uda Antenna Calculator and based on 10mm diameter aluminium tube.
The over all reduction factor of 0.958 is a result of the 10mm
dipole element diameter and will be different for other tube
diameters. The reduction factor is applied to the
calculated half wavelength of 255MHz to given overall dipole element
length of 563mm less the gap of 10mm resulting in the A element lengths
of 276.5mm.
The 1:1 transformer was in this
instant four toroidal core over the 50ohm coax near the feed point.
