Peter Miles
Northam, Western Australia QRZ Page: VK6YSF
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255MHz
5 ELEMENT YAGI-UDA ANTENNA
5 Element Yagi-Uda
antenna for 250MHz to 260MHz . September 2022
In the pursuit of a practical and
user-friendly Yagi-Uda antenna construction method suitable for VHF
and UHF applications, as well as the intention to monitor pirate
activity on a United States Navy communication satellite, a decision
was made to construct a small 5 element Yagi-Uda antenna tuned to
255MHz.
The primary objective was to devise an element mounting technique
that facilitated effortless installation and replacement of the
elements, while enabling convenient adjustment of the element
attachment point on the boom, all without the need for drilling
holes into the boom. Furthermore, a key consideration was to ensure
that the antenna could be easily repurposed or recycled once it was
no longer needed.
By adopting these criteria, the aim was to create an antenna design
that provided flexibility, ease of construction, and high
performance.
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 1
Element
to Boom mounting arrangement..
Photo 2 Element to Boom mounting
assembly.
Photo 3 Element to Boom mounting
assembly.
Photo 4 Stainless steel
hose clamp and counter
sunk headed set screw.
Photo 5 Element mounting bracket.
Photo
6CompletedYagi-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 using the Yagi
Uda Antenna Calculator and based on a 10mm diameter aluminium tube. The overall reduction factor of 0.958 is a result of the 10mm dipole element diameter and will vary for other tube diameters. This reduction factor is applied to the calculated half-wavelength of 255MHz to give an overall dipole element length of 563mm, minus the 10mm gap, resulting in the A element length of 276.5mm.
The 1:1 transformer was in this
instant four toroidal core over the 50ohm coax near the feed point.
