antenna for 430MHz to 440MHz . February 2023
Investigating a practical and
easy Yagi antenna construction
technique for VHF and UHF with good performance and also keen to
small 6 element 435MHz Yagi antenna for local operations with an
option to add additional elements for future performance increase.
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 more easily recycled into another
6 element Yagi.
435 MHz, (useful from 431 to 442)
(Plus additional length for mounting).
Gain: 9.0 dBd
from Reflector (mm)
Distance Reflector - Dipole: 145 mm
Table 1 Yagi dimension
Folded dipole dimensions in mm for
435MHz in below table 2. The fold spacing of 77mm for the inside of the element material
and is determined by the
The material is 12mm Aluminium tube and was chosen to suit tube
bending tool, changes to the tube diameter will affect other
dimensions in the below table.
the fold spacing is relatively large when compared with the
wavelength, the folded dipole is a bit more like a loop antenna with
length of the loop closer too a full wavelength and the end to
end being much less than the half wave length normally associated
with a dipole antenna. Photo 1 of
the complete 435MHz
Yagi shows the relatively short length of the folded dipole compared with
the other elements.
75ohm coax cable length is known a Series Section Transformer and
works in series with a 50ohm feed line to the radio. The length of
the Series Section Transformer is 685mm and the determinations
of this length is discussed below.
Table 2 Folded
dipole dimension details. All dimensions are in mm.
Dipole showing the installation of the RG59 75ohm coax. Semi-rigid PVC
agricultural tube and two heat-shink tubes readied to seal the
75ohm coax terminated to spade lugs with the spade lugs drilled with 3mm
holes for pop riveting to the tube.
75ohm coax terminated to spade lugs pop riveting to the tube.
Folded Dipole with heat-shrink x 2 over coax connection and rubber grommet for
RG59 75ohm coax entry.
Series Section Transformer length is determined in the below NanoVNA
Series Section Transformer
coax is a section of coax that has a different characteristic impedance
from the main coax feed and in this instant the the main coax line is
50ohms and the Series Transformer Section is 75ohm RG59 coax.
length is determined by first calibrating the NanoVNA to the end of
the 'Blue' coax in the below set-up and connecting a BNC 'T'
connector with a 50ohm load on one side of the 'T' connector and the
RG59 coax section under test to the other side. The aim is to find a
practical length that is centred on a low SWR for 435MHz.
practical process is to choose length that is over a full wavelength
for 435MHz and cut back bit by bit until the SWR dip centres on
435MHz. The Series Section Transformer resultant length for the RG59
coax cable under test was 685mm.
set up for determining the Series Section Transformer length.
Series Transformer Section
is not required to run through the folded dipole tube and will work
the same if connect and lead away along the boom.
This arrangement was test revealing the same SWR result.
4:1 coax balun was also connected and produced similar SWR results
as the Series Transformer Section method.
The element mounting
assemble shown in Figure 1 and Photo 7 consists of a stainless steel
hose clamp with a 5mm stud hole drilled in the strap and a counter sunk headed set screw mounted with the flat head against the boom
as shown in Photo 7. 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
in Photo 7,8 and 9 is fabricated from 12 x 12mm aluminium channel. The aluminium channel has ‘V’ cut notch to allow
various diameter elements to be attached and a lower notch cut out
so that it mount flat against the boom and clear of the hose clamp strap.
'V' cut needs to be accurately cut to achieve a symmetrical element
Figure 1 Element
to Boom mounting arrangement.
Figure 2 Element
to Boom mounting bracket.
Photo 7 Element to Boom
Photo 8 Element to Boom mounting
Photo 9 Folded
dipole using identical Boom mounting
Figure 3 Modelling
with MMANA antenna modelling application.
Once the antenna is full
assembles the SWR was measured with a short length of 50ohm coax
connected to the antenna's Series
Transformer Section coax with final adjustment made by moving the
antenna's reflector element for the best SWR value.
Photo 9 NanoVNA
SWR sweep from 425MHz to 445MHz.
Chart 1 Gives a
clearer view of the 435MHz Yagi antenna's SWR from 430MHz to 444MHz with
range from 431MHz to 442MHz
Yagi-Uda antenna dimension calculator
on Rothammel / DL6WU