6 Element Yagi-Uda 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 construct a 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 antenna.

Photo 1  Complete 435MHz 6 element Yagi.

Antenna details

Frequency:                  435 MHz, (useful from 431 to 442)

Wavelength:                690 mm

Rod Diameter:            10 mm

Boom Diameter:          20 mm

Boom Length:             682 mm (Plus additional length for mounting).

Elements:                   6

Gain:                          9.0 dBd (approximately.)

Table 1 Yagi dimension details.

Folded Dipole  

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 bending tool. 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.  

Because 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.

The 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.

Photo 2  Folded Dipole showing the installation of the RG59 75ohm coax. Semi-rigid PVC agricultural tube and two heat-shink tubes readied to seal the connection. 

Photo 3  RG59 75ohm coax terminated to spade lugs with the spade lugs drilled with 3mm holes for pop riveting to the tube. 

Photo 4  RG59 75ohm coax terminated to spade lugs pop riveting to the tube. 

Photo 5  Completed Folded Dipole with heat-shrink x 2 over coax connection and rubber grommet for RG59 75ohm coax entry.

The Series Section Transformer length is determined in the below NanoVNA set up.

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.

The 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. 

The 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.

Photo 6  NanoVNA set up for determining the Series Section Transformer length.

The 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.

A 4:1 coax balun was also connected and produced similar SWR results as the Series Transformer Section method. 

Construction

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 bracket shown 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.  The 'V' cut needs to be accurately cut to achieve a symmetrical element mounting. 

  Figure 1 Element to Boom mounting arrangement.

  Figure 2 Element to Boom mounting bracket.

  Photo 7 Element to Boom mounting assembly.

  Photo 8 Element to Boom mounting assembly.

  Photo 9 Folded dipole using identical Boom mounting assembly.

Modelling

Figure 3 Modelling with MMANA antenna modelling application.

Testing

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 a useful range from 431MHz to 442MHz

Yagi-Uda antenna dimension calculator

https://www.changpuak.ch/electronics/yagi_uda_antenna.php

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