6 Element Yagi-Uda antenna for 430MHz to 440MHz . February 2023

The primary objective was to explore a practical and straightforward construction technique for VHF and UHF Yagi antennas, ensuring satisfactory performance. Additionally, there was a particular interest in building a compact 6-element Yagi antenna for local operations, while keeping the option open to incorporate additional elements for potential future performance enhancements.

To achieve these goals, a key focus was placed on developing an element mounting technique that facilitated easy installation and replacement of the elements. It was also essential to enable effortless adjustment of the element attachment point on the boom without the need to drill holes into the boom itself. Moreover, careful consideration was given to designing the antenna in a manner that would facilitate its recycling into another antenna when it is no longer required.

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 dBd (approximately).

Table 1 Yagi dimension details.

Folded Dipole  

The dimensions of the folded dipole for 435MHz are detailed in Table 2, denoted in millimetres. The fold spacing, set at 77mm, was determined based on the bending tool used for the internal part of the element material. In this instance, a 12mm Aluminium tube was chosen to align with the tube bending tool. It should be noted that modifying the tube diameter will have repercussions on other dimensions specified in the table.

 
Because the fold spacing is relatively large in comparison to the wavelength, the folded dipole exhibits characteristics that resemble a loop antenna. The length of the loop is closer to a full wavelength, while the overall end-to-end length is significantly shorter than the conventional half wavelength commonly associated with a dipole antenna. Photo 1 presents a comprehensive view of the complete 435MHz Yagi, showcasing the relatively compact size of the folded dipole in comparison to the other elements.

 
To achieve the desired impedance matching, a 75ohm coaxial cable is employed as a Series Section Transformer in series with a 50ohm feed line connected to the radio. The length of the Series Section Transformer is determined to be 685mm, with a detailed explanation of the reasoning behind this specific length provided in subsequent text.

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.

The Series Section Transformer coax is employed as a segment of coaxial cable with a characteristic impedance that differs from the main coaxial feed. In this case, the main coax line has a characteristic impedance of 50ohms, while the Series Section Transformer utilizes a 75ohm RG59 coaxial cable.

Determining the appropriate length of the Series Section Transformer involves initial calibration of the NanoVNA to the end of the 'Blue' coax in the provided set-up. A BNC 'T' connector is then connected, with a 50ohm load attached to one side of the 'T' connector, and the RG59 coaxial cable section being tested connected to the other side. The objective is to achieve a practical length that results in a low standing wave ratio (SWR) centred around 435MHz.

The practical approach entails selecting a length that exceeds a full wavelength for 435MHz and progressively trimming it down until the SWR dip aligns with the frequency of 435MHz. The determined length for the Series Section Transformer utilizing the RG59 coaxial cable under test was found to be 685mm.

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

If 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 assembly shown in Figure 1 and Photo 7 consists of a stainless steel hose clamp with a 5mm stud hole drilled in the strap. A countersunk-headed set screw is then mounted with the flat head against the boom, as depicted in Photo 7. The hose clamp stud mount eliminates the need for drilling any holes into the boom and allows for infinite lateral adjustment along the boom.

 
The element mounting bracket, as seen in Photo 7, 8, and 9, is made of 12 x 12mm aluminium channel. This aluminium channel features a 'V'-cut notch that facilitates the attachment of elements of various diameters. Additionally, a lower notch is cut out to ensure the bracket mounts flat against the boom and remains clear of the hose clamp strap. It is crucial to accurately cut the 'V' notch to achieve 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

Antenna Gain Range Testing

This is the most important antenna measurement because even if all other measurements such as SWR and resonance are satisfactory, if the antenna fails to achieve at least an approximation of the desired or predicted gain, it can be considered a failure. Measuring antenna gain is perhaps one of the most challenging tasks to accomplish successfully, as it requires a large and unobstructed area, especially free from metallic obstacles that can significantly distort the antenna's ideal radiation pattern. Figure 4 below illustrates an example of an antenna gain range test using the popular NanoVNA.

Figure 4  Shows the basic antenna gain range test set-up.

Source Antenna is the 435MHz Source dipole antenna.

Reference Antenna is the 435MHz Reference dipole antenna. A measurement will be taken with this antenna to determine the base line. This antenna is replaced with the Yagi antenna and the return loss measured that will show the gain in dB with respect to the Reference dipole antenna. 

NanoVNA set to LOGMAG with a display of typically -2 ~ +18dB and calibrated to remove the lead characteristics from the measurements and with the reference antenna and set the base line to 0 as per Fig 4.

Photo 10 NanoVNA showing the Fig 4 set-up and calibrated for the base line to be zero.

Figure 5  Shows the basic antenna gain range test set-up with the antenna under test in place.

d Is the distance between the Source dipole antenna and the Reference dipole antenna and while not critical needs to be between 2 and 3 wavelengths apart. In the set up the two antennas were placed 2mtr (approximately 3 wavelengths) apart.

The ideal separation for antenna gain testing depends on various factors such as the frequency of operation, the type of antennas being tested, and the testing environment. Generally, a separation of at least 2-3 wavelengths is recommended between the transmitting and receiving antennas to minimize interference and achieve accurate measurements. larger antenna separations can give false readings due to ground reflection and other multi-path effects. 

The distance (d) between the source distance to the antenna under test ( 435MHz Yagi antenna) is taken from the source dipole to the Yagi's driven element.

The suitable height above ground for antenna gain testing depends on various factors, including the type of antenna, the desired testing accuracy, the operating frequency, and the testing environment. As a general guideline, a height of at least 1 to 2 wavelengths above ground is recommended to minimize ground effects and reflections.

The antennas in this set-up are positioned 1.5 meters above the ground, which is slightly over 2 wavelengths at 435 MHz.

Measurements with horizontal polarization are less affected by ground bounce and can provide more accurate and consistent gain values. Horizontal polarization also helps simulate more ideal free-space conditions, which is important for accurate gain characterization.

Target performance.

A general guideline is that a well-designed and properly constructed 6-element Yagi antenna can typically provide a gain of around 8 to 10 dBd (decibels over dipole) or approximately 10 to 12 dBi (decibels over isotropic).

Test Results.

The test results recorded a 10 dB return gain (9.98) for the 6-element Yagi antenna compared to the 435MHz Reference dipole antenna. This gain is defined as 10 dBd for the 6-element Yagi. Considering that a dipole antenna in free space has a gain of 2.15 dB over the theoretical isotropic antenna, the 6-element Yagi demonstrates an approximate gain of 12 dBi. This gain closely matches the ideal gain predicted for the 6-element Yagi antenna.

Photo 11 NanoVNA showing the Fig 5 set-up and displaying Yagi antenna's gain compared with the reference dipole antenna.

Yagi-Uda antenna dimension calculator

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JavaScript Version 12.01.2014, based on Rothammel / DL6WU

Video of the design, modelling, construction, and testing of a 435MHz Yagi Antenna.

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