5 element omnidirectional horizontally polarized (Cloverleaf or Big Wheel) 435mhz antenna . June 2024

This is the second version of constructing a 5 element horizontally polarised omnidirectional (HPOD) antenna also known as a Clover Leaf antenna or sometimes referred to as a Big Wheeled antenna. The first version was instigated as I had been requested to construct a 70cm antenna suitable for a new beacon in the city of Kalgoorlie VK6RTU and after searching the internet I hound a paper called Omnidirectional Horizontally Polarized Uhf Antenna Design where various Clover leaf type antennas were modelled and constructed to determine the most uniform omnidirectional radiation pattern. The conclusion was that a 5 Petal Clover Leaf antenna as noted on page 99 was after extensive consideration of antenna geometries and optimisation of design parameters, produced a prototype 5-petal “big wheel” or “Clover Leaf” antenna with each petal length of l = 0.9λ and parallel circular plates central configuration. According to all of the simulations, the 5-petals is the minimum number of petals that can produce the omnidirectional pattern required’.

The version produced for the VK6RTU beacon as shown below differs in a few way, however follows the basic idea with 5 radiating petals and produced all of the characteristics of the papers conclusions.

Photo 1 The new version of the Horizontally Polarised Omnidirectional (HPOD) antenna also known as a Clover Leaf antenna.

Photo 2 The original version of the Clover Leaf antenna as installed at the VK6RTU beacon at Kalgoorlie.

The current version of the antenna is based on all dimensions determined for the first version and is primarily interested in an achieving superior physical construction techniques and carry out more detail performance measurements. The antenna will be installed and reviewed for satellite and ISS communication. 

Antenna details

Frequency:                  420 to 450MHz

Wavelength:                690 mm

VSWR:                       Better than 2:1 between 425MHz and 445MHz

Polarization:                Horizontal

Gain:                          Neutral gain compared to a dipole at 0 dBd (approximately).

                                  Less than -17dBd compared to a cross polarized dipole

Construction

 
The antenna is constructed around a standard male N connector with a 100mm diameter aluminium radial mounting disk with a 16mm centre hole to attach to the N connector with the standard coax gland screw cap as shown in Photos 5 and 6. The elements have been attached with 3mm diameter pop-rivets to the mounting disks.

Figure 1 5 Petal Cloverleaf general layout.

Figure 2 Cloverleaf element dimensions.

Figure 3 Cloverleaf element dimensions table.

Figure 4 Cloverleaf antenna hub plates dimensions.

Photo 3. Shows petal element bending jig.

 

Photo 4. Shows completed petal element.

Photo 5. Shows 5mm Stainless Steel stud and N connector pin assembly

Photo 6. Main attachment hub

Photo 7. Shows petal element hub attachment with 3mm pop rivits.

The smaller upper disk is fitted to a standard male N connector's coax centre pin with a 6mm diameter stainless steel stud and positioned in the N connector and secured with two part epoxy. A small rubber grommet has been fashioned to fit tightly in the end of the N connector body with a smearing of marine grade silicon to make to assembly water proof.

 
The antenna mounting is a standard antenna mirror mount bracket with a female to female N connector bulkhead socket fitted for a range of similar antennas that are constructed around male N connectors. See: Generic Antenna Mount.

Antenna testing

Testing was completed using a NanoVNA for all antenna range measurements including both SWR and antenna gain performance.

The SWR measurements showed a very broadband match with an SWR of less than 3:1 from approximately 410MHz to 490MHz and very good match at the target frequency of 438MHz.

The Smith Chart display indicates that the antenna is resonant at 459MHz. This result is consistent with the antenna in the paper and the Mk 1 version of the Clover Leaf antenna.

Photo 10. NanoVNA display of an SWR of less than 3:1 from approximately 410MHz to 490MHz.

Photo 11. NanoVNA display of an SWR of less than 2:1 from approximately 430MHz to 445MHz and very good match at the target frequency of 438MHz of 1.33:1.

Photo 12. Shows the Smith Chart display that indicates the antenna’s resonant frequency is around 458MHz.

Antenna Gain Range Testing

This is the most important antenna measurement because even if all other measurements such as SWR and resonance are satisfactory, however 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 NanoVNA.

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

Figure 4 shows the basic set up with d indicating 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 distances 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.

Source Antenna is a 435MHz Source dipole antenna.

Reference Antenna is also a 435MHz Reference dipole antenna. A measurement will be taken with this antenna to determine the base line. This antenna is replaced with the Clover Leaf 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 -2.5 ~ +2.5dB 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 12 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.

Target performance.

It is generally expected that a properly constructed Clover Leaf antenna can typically exhibit an approximately neutral gain to slightly positive against a dipole.   

Test Results.

The test results recorded a slight return gain (0.86dBd) for the Clover Leaf antenna compared to the 435MHz Reference dipole antenna. This gain is defined as neutral dBd for the Clover Leaf antenna. 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 2.15 dBi. This gain closely matches the expected gain results for this antenna.

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

The source dipole was then rotated to a vertical position to measure cross polarized gain with a more or less expected measurements indicate negative gain of – 17 to 18dBd.

Photo 12 NanoVNA showing the Fig 5 set-up and displaying Clover Leaf antenna's gain compared with the reference dipole antenna with the source antenna in the vertical cross polarized position. Result measurements indicate negative gain of –17 to 18dBd.

Conclusion.

The Mk 2 Clover Leaf antenna, designed for the 70cm band (420 to 450 MHz), is a horizontally polarized antenna suitable for SSB and beacon use. It has a neutral gain of 0 dBd compared to a dipole of the same frequency and polarization. This antenna is very broadband and, although complex to construct, produces consistent and replicable performance results.

Photo 13 The Horizontally Polarised Omnidirectional (HPOD) antenna also known as a Clover Leaf antenna installed.

Video of the construction and testing of the 435MHz Clover Leaf Antenna.

Omnidirectional Horizontally Polarized Uhf Antenna Design UHFOmniDesignMQP.pdf

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Page last revised 06 October, 2024