J Dipole driven element of half folded dipole as a substitute for a standard folded dipole on a 70cm Yagi antenna. July 2024

With the goal of producing and easier to construct driven element for a 70cm band Yagi than the more difficult to construct folded dipole the Half Folded Dipole or J Dipole popularised by Kent Britain W5VJB was selected as a more simpler to construct alternative.

This interpretation of Kent’s design is an all Aluminium construction and consistent with my previous construction techniques used for the 5 element 70cm band Yagi that allow for ease of construction, experimentation and optimisation of the antenna.

The half folded element having a J shape which is grounded at the centre of the long section (A) of the element should have an impedance of approximately 150 Ohms in free space on its own, however when loaded down by the Yagi reflector and driven elements can produce close to 50 ohms feed point impedance. The spacing adjustment of the nearby reflector and driven elements can produce a close match to the feed point impedance without loss of antenna gain. Also the length of the C section of the J Dipole can trimmed to achieve resonance and matched SWR.

See Figure 1 and Table 1 for J Dipole dimensions.

Figure 1 J Driven Element 

Table 1 J Driven Element dimensions. 

 Photo 1  Complete 70cm 5 element Yagi with J Driven element installed.  

 Photo 2  Close up the complete 70cm 5 element Yagi with J Driven element.  

 Photo 3  Assembled J Driven element detailing the N Connector attachment.  

Photo 4  Close up of the assembled J Driven element detailing the N Connector attachment. 

Photo 5  Assembled J Driven element detailing the N Connector attachment. 

 Photo 6  Assembled J Driven element detailing the mounting saddle. 

Testing

Once the antenna is full assembles the SWR was measured with a short length of 50ohm coax connected to the antenna with final adjustment made by moving the antenna's reflector element for the best SWR value.

 Photo 7 NanoVNA SWR sweep from 425MHz to 440MHz with a useful range from 431MHz to 439MHz

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 2 below illustrates an example of an antenna gain range test using the popular NanoVNA.

Figure 2  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 -4 ~ +16dB and calibrated to remove the lead characteristics from the measurements and with the reference antenna and set the base line to 0 as per Photo 8 and Figure 2.

  Photo 8 NanoVNA showing the Fig 4 set-up and calibrated for the base line to be zero.

Figure 3  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 8 dB return gain (7.7) for the 6-element Yagi antenna compared to the 435MHz Reference dipole antenna. This gain is defined as 8 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 10 dBi. This gain closely matches the ideal gain predicted for the 6-element Yagi antenna.

  Photo 9 NanoVNA showing the Fig 5 set-up and displaying Yagi antenna's gain compared with the reference dipole antenna.

Conclusion

The Half Folded dipole or J Dipole driven element is clearly a simpler to construct option compared to the standard folded dipole used in the previous 70cm Yagi antenna. See Yagi 6El 436MHz.

J Dipole driven element however based on initial SWR measurements (Photo 7) indicates a reduced bandwidth of 431MHz to 439MHz at an SWR of less than 2.5:1 when compared to the original Folded Dipole driven element. The J Dipole driven element offers much more flexibility to adjust for minimum SWR for a much greater range of frequencies in that the approximate 8 MHz bandwidth can be landed a much broader range of frequencies allowing access with adjustments to the entirety of the 70cm band.

The overall Yagi gain appeared to be slightly less with the J Dipole driven element compared to similar gain test carried out for the same Yagi antenna using the standard dipole driven element. The reduction in gain was recorded as less than 1 dB of gain and is likely with the margin of error for backyard antenna gain measurements.

Video of the installation and testing of the 435MHz Yagi antenna J-Driven element.

References:

Previous 70cm Yagi antenna with standard Folded Dipole driven element: Yagi 6El 436MHz.

Cheap Yagi and matching by Kent Britain WA5VJB:  cheapyagi.pdf (wa5vjb.com) 

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Page last revised 06 October, 2024