Peter Miles
Northam, Western Australia QRZ Page: VK6YSF
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J
DRIVEN ELEMENT FOR A 435MHz
(70cm BAND) YAGI-UDA ANTENNA
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 1J
Driven Element
MHz
A
mm
B
mm
C
mm
D
mm
Total
Element Length mm
430
637
318
159
20
to 30
478
435
629
315
157
20
to 30
472
440
622
311
156
20
to 30
467
Table 1 J Driven Element
dimensions.
Photo 1 Complete
70cm 5 element Yagi with J Driven element installed.
Photo
2 Close up thecomplete
70cm 5 element Yagi with
J Driven element.
Photo
3 Assembled
J Driven element detailing the N Connector attachment.
Photo 4 Close up of theassembled
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.
Ageneral 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.