The Dual Band J-pole antenna (6Mtr  and  2Mtr bands) constructed and used at my previous residence in Melbourne, Victoria from 2001 - 2007

Considering the limited space available and the objective of minimizing visual clutter caused by multiple antennas, the need arose for antennas capable of covering multiple bands. After contemplating various options, the idea of utilizing a dual-band J pole antenna emerged as a viable solution. The simplicity of the J pole design and its ground independence, which eliminates the need for ground plane radials, made it an appealing choice.

The intended antenna design would provide coverage across the entire 6-meter band (50 - 54MHz) and the entire 2-meter band (144 - 148MHz), with a slight bias towards the upper halves of both bands.

The J pole antenna consists of a vertical element measuring half the wavelength, accompanied by a folded quarter-wave match stub. It's important to note that the stub itself does not transmit or receive any energy; rather, it acts as an impedance transformer, allowing the desired feed impedance to be extracted.

By employing this design, the objective is to achieve a compact, efficient, and multi-band antenna system while maintaining an aesthetically pleasing appearance and minimizing the impact on the surrounding environment.

Practical construction

The primary component of the antenna is a 5-meter long aluminium tube with a diameter of 30mm.

For the 6-meter band, the calculated stub length for a frequency of 52.5MHz is 30 x 1520mm, including 150mm for mounting. The stub is positioned 4090mm from the top of the main element to the top of the stub mounting plate. The mounting plate for the 6-meter band stub measures 150 x 75mm.

The 2-meter band, the calculated stub length for a frequency of 146.5MHz is 16 x 583mm, including 100mm for mounting. The stub is mounted 1448mm below the top of the main element, up to the top of the mounting plate for the 2-meter band stub, which measures 100 x 30mm.

To attach the 6-meter and 2-meter band stubs, a fabricated mounting plate is used along with stainless steel hose clamps. This approach offers several advantages, including no need to drill holes in the main elements, ensuring the structural integrity of the antenna. It also allows for infinite adjustment opportunities and facilitates easier recycling of the antenna for future projects. Additionally, the installation appears neat and robust.

The RG213 coaxial cable is routed inside the main element and exits above the feed point connection. It is essential to protect the coaxial cable by using rubber grommets at the points where it exits the tube.

FIG 1

DIAGRAM OF THE DUAL BAND J-POLE

Frequency:                       50 - 54Mhz and 144 - 148Mhz

Polarisation:                     Vertical

(A) Main 6M element:       4090mm + mounting material

(B) 6M stub:                    1370mm + mounting material

(C) Main 2M element:       1448mm

(D) 2M stub:                    483mm + mounting material

(E) Feed point for 6M:      Approximately 90mm. Subject to adjustment 

                                      for lowest VSWR

(F) 6M stub spacing:        75mm

(G) Feed point for 2M:      Approximately 40mm. Subject to adjustment 

                                      for lowest VSWR

(H) 2M stub spacing:        30mm

The dimension of the J pole may also be arrived at by using the below formula:

Length (Mtr) = 300/f x k x el + mounting section

f =   Operating frequency e.g. 52.5MHz

k =  The effect of the diameter of the material in relation to the wave

        length (0.955)

el = Chosen element e.g. 3/4 wave length main element, 1/2 wave tuning

        stubs or 1/4 wave matching stubs, 0.75, 0.50 Or 0.25 respectively.

mounting section = The amount of extra physical material required for mounting.

The above radiation plots were produced using MMANA Antenna Analyser software by JE3HHT, Makoto (Mako) Mori. 

References

The ARRL Antenna Book.

The 1990 ARRL Hand Book.

For more information on the J-Pole see: http://en.wikipedia.org/wiki/J-pole

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