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

Due to space limitations and the need to keep the visual pollution of too many antennae to a minimum I required antennae that are able to cover multiple bands.

A dual band J pole had been in the back of my mind for a number of years, the simplicity of this design and the fact that it was ground independent, that is requiring no ground plan radials made the J pole the choice. The proposed antenna would be operational over the entire 6Mtr band (50 - 54MHz) and the entire 2Mtr band (144 - 148MHz), slightly favouring the upper half of both bands.

The J pole is simply a half wave length vertical element with a folded quarter wave match stub. The sub doesn’t receive or radiate any energy; it is simply a way of picking off a desired feed impedance. An impedance transformer.  

Practical construction

The main element of the antenna is a 5Mtr length of 30mm diameter aluminium tube.

The 6Mtr band stub calculated for a frequency of 52.5MHz is 30 x 1520mm including 150mm for mounting. The stub is mounted 4090mm down from the top of the main element to the top of the stub mounting plate. The 6Mtr band stub mounting plate is 150 x 75mm

The 2Mtr band stub calculated for a frequency of 146.5MHz is 16 x 583mm including 100mm for mounting. The stub is mounted 1448mm down from the top of the main element to the top of the 2Mtr stub mounting plate. The 2Mtr band stub mounting plate is 100 x 30mm

The 6Mtr & 2Mtr band stubs are attached with a fabricated mounting plate using stainless steel hose clamps. This has several advantages. No drilling into the main elements that may weaken the structural integrity of the antenna, Infinite adjustment opportunity and if and when the antenna is no longer require it can be more easily recycled into the next project. Above all, it appears neat and sturdy.

The RG213 coax is installed within the main element and exits above the feed point connection. It is important to protect the coax with rubber grommets 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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