The Super J-Pole antenna for 430 - 450MHz was constructed and used for FM simplex and repeater operation at my previous residence in Melbourne, Victoria from 2002 - 2007

The Super J Pole antenna is a co-linear vertical consisting of a number of half wave length vertical elements separated with half-wave length stubs (Tuning stub) feed with a folded matching stub. The sub doesn't receive or radiate any energy; it is simply a way of picking off the desired feed impedance.

Additional half wave length section can be added with addition of a half wave length tuning section (Tuning stub). The addition of half vertical elements could be added indefinitely, subject to reality of diminishing returns. This arrangement can be applied practically and usefully on bands within the VHF and UHF spectrum. The use of the J pole feed arrangement creates an antenna that is relatively ground independent compared to many other designs.  

DIAGRAM OF THE SUPER J-POLE

Frequency:                       435Mhz

Polarisation:                     Vertical

(A) Main element:            502mm           (0.75 Wavelength x 300/f x k)

(B) Matching stub:           167mm           (0.25 Wavelength x 300/f x k)

(C) Additional section      334mm          (0.50 Wavelength x 300/f x k)

(D) Tuning stub:               334mm          (0.50 Wavelength x 300/f x k)

(E) Feed point:                 Approximately 10 - 30mm from the bottom.

                                          Subject to adjustment for lowest VSWR

The dimensions of the Super J-Pole have been determined as follows:

Length (Mtr) = 300/f x k x el  

f = Operating frequency e.g. 435.5MHz 

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

       (0.97) 

el = Chosen element e.g.  Wavelength main element, wave tuning stubs

        or wave matching  

       stubs,   0.75, 0.50 or 0.25 respectively.

The associated photos are of my successful in service (2001 - 2006) version that covers the entire 70cm (430 - 450 MHz) band.  

Practical construction

Construction is very simple. After determining the various element lengths, mark them out accordingly on a continuous length of copper wire and bend to achieve the antenna shape. The tuning section that separating the additional half wave length elements can be bent around to form a semicircle (see photos). Tape the wire formed antenna to a suitable length of dowel or fibreglass rod for stability. 

After successfully impedance matching the coax feed point with a VSWR meter the antenna can be mounted inside a suitable length of conduit to protect the antenna from the effects of weathering.   

It is recommended that the coax be attached through some sort of feed line choke arrangement to mitigate possible common mode currents in the transmission line. 

 

Adjustable feed point.	Tuning stub	Complete assembly

Note: The above photo of the adjustable feed point shows the coax connection polarity per the original construction as the polarity was viewed then to be unimportant. I have been convinced since that it is crucially important that the  coax connection polarity be the reverse of that shown in the photo, that is the centre of the coax attached to the main element and the shield to the J pole stub.

See: J-Pole Coax Polarity

The above 3D radiation plot of the Super J Pole was modeled using MMANA Antenna Analyser software by JE3HHT, Makoto (Mako) Mori. 

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

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Page last revised 17 September, 2024