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Antenna No 64

 

J-antenna for 4m

 

I built the following J-antennas for friends of mine.  One antenna is to be used at a holiday QTH and another is to be mounted in an attic for occasional use.  It enables a group of 4m FM users to experiment on the band and stay in touch.

 

The J-antenna is essentially an end-fed vertical dipole with an attached quarter wave matching section that can be positioned vertically as well as horizontally.  The latter is ideally suited for restricted height zones such as an attic.

 

Using the QW stub section it is possible to find a point for best SWR match, say anything between 28 Ohm and 200 Ohm depending on your coax feedline and impedance transformer, if used.  I decided to try out a 50 Ohms balanced match using a 1:1 coax balun.

 

For the outdoor J-antenna a 50 Ohms match was quickly found leading to a SWR of 1.2;  the indoor antenna is a bit more tricky due (unknown) influence of roof / wall / floor materials.  An SWR of 1.6 was obtained on the pre-set markings (from my initial test).  By sliding the balun eyelets further up the QW stub section on location this can be improved on.  A portable network analyser would have been a great help but I did not have one at hand.

 

I used 10x1.5mm aluminium tubing which allows for a strong, self supporting antenna.  The tubing is easily bent by hand using a jig.  For temporary outdoor setup (e.g. holiday QTH) the antenna can be strapped to a fiber pole.

 

 

Dimensions and simulation of the traditional J-antenna.

 

Dimensions of the traditional 4m J-antenna.  Tubing heart distance is 7.0cm.

Simulation of the currents distribution for this antenna.


 

 

Practical construction and testing of the J-antenna.

 

For ease of transportation I made a separate vertical and stub section.

Detail of the QW section with insert to fix the vertical section.

The assembled J-antenna with coax balun.

J-antenna strapped to a fiber mast for testing.

SWR plot obtained during testing.


 

 

Dimensions and simulation of the J-antenna with horizontal QW section.

 

Dimensions of the 4m J-antenna with horizontal quarter wave section, tubing heart distance is 7.0cm.

Simulation of the currents distribution for this antenna.

Simulation of the far field plot.  Note the slight asymmetry caused by the horizontal stub section.

The assembled J-antenna with horizontal QW stub section.

Detail of the QW stub with polyethylene spacers.

The J-antenna being tested on a fiber mast at about 5m agl.


 

 

Coax balun 1:1

 

Principle diagram of the 1:1 coax balun made out of 50 Ohms coax e.g. RG-213, Ecoflex-10.

The 1:1 coax balun (eyelets at the top) and a section of 50 Ohms coax cable for testing.

The coax shields are connected at about 1/4 wavelength down.  The air gap between coax is about 8mm.

The balanced end of the balun with [temporary] sliding straps to find the lowest SWR point.


 

 


 


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