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Antenna Scatterers Analysis Program

Examples and Successful Use

J-Pole, Preprocessors

and Feedlines 2


J-Pole Using Preprocessor

The J-pole antenna will be used as an example of both a case where the presence of a feedline does make a significant pattern difference as well as an example of how a simple geometry processor might be used. The model of a simple J-pole antenna is shown below.

Basic J-pole

The J part of the antenna consists of 3 straight segments that must be divided into sections for analysis. Although this could be done manually for this problem, there are many types of problems that are not this simple. The example preprocessor "preasap" (available in the download area) will be used in this case. The instructions for how to set up the preprocessor input file are contained in the 'C' source code. Basically, the input file consists of a list of continuous wires, each wire defined by a set of points (that don't have to be in a straight line). Preasap will break each wire into segments small enough to analyze.

For the purposes of the example, 300 MHz will be used so that the geometry of the J-pole in wavelengths will match the dimensions in meters. The input file to "preasap" for this case is shown below.


1.0
 1

4 0

  0 0 0.75
  0 0 0
  0.05 0 0
  0.05 0 0.25

2 0

  0 0 -0.1
  0 0 -0.25

0 0 

The main 'J' section is the list of four points:

  0 0 0.75
  0 0 0
  0.05 0 0
  0.05 0 0.25

While another short wire "floats" below the main radiating element to become the feedline or support.

   0 0 -0.1
  0 0 -0.25

Since a ground plane is not going to be used for this problem, the geometry can extend below z=0. Note that preasap can generate individual wires that are not straight but that it can't join two independent wires (that is a programming job for another day). Therefore a gap is left between the 'J' element and the floating wire so that it can be joined "manually" later. When preasap is run it produces the following output file.

GXYZ
0  0  0.75	  / 	w1 s1 
0  0  0.375	  / 	w1 s2 
0  0  0	  / 	w1 s3 
0.025  0  0	  / 	w1 s4 
0.05  0  0	  / 	w1 s5 
0.05  0  0.125	  / 	w1 s6 
0.05  0  0.25	  / 	w1 s7 
0  0  -0.1	  / 	w2 s8 
0  0  -0.175	  / 	w2 s9 
0  0  -0.25	  / 	w2 s10 
XXXX
DNODE
  1 2  / wire: 1
2 3  / wire: 1
3 4  / wire: 1
4 5  / wire: 1
5 6  / wire: 1
6 7  / wire: 1
  8 9  / wire: 2
9 10  / wire: 2
XXXX

The preprocessor output uses the GXYZ and DNODE forms of the geometry description so that comments can be added identifying the wires and segments to make the job of joining the segments easier. The necessary extra information is added to the ASAP input file:

C      A J-POLE INVESTIGATION
C       OF FEEDLINE AND SUPPORT EFFECTS
C
C      FIRST INVESTIGATE BARE J-POLE WITH
C       NO FEEDLINE
WIRE(RADIUS=0.001)
GXYZ
0  0  0.75	  / 	w1 s1 
0  0  0.375	  / 	w1 s2 
0  0  0	  / 	w1 s3 
0.025  0  0	  / 	w1 s4   -- this is the center at bottom of U
0.05  0  0	  / 	w1 s5 
0.05  0  0.125	  / 	w1 s6 
0.05  0  0.25	  / 	w1 s7 
XXXX
DNODE
  1 2  / wire: 1
2 3  / wire: 1
3 4  / wire: 1
4 5  / wire: 1
5 6  / wire: 1
6 7  / wire: 1
XXXX
FEED(4)
PLOT(FARF/PHI=0)
OUTPUT(FARF=0,0,0,360)
CHANGE
PLOT(FARF/THETA=90)
END
C      JOIN FEEDLINE WIRE TO BOTTOM OF J SECTION
C        quarterwave long
C       
WIRE(RADIUS=0.001)
GXYZ
0  0  0.75	  / 	w1 s1 
0  0  0.375	  / 	w1 s2 
0  0  0	  / 	w1 s3         -- bottom of main element
0.025  0  0	  / 	w1 s4 
0.05  0  0	  / 	w1 s5 
0.05  0  0.125	  / 	w1 s6 
0.05  0  0.25	  / 	w1 s7 
0  0  -0.1	  / 	w2 s8 -- top end of feed/support
0  0  -0.175	  / 	w2 s9 
0  0  -0.25	  / 	w2 s10 
XXXX
DNODE
  1 2  / wire: 1
2 3  / wire: 1
3 4  / wire: 1
4 5  / wire: 1
5 6  / wire: 1
6 7  / wire: 1
  8 9  / wire: 2
9 10  / wire: 2
   3 8   /   join the main element to the feed/support
XXXX
FEED(4)
PLOT(FARF/PHI=0)
OUTPUT(FARF=0,0,0,360)
END
C      JOIN FEEDLINE WIRE TO BOTTOM OF J SECTION 
C         halfwave long
C       
WIRE(RADIUS=0.001)
GXYZ
0  0  0.75	  / 	w1 s1 
0  0  0.375	  / 	w1 s2 
0  0  0	  / 	w1 s3         -- bottom of main element
0.025  0  0	  / 	w1 s4 
0.05  0  0	  / 	w1 s5 
0.05  0  0.125	  / 	w1 s6 
0.05  0  0.25	  / 	w1 s7 
0  0  -0.1	  / 	w2 s8 -- top end of feed/support
0  0  -0.25	  / 	w2 s9 
0  0  -0.5	  / 	w2 s10 --  lengthen the feed to halfwave
XXXX
DNODE
  1 2  / wire: 1
2 3  / wire: 1
3 4  / wire: 1
4 5  / wire: 1
5 6  / wire: 1
6 7  / wire: 1
  8 9  / wire: 2
9 10  / wire: 2
   3 8   /   join the main element to the feed/support
XXXX
FEED(4)
PLOT(FARF/PHI=0)
OUTPUT(FARF=0,0,0,360)
STOP

The feedpoint is normally across the quarterwave feed stub at the bottom. The precise position used is dependent on matching to the feedline. In the case of this example, the feed point impedance is not of concern so the feedpoint will be chosen for convenience. This point will be in the center of the bottom part of the 'U' section. It will be shown later that this choice of feedpoint does make a difference in the antenna's radiation pattern. The bottom section "support" or "feedline" will be modeled as a wire.

For the first part of the input file the floating segment was deleted to make the case for no feedline while the second part of the file joins the floating segment. The third section increases the length of the feedline to a halfwave. The following plot shows a pattern cut for the plane the 'J' lies in for each of the three feedline cases.

Plot of J-Pole Patterns for various feedline length

The above plot was done by importing the data into MathCad (the angle has been rotated to orient the pattern to be conceptually equivalent to the geometry).

It is interesting to note that the case with no feedline is also distorted. This is because, although it is not often recognized in the literature, that the J-pole is really a 3/4 wave antenna with a quarterwave matching stub. Adding feedline (or conductive support) makes the antenna longer. The feedpoint location on the structure also makes a difference in the pattern. Several test runs for j-poles was made and has been summarized on another page.

The line printer 'plot' output from the ASAP output file for the J-pole case without feedline is shown below as an example of the fact that the line print plot is useful for quick looks at the pattern.

Note that the pattern is rotated on the page compared to the plot shown above.


                                .                      . . . . . . .                      .                                        
                                 .                .          .          .                .                                         
                                  .          .               .            * *.          .                                          
                                 120      .                  .       ******  *****     060                                         
                                   .   .                     .    ***             ***  .                                           
                                    ..                       .  ***                 **.                                            
                                   ..                        .**                      *.                                           
                                 .   .                       **                      . **.                                         
                               .      .                 . . *.. . .                 .   ** .                                       
                             .         .           .       * .         .           .     *   .                                     
                            .          .       .          *  .             .       .      *   .                                    
                           .            .   .            *   .                .   .       *    .                                   
                         .               ..             **   .                  ..         *     .                                 
                        .               . .             *    .                  . .        *      .                                
                       .              .   .            *     .                  .   .       *      .                               
                      .             .      .          *      .                 .      .     *       .                              
                     .             .        .         *  . ..... .            .        .    *        .                             
           .        .            .          .       .*       .        .       .          .  *         .        .                   
             .     .            .            .   .   *       .           .   .            . *          .     .                     
            150 . .            .              .      *       .              .              .*           . . 030                    
                  .           .             .  .    *        .             .  .             *           .                          
                 .  .        .             .   .    *        .             .   .            *.        .  .                         
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               .          .  .        .           .*  .      .      .   .           .      * .  .          .                       
              .          .     .     .             *         .         .             .    **     .          .                      
              .         .        .  .            . *.        .        .  .            .  .*       .         .                      
             .          .          .            .  *.        .        .   .            .  *       .          .                     
             .         .          .  .        .    **.       .       .      .        .  .*         .         .                     
             .         .          .    .     .      * .      .      .        .     .    *          .         .                     
            .          .         .        . .       *  .     .     .          . .      * .         .          .                    
            .         .          .          .       *  .   . . .   .          .       ** .          .         .                    
            .         .         .          .  .     **  .    .    .         .  .     *    .         .         .                    
            .         .         .         .     .    *.  .   .   .  .     .     .   *     .         .         .                    
           .         .          .         .       .  **  .   .   .   .  .       . **      .          .         .                   
           .         .         .          .         . **  .  .  .     .         .**        .         .         .                   
           .         .         .         .          .  **  . . .   .  .        **.         .         .         .                   
           .         .         .         .         .    **   .   .     .     **  .         .         .         .                   
           .         .         .         .         .  ****** . .       .  ***    .         .         .         .                   
       180 1........4/5.......3/5.......2/5......*****.....**+.....*****5*......2/5.......3/5.......4/5........1 000               
           .         .         .         .     *** .       . . .***    .         .         .         .         .                   
           .         .         .         .   **    .     .   .   . **  .         .         .         .         .                   
           .         .         .         .  **      .  .   . . .   . **          .         .         .         .                   
           .         .         .          **        .     .  .  .     ***       .          .         .         .                   
           .         .          .        **       .  .   .   .   .   .  **      .         .          .         .                   
            .         .         .        *.     .     .  .   .   .  .    **     .         .         .         .                    
            .         .         .       *  .  .         .    .    .       **.  .          .         .         .                    
            .         .          .     **   .          .   . . .   .       *  .          .          .         .                    
            .          .         .     *  . .          .     .     .        * . .        .         .          .                    
             .         .          .   **     .        .      .      .       *.     .    .          .         .                     
             .         .          .  .*       .      .       .       .      .*       .  .          .         .                     
             .          .          .  *         .   .        .        .   .  *         .          .          .                     
              .         .        .  . *          .  .        .        .  .   *        .  .        .         .                      
              .          .     .     .*            .         .         .     *       .     .     .          .                      
               .          .  .        *           .   .      .      .   .    *      .        .  .          .                       
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                .       .  .          **.        .           .           .   *    .            .  .       .                        
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                 .  .        .         **  .   .             .             .*  .             .        .  .                         
                  .           .         **  .  .             .             .* .             .           .                          
            210 . .            .         **   .              .             *.              .            . . 330                    
             .     .            .         ** .   .           .           **  .            .            .     .                     
           .        .            .          **      .        .        .**     .          .            .        .                   
                     .             .        . ****       . ..... .  ****      .        .             .                             
                      .             .      .      *******************          .      .             .                              
                       .              .   .                  .                  .   .              .                               
                        .               . .                  .                  . .               .                                
                         .               ..                  .                  ..               .                                 
                           .            .   .                .                .   .            .                                   
                            .          .       .             .             .       .          .                                    
                             .         .           .         .         .           .         .                                     
                               .      .                 . . ... . .                 .      .                                       
                                 .   .                       .                       .   .                                         
                                   ..                        .                        ..                                           
                                    ..                       .                       ..                                            
                                   .   .                     .                     .   .                                           
                                 240      .                  .                  .      300                                         
                                  .          .               .               .          .                                          
                                 .                .          .          .                .                                         
                                .                      . . . . . . .                      .                                        

For more information about the effects of the support/feedline and feedpoint on the pattern of a J-pole antenna the results of a short study have been summarized on another page.


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Last modified on: 3 Nov 2007