Triax FM 8S

The Triax FM 8S is an eight-element Yagi for the FM broadcast band made in Europe. It has two reflectors and a 2.31 m (91″) boom.

I modeled the antenna with the AO 9.50 Antenna Optimizer program. The red dot marks the 300Ω feedpoint. The very close spacing of the first director raises the feedpoint impedance.

Marko Weck uses this dual-polarization setup in Kangasala, Finland.

Modeling Results

Calculated performance is for 30 analysis segments per conductor halfwave. Forward gain includes mismatch and conductor losses. F/R is the ratio of forward power to that of the worst backlobe in the rear half-plane.

Frequency  Impedance    SWR   Mismatch  Conductor   Forward    F/R 
   MHz        ohms             Loss dB   Loss dB   Gain dBd     dB 
    88     94.3+j26.2   3.21     1.40      0.02      5.33      9.80
    89      122+j59     2.57     0.93      0.01      5.91     11.70
    90      156+j87     2.14     0.61      0.01      6.29     13.57
    91      193+j107    1.85     0.41      0.01      6.55     15.43
    92      234+j118    1.66     0.27      0.01      6.73     17.31
    93      273+j119    1.53     0.19      0.01      6.86     19.33
    94      308+j110    1.43     0.14      0.01      6.98     21.51
    95      336+j95     1.38     0.11      0.01      7.09     24.10
    96      356+j79     1.34     0.09      0.01      7.20     23.78
    97      371+j64     1.33     0.09      0.01      7.32     23.67
    98      384+j50     1.33     0.09      0.01      7.45     23.80
    99      398+j35     1.35     0.10      0.01      7.58     24.11
   100      416+j15     1.39     0.12      0.01      7.71     24.67
   101      435-j22     1.46     0.15      0.02      7.82     21.65
   102      439-j82     1.55     0.21      0.02      7.92     19.11
   103      397-j157    1.70     0.30      0.02      7.96     17.10
   104      305-j201    1.92     0.46      0.03      7.90     15.49
   105      201-j187    2.31     0.74      0.04      7.66     14.17
   106      125-j132    2.93     1.20      0.06      7.18     13.30
   107     80.3-j65.8   3.93     1.89      0.08      6.37     12.86
   108     54.2-j3.2    5.54     2.86      0.11      5.14     12.87

Antenna File

Triax FM-8S
Free Space Symmetric
98 MHz
18 6063-T832 wires, meters
1  0     -0.835 0.34  0      0.835 0.34  .01
1  0     -0.835 -.34  0      0.835 -.34  .01
1  0.38  -.7225 -.03  0.38   .7225 -.03  .01
1  0.38   .27    .03  0.38   .7225  .03  .01
1  0.38  -.7425 -.01  0.38  -.7425  .01  .01
1  0.38   .7425 -.01  0.38   .7425  .01  .01
1  0.38  -.7425 -.01  0.38  -.7225 -.03  .01
1  0.38  -.7425  .01  0.38  -.7225  .03  .01
1  0.38   .7425 -.01  0.38   .7225 -.03  .01
1  0.38   .7425  .01  0.38   .7225  .03  .01
1  0.38  -.7225  .03  0.38  -.27    .03  .01
1  0.38  -.27    .03  0.38     0  -.015  .01
1  0.38   .27    .03  0.38     0  -.015  .01
1  0.45  -0.6575   0  0.45   0.6575   0  .01
1  0.90  -0.6425   0  0.90   0.6425   0  .01
1  1.315  -.618    0  1.315  .618     0  .01
1  1.77   -.618    0  1.77   .618     0  .01
1  2.305  -.596    0  2.305  .596     0  .01
1 source
Wire 3, center

Mounting brackets not modeled.

May 30, 201588108 MHz