Triax FM 5

The Triax FM 5 is a five-element Yagi for the FM broadcast band made in Europe. It has a 1.87 m (74″) boom.

I modeled the antenna with the AO 9.50 Antenna Optimizer program. The red dot marks the 300Ω feedpoint.

John Faulkner, G1VVP, combined two FM 5s on one boom to receive either polarization at Sutton-in-Ashfield, Nottinghamshire, England.

Antenna Variation

Triax has made minor changes to the FM 5 over the years. Older antennas use the ferrite balun shown on the left, while newer antennas use the halfwave PCB balun on the right. The folded dipole of older antennas is somewhat pinched near the center, as shown in the previous images. Newer folded dipoles have uniformly spaced conductors. Julian Hardstone, who provided the balun photos, has seen dimensions vary a couple millimeters from antenna to antenna.

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     169 - j3     1.78     0.36      0.01      5.23      6.97
    89     194 + j24    1.57     0.22      0.01      5.59      8.77
    90     220 + j45    1.43     0.14      0.01      5.78     10.45
    91     247 + j61    1.35     0.10      0.01      5.86     11.88
    92     272 + j73    1.31     0.08      0.01      5.88     12.90
    93     297 + j81    1.31     0.08      0.01      5.88     13.38
    94     322 + j86    1.33     0.09      0.01      5.88     13.44
    95     345 + j88    1.36     0.10      0.01      5.87     13.21
    96     368 + j87    1.39     0.12      0.01      5.88     12.85
    97     389 + j80    1.42     0.13      0.01      5.89     12.46
    98     407 + j69    1.44     0.14      0.01      5.93     12.09
    99     419 + j53    1.44     0.14      0.01      5.98     11.80
   100     423 + j34    1.43     0.14      0.01      6.06     11.61
   101     416 + j13    1.39     0.12      0.01      6.15     11.52
   102     397 - j1     1.32     0.08      0.01      6.28     11.62
   103     367 - j4     1.22     0.04      0.01      6.44     11.95
   104     333 + j9     1.11     0.01      0.01      6.63     12.62
   105     299 + j41    1.15     0.02      0.02      6.81     13.85
   106     275 + j94    1.40     0.12      0.02      6.96     16.05
   107     277 + j175   1.83     0.39      0.03      6.98     18.18
   108     352 + j284   2.37     0.78      0.04      6.80     20.20

Antenna File

Triax FM 5
Free Space Symmetric
98 MHz
15 6063-T832 wires, meters
1     0    -.79      0        0     .79      0      .01
1    .76   -.7225  -.03      .76    .7225  -.03     .01
1    .76    .27     .03      .76    .7225   .03     .01
1    .76   -.7425  -.01      .76   -.7425   .01     .01
1    .76    .7425  -.01      .76    .7425   .01     .01
1    .76   -.7425  -.01      .76   -.7225  -.03     .01
1    .76   -.7425   .01      .76   -.7225   .03     .01
1    .76    .7425  -.01      .76    .7225  -.03     .01
1    .76    .7425   .01      .76    .7225   .03     .01
1    .76   -.7225   .03      .76   -.27     .03     .01
1    .76   -.27     .03      .76     0     -.015    .01
1    .76    .27     .03      .76     0     -.015    .01
1    .913  -.635     0       .913   .635     0      .01
1   1.323  -.635     0      1.323   .635     0      .01
1   1.873  -.62      0      1.873   .62      0      .01
1 source
Wire 2, center

Element mounting brackets not modeled.

May 30, 201588108 MHz