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.
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.
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
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.
88–108 MHz