The Antennacraft FM6 is a six-element Log-Yagi array with three driven elements on a 70″ boom. Radio Shack once sold the antenna as catalog number 15-2163.
I modeled the antenna with the AO 9.50 Antenna Optimizer program. This image shows the antenna geometry.
This shows phasing line and feedpoint detail. Blue dots mark analysis segments. The red dot is the feedpoint.
Calculated performance figures are for 50 analysis segments per 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
88 246 - j85 1.45 0.15 0.03 5.76 16.48
89 266 - j70 1.32 0.08 0.03 5.77 18.73
90 283 - j67 1.27 0.06 0.03 5.75 20.14
91 298 - j75 1.28 0.07 0.02 5.72 20.68
92 307 - j89 1.34 0.09 0.02 5.68 20.65
93 310 - j110 1.43 0.14 0.02 5.65 20.30
94 304 - j133 1.55 0.21 0.02 5.60 19.81
95 291 - j156 1.69 0.29 0.02 5.56 19.23
96 271 - j175 1.85 0.40 0.02 5.50 18.62
97 245 - j189 2.03 0.54 0.02 5.43 17.97
98 217 - j195 2.24 0.69 0.02 5.35 17.29
99 190 - j194 2.47 0.86 0.02 5.26 16.58
100 164 - j187 2.72 1.05 0.02 5.17 15.86
101 142 - j176 2.98 1.23 0.02 5.09 15.08
102 125 - j161 3.21 1.40 0.02 5.03 14.31
103 111 - j144 3.39 1.53 0.02 5.00 13.52
104 103 - j125 3.46 1.58 0.03 5.05 12.74
105 101 - j104 3.36 1.50 0.03 5.21 11.96
106 108 - j79 2.99 1.24 0.03 5.55 11.18
107 134 - j52 2.32 0.75 0.03 6.09 10.42
108 208 - j39 1.49 0.17 0.04 6.70 9.71
The gain figures do not include balun loss. Subtract 0.85 dB to account for the loss of a Radio Shack 15-1140 or 15-1230, ferrite baluns with long leads often used with the FM6. Replace either with a halfwave coaxial balun to reduce the loss to about 0.1 dB.
Altering each tip of three elements as follows greatly lowers the backlobe:
Reflector lengthen 11/16″ Driven Element #1 shorten 111/16″ Driven Element #2 Driven Element #3 Director #1 Director #2 shorten 213/16″
It's easy to shorten elements with a hacksaw. Lengthening the reflector is more difficult, but it substantially improves the pattern at the low end of the band. Perhaps the easiest way is to flatten the tubing at the tip, drill a hole, wrap a thick aluminum wire around a bolt, and secure it under a washer and nut. Extend and space both ends of the wire to simulate a thicker conductor and trim them 11⁄16″ beyond the tip.
Shunting the feedpoint with a 700-nH inductance reduces mismatch loss. A suitable inductor is six turns of #14 bare copper wire, 1″ inside diameter and 1″ long. Mount it between the feedpoint terminals using the shortest possible leads. The inductor does not affect the pattern. You can neglect it if somewhat lower gain is acceptable.
Frequency Impedance SWR Mismatch Conductor Forward F/R
MHz ohms Loss dB Loss dB Gain dBd dB
88 388 + j257 2.17 0.64 0.04 5.00 26.38
89 385 + j198 1.87 0.42 0.04 5.14 27.13
90 375 + j156 1.67 0.28 0.03 5.21 26.87
91 364 + j127 1.53 0.20 0.03 5.25 26.68
92 355 + j104 1.43 0.14 0.03 5.27 26.52
93 347 + j86 1.35 0.10 0.03 5.29 26.52
94 342 + j69 1.29 0.07 0.02 5.31 26.51
95 337 + j54 1.23 0.05 0.02 5.33 26.55
96 333 + j40 1.18 0.03 0.02 5.36 26.64
97 329 + j26 1.13 0.02 0.02 5.39 26.78
98 326 + j12 1.09 0.01 0.02 5.41 26.96
99 322 - j2 1.07 0.01 0.02 5.45 27.20
100 317 - j16 1.08 0.01 0.02 5.49 27.52
101 314 - j29 1.11 0.01 0.02 5.53 27.94
102 312 - j43 1.16 0.02 0.02 5.57 28.45
103 310 - j59 1.22 0.04 0.02 5.62 28.14
104 307 - j76 1.29 0.07 0.02 5.66 27.75
105 305 - j98 1.38 0.11 0.02 5.70 27.44
106 301 - j125 1.51 0.19 0.03 5.72 27.29
107 289 - j161 1.72 0.31 0.03 5.69 27.15
108 263 - j201 2.04 0.54 0.03 5.58 27.06
Antennacraft FM6 Free Space 98 MHz 38 6063-T832 wires, inches r = 1.875 / 2 ; half of rivet spacing for insulated elements h = 12.375 / 2 ; half of DE spacing y1 = r * (1 - 3.375 / h) ; y at first phasing-line bend y2 = r * (1 - 3.875 / h) ; y at second bend y3 = -y2 ; y at third bend y4 = -y1 ; y at fourth bend x1 = -17.75 + 3.375 ; x at first phasing-line bend, rear line x2 = -17.75 + 3.875 ; x at second bend x3 = -5.375 - 3.875 ; x at third bend x4 = -5.375 - 3.375 ; x at fourth bend x5 = -5.375 + 3.375 ; x at first phasing-line bend, forward line x6 = -5.375 + 3.875 ; x at second bend x7 = 7 - 3.875 ; x at third bend x8 = 7 - 3.375 ; x at fourth bend s1 = .375 / 2 ; half of rear phasing-line crossover spacing s2 = .5625 / 2 ; half of front phasing-line crossover spacing 1 -33.75 -33 0 -33.75 -2 0 0.375 1 -33.75 -2 0 -33.75 2 0 1.1 1 -33.75 2 0 -33.75 33 0 0.375 1 -17.75 -29.25 0 -17.75 -r 0 0.375 1 -17.75 r 0 -17.75 29.25 0 0.375 1 -5.375 -28.125 0 -5.375 -r 0 0.375 1 -5.375 r 0 -5.375 28.125 0 0.375 1 7 -21.6875 0 7 -r 0 0.375 1 7 -r 0 7 -r -0.75 0.125 steel 1 7 -r -0.75 7 r -0.75 #18 copper 1 7 r 0 7 r -0.75 0.125 steel 1 7 r 0 7 21.6875 0 0.375 1 10.25 -25 0 10.25 -2 0 0.375 1 10.25 -2 0 10.25 2 0 1.1 1 10.25 2 0 10.25 25 0 0.375 1 33.75 -25 0 33.75 -2 0 0.375 1 33.75 -2 0 33.75 2 0 1.1 1 33.75 2 0 33.75 25 0 0.375 1 -17.75 r 0 x1 y1 0 .11 1 x1 y1 0 x2 y2 -s1 .11 1 x2 y2 -s1 x3 y3 -s1 .11 1 x3 y3 -s1 x4 y4 0 .11 1 x4 y4 0 -5.375 -r 0 .11 1 -17.75 -r 0 x1 -y1 0 .11 1 x1 -y1 0 x2 -y2 s1 .11 1 x2 -y2 s1 x3 -y3 s1 .11 1 x3 -y3 s1 x4 -y4 0 .11 1 x4 -y4 0 -5.375 r 0 .11 1 -5.375 r 0 x5 y1 0 .11 1 x5 y1 0 x6 y2 s2 .11 1 x6 y2 s2 x7 y3 s2 .11 1 x7 y3 s2 x8 y4 0 .11 1 x8 y4 0 7 -r 0 .11 1 -5.375 -r 0 x5 -y1 0 .11 1 x5 -y1 0 x6 -y2 -s2 .11 1 x6 -y2 -s2 x7 -y3 -s2 .11 1 x7 -y3 -s2 x8 -y4 0 .11 1 x8 -y4 0 7 r 0 .11 1 source Wire 10, center I used YO 7.70 to model the 4" x 1.625" x 0.5" x 1/32" parasitic-element mounting brackets as U-channels. This yielded the 1.1" diameter of the 4" center sections that represent the brackets. Unlike some larger Antennacraft models, each phasing-line wire remains on top or on the bottom for the entire line length. The steel conductors near the feedpoint model the terminal bolts.
88–108 MHz