The Antennacraft FM6 is a six-element log-Yagi array with three driven elements on a 68″ boom. Radio Shack once sold the antenna as catalog number 15-2163.
I modeled the antenna with the AO 9.67 Antenna Optimizer program. This image shows the antenna geometry.
This shows phasing line and feedpoint detail. The model uses a jumper wire between the feedpoint terminal bolts. The long leads of the ferrite balun typically used with this antenna add inductance that slightly reduces SWR in the upper part of the FM band. Blue dots mark analysis segments. The red dot is the feedpoint.
Calculated performance is for 28 analysis segments per conductor halfwave with additional segments for the phasing lines. 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
88 237-j100 1.55 0.21 0.03 5.76 14.83
89 258-j80 1.38 0.11 0.03 5.81 17.40
90 278-j76 1.31 0.08 0.03 5.79 19.29
91 293-j82 1.32 0.08 0.02 5.75 20.37
92 303-j96 1.37 0.11 0.02 5.71 20.75
93 305-j115 1.46 0.15 0.02 5.66 20.61
94 301-j136 1.57 0.22 0.02 5.61 20.22
95 288-j157 1.70 0.30 0.02 5.55 19.71
96 269-j175 1.85 0.41 0.02 5.49 19.14
97 244-j188 2.04 0.54 0.02 5.42 18.49
98 218-j195 2.23 0.68 0.02 5.34 17.83
99 192-j195 2.46 0.85 0.02 5.25 17.14
100 167-j189 2.69 1.02 0.02 5.16 16.42
101 146-j179 2.94 1.20 0.02 5.07 15.67
102 128-j167 3.18 1.38 0.02 5.00 14.91
103 114-j151 3.38 1.52 0.02 4.95 14.14
104 104-j134 3.52 1.61 0.02 4.96 13.35
105 98.9-j115 3.53 1.62 0.02 5.05 12.56
106 101-j93 3.30 1.47 0.02 5.29 11.72
107 112-j69 2.83 1.13 0.03 5.70 10.96
108 147-j45 2.09 0.58 0.03 6.28 10.21
Antennacraft FM6 Free Space 98 MHz 38 6063-T832 wires, inches r = 1.875 / 2 ; rivet half-spacing yo = r * (1 - 3.375 / 6.1875) ; y at outer phasing-line bends yi = r * (1 - 3.875 / 6.1875) ; y at inner bends 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 ; rear phasing-line crossover half-spacing s2 = .5625 / 2 ; front phasing-line crossover half-spacing t = -.75 ; terminals 1 -33.75 -33 0 -33.75 -2 0 0.375 1 -33.75 -2 0 -33.75 2 0 1.28 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 t 0.125 zinc 1 7 -r t 7 r t #16 copper 1 7 r 0 7 r t 0.125 zinc 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 1.28 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 1.28 1 33.75 2 0 33.75 25 0 0.375 4 -17.75 r 0 x1 yo 0 .11 1 x1 yo 0 x2 yi -s1 .11 10 x2 yi -s1 x3 -yi -s1 .11 1 x3 -yi -s1 x4 -yo 0 .11 4 x4 -yo 0 -5.375 -r 0 .11 4 -17.75 -r 0 x1 -yo 0 .11 1 x1 -yo 0 x2 -yi s1 .11 10 x2 -yi s1 x3 yi s1 .11 1 x3 yi s1 x4 yo 0 .11 4 x4 yo 0 -5.375 r 0 .11 4 -5.375 r 0 x5 yo 0 .11 1 x5 yo 0 x6 yi s2 .11 8 x6 yi s2 x7 -yi s2 .11 1 x7 -yi s2 x8 -yo 0 .11 4 x8 -yo 0 7 -r 0 .11 4 -5.375 -r 0 x5 -yo 0 .11 1 x5 -yo 0 x6 -yi -s2 .11 8 x6 -yi -s2 x7 yi -s2 .11 1 x7 yi -s2 x8 yo 0 .11 4 x8 yo 0 7 r 0 .11 1 source Wire 10, center I used YO 8.00 to model the 4" x 1.625" x 0.5" x 0.05" parasitic-element mounting brackets as U-channels. This yielded the 1.28" diameter of the 4" center sections that represent the brackets.
The directive pattern is sensitive to element angle. Make sure the element mounts are securely locked and the tubing is perpendicular to the boom.
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 or L-network balun to reduce the loss to < 0.2 dB across the band.
Shunting the feedpoint with an inductance of 810 nH reduces mismatch loss. Use 7 turns of #14 bare copper wire, 1″ diameter, 2″ long, with 1⅝″ leads. The coil does not affect the pattern. You can neglect it if somewhat lower gain is acceptable. See the curves below. Use this Windows program to design a different coil.
To greatly improve the pattern, alter the elements to obtain these lengths from boom center to element tip:
Reflector 343⁄16″ Driven Element #1 279⁄16″ Driven Element #2 281⁄8″ Driven Element #3 2111⁄16″ Director #1 25″ Director #2 221⁄4″
Check the length of each element on both sides of the boom. Three elements must be modified: the reflector must be lengthened, and driven element #1 and director #2 must be shortened. It's easy to shorten elements with a tube cutter or hacksaw. It's more trouble to lengthen the reflector, but doing so greatly improves the pattern at the low end of the band. Rob Keeney suggests using the tips cut from the director. If you force them over the reflector tips, the seam will expand. Secure and weatherproof with heat-shrink tubing.
Dave Latchum in Sanford, Florida, first cut and filed the flat part of a reflector tip to enable a good fit.
Then he forced a director tip over it.
Finally, Dave added two layers of heat-shrink tubing. To minimize dielectric loss and detuning, use only enough to secure and weatherproof the joint. The high electric field at element tips may make them sensitive to dielectrics.
Carl Van Camp used a shorter, single layer of heat-shrink.
This shows how Dave mounted the coil about ½″ below the boom.
Carl's coil. Calculated inductance includes the lead length.
Rob Keeney modified this FM6 in Summerville, South Carolina.
Carl Van Camp uses this modified FM6 in Chicago, Illinois.
Steve Capowski modified this FM6 in Wappingers Falls, New York. Steve flattened the cut element ends with welding pliers.
88–108 MHz