Antennacraft FM6

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 9.53 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.

Feedpoint

Normally I jumper a wire across the feedpoint terminals and place a source at its center. This model is unrealistic for the FM6 because the boom blocks such a wire. A Radio Shack balun often used with the antenna has twin-lead split 3″ from the terminal lugs. I model this geometry instead. The resulting inductive reactance alters SWR, mismatch loss, and forward gain.

Modeling Results

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     244 - j58    1.34     0.09      0.03      5.86     15.73
    89     267 - j43    1.21     0.04      0.03      5.86     18.12
    90     287 - j42    1.16     0.02      0.03      5.83     19.73
    91     300 - j50    1.18     0.03      0.03      5.79     20.49
    92     307 - j66    1.25     0.05      0.02      5.76     20.62
    93     306 - j86    1.33     0.09      0.02      5.72     20.34
    94     298 - j107   1.43     0.14      0.02      5.69     19.87
    95     282 - j126   1.54     0.20      0.02      5.66     19.31
    96     260 - j141   1.68     0.29      0.02      5.62     18.71
    97     233 - j151   1.85     0.40      0.02      5.58     18.05
    98     207 - j153   2.02     0.53      0.02      5.52     17.38
    99     181 - j149   2.22     0.68      0.02      5.45     16.67
   100     157 - j141   2.44     0.84      0.02      5.38     15.95
   101     137 - j129   2.67     1.01      0.03      5.31     15.21
   102     121 - j114   2.90     1.17      0.03      5.24     14.45
   103     108 - j96    3.09     1.31      0.03      5.21     13.68
   104     101 - j78    3.21     1.40      0.03      5.22     12.91
   105    97.7 - j57.2  3.20     1.39      0.03      5.32     12.13
   106     104 - j34    2.94     1.21      0.03      5.59     11.33
   107     124 - j9     2.43     0.83      0.03      6.02     10.60
   108     180 + j6     1.66     0.28      0.04      6.61      9.91

Antenna File

Antennacraft FM6
Free Space
98 MHz
40 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
f = .3125 / 2			   ; feedpoint half-spacing
b = .75				   ; feedpoint bolt length
v = b + 3			   ; feedpoint
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        -b        0.125   zinc
1      7        -r        -b         7        -f        -v         #18    copper
1      7         f        -v         7        -f        -v         #18    copper
1      7         r        -b         7         f        -v         #18    copper
1      7         r         0         7         r        -b        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        1.1
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
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 11, 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.

Improving Performance

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.

Shunting the feedpoint with a 860-nH inductance reduces mismatch loss. A suitable inductor is seven turns of #14 bare copper wire, 1″ inside diameter and 1″ long (wire center to wire center), with leads about 1″ long. The coil does not affect the pattern. You can neglect it if somewhat lower gain is acceptable.

Altering each tip of three elements as follows greatly lowers the backlobe:

Reflector             lengthen 11/16″
Driven Element #1     shorten  21/16″
Driven Element #2
Driven Element #3
Director #1
Director #2           shorten  27/8

It's easy to shorten elements with a 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 ensure a good fit.

Then he forced a director tip over it.

Finally, Dave added two layers of heat-shrink tubing.

This shows how Dave mounted the coil about ″ below the boom. Calculated inductance includes the lead length.

Frequency  Impedance    SWR   Mismatch  Conductor   Forward    F/R
   MHz        ohms             Loss dB   Loss dB   Gain dBd     dB
    88     528 + j190   2.07     0.57      0.05      5.15     26.55
    89     471 + j111   1.71     0.31      0.04      5.30     27.57
    90     423 + j71    1.48     0.17      0.04      5.37     27.24
    91     386 + j47    1.33     0.09      0.03      5.40     26.98
    92     358 + j32    1.22     0.04      0.03      5.40     26.79
    93     336 + j21    1.14     0.02      0.03      5.40     26.69
    94     319 + j12    1.08     0.01      0.03      5.39     26.63
    95     305 + j4     1.02     0.00      0.03      5.39     26.62
    96     293 - j4     1.03     0.00      0.02      5.39     26.65
    97     281 - j12    1.08     0.01      0.02      5.40     26.77
    98     271 - j18    1.13     0.02      0.02      5.40     26.90
    99     262 - j25    1.18     0.03      0.02      5.42     27.10
   100     253 - j32    1.23     0.05      0.02      5.44     27.40
   101     245 - j39    1.28     0.07      0.02      5.46     27.74
   102     237 - j46    1.34     0.09      0.02      5.49     28.19
   103     230 - j54    1.40     0.12      0.02      5.52     28.38
   104     223 - j63    1.47     0.16      0.02      5.55     27.94
   105     214 - j75    1.56     0.21      0.03      5.57     27.55
   106     205 - j91    1.69     0.29      0.03      5.59     27.49
   107     188 - j107   1.89     0.44      0.03      5.55     27.34
   108     165 - j123   2.22     0.68      0.04      5.42     27.18


January 19, 201488108 MHz