Antennacraft FM6
The Antennacraft FM6 is a six-element Log-Yagi design with three driven elements on a six-foot boom. Radio Shack once sold the antenna as catalog number 15-2163.
I modeled the antenna with the AO 7.03 Antenna Optimizer program. This image shows the antenna geometry.
This shows phasing line and feedpoint detail. Blue dots indicate analysis segment boundaries. The wires in the vertical plane model the terminal bolts and balun leads. The red dot indicates the feedpoint.
Modeling Results
Below are calculated performance figures for a segmentation density of 40 segments per halfwave. Mismatch loss is due to SWR. Wire loss is due to conductor resistance. Mismatched gain is forward gain including wire and mismatch losses. F/B is the ratio of forward power to that directly to the rear. The SWR reference impedance is 300 ohms. If you use 75-ohm coax and a Radio Shack 15-1140 or 15-1230 balun, subtract 0.85 dB from the gain figures to account for balun loss.A 1-microhenry inductor across the feedpoint improves calculated SWR and gain over most of the band as shown in the second column. You can make a suitable inductor from six turns of #14 bare copper wire, 1" diameter and 1" long. Mount it between the feedpoint terminals.
Stock With Inductor
88.000 MHz: Impedance 250 - j63 ohms 303 + j63 ohms
SWR 1.34 1.23
Mismatch Loss 0.09 dB 0.05 dB
Wire Loss 0.04 dB 0.04 dB
Mismatched Gain 5.86 dBd 5.91 dBd
F/B 16.43 dB 16.45 dB
90.000 MHz: Impedance 288 - j47 ohms 314 + j106 ohms
SWR 1.18 1.41
Mismatch Loss 0.03 dB 0.13 dB
Wire Loss 0.03 dB 0.03 dB
Mismatched Gain 5.83 dBd 5.73 dBd
F/B 20.09 dB 20.13 dB
92.000 MHz: Impedance 308 - j70 ohms 349 + j108 ohms
SWR 1.26 1.41
Mismatch Loss 0.06 dB 0.14 dB
Wire Loss 0.03 dB 0.03 dB
Mismatched Gain 5.76 dBd 5.68 dBd
F/B 20.65 dB 20.70 dB
94.000 MHz: Impedance 300 - j112 ohms 391 + j71 ohms
SWR 1.45 1.40
Mismatch Loss 0.15 dB 0.12 dB
Wire Loss 0.03 dB 0.03 dB
Mismatched Gain 5.70 dBd 5.73 dBd
F/B 19.83 dB 19.88 dB
96.000 MHz: Impedance 263 - j148 ohms 410 - j5 ohms
SWR 1.71 1.37
Mismatch Loss 0.31 dB 0.11 dB
Wire Loss 0.03 dB 0.03 dB
Mismatched Gain 5.62 dBd 5.83 dBd
F/B 18.66 dB 18.70 dB
98.000 MHz: Impedance 210 - j161 ohms 374 - j94 ohms
SWR 2.05 1.43
Mismatch Loss 0.55 dB 0.14 dB
Wire Loss 0.03 dB 0.03 dB
Mismatched Gain 5.52 dBd 5.94 dBd
F/B 17.35 dB 17.39 dB
100.000 MHz: Impedance 160 - j150 ohms 291 - j143 ohms
SWR 2.48 1.62
Mismatch Loss 0.87 dB 0.25 dB
Wire Loss 0.03 dB 0.03 dB
Mismatched Gain 5.37 dBd 6.00 dBd
F/B 15.93 dB 15.96 dB
102.000 MHz: Impedance 122 - j123 ohms 208 - j137 ohms
SWR 2.93 1.91
Mismatch Loss 1.20 dB 0.45 dB
Wire Loss 0.03 dB 0.03 dB
Mismatched Gain 5.24 dBd 6.00 dBd
F/B 14.42 dB 14.44 dB
104.000 MHz: Impedance 102 - j87 ohms 154 - j96 ohms
SWR 3.22 2.21
Mismatch Loss 1.41 dB 0.66 dB
Wire Loss 0.03 dB 0.03 dB
Mismatched Gain 5.23 dBd 5.98 dBd
F/B 12.88 dB 12.88 dB
106.000 MHz: Impedance 105 - j43 ohms 137 - j37 ohms
SWR 2.93 2.23
Mismatch Loss 1.20 dB 0.68 dB
Wire Loss 0.03 dB 0.03 dB
Mismatched Gain 5.60 dBd 6.13 dBd
F/B 11.35 dB 11.34 dB
108.000 MHz: Impedance 185 - j1 ohms 204 + j48 ohms
SWR 1.62 1.54
Mismatch Loss 0.25 dB 0.20 dB
Wire Loss 0.04 dB 0.04 dB
Mismatched Gain 6.64 dBd 6.70 dBd
F/B 9.91 dB 9.90 dB
Antennacraft specifies the following:
Gain 6.6 dB over half-wave tuned dipole F/B 18.9 dB
Patterns
Augmented Antenna
Augmenting the Yagi with an additional director can substantially improve performance. Install a 3/8"-diameter, 46"-long element 32-3/4" in front of the last director. The following results are for the element mounted directly to a nonconducting boom extension, such as a piece of wood. A metallic support should work just about as well. Adding the 1-microhenry inductor across the feedpoint improves gain even more.
Stock Augmented Augmented + Inductor
88.000 MHz: Mismatched Gain 5.86 dBd 6.17 dBd 6.25 dBd
F/B 16.43 dB 15.83 dB 15.85 dB
90.000 MHz: Mismatched Gain 5.83 dBd 6.15 dBd 6.17 dBd
F/B 20.09 dB 20.31 dB 20.35 dB
92.000 MHz: Mismatched Gain 5.76 dBd 6.15 dBd 6.19 dBd
F/B 20.65 dB 23.65 dB 23.69 dB
94.000 MHz: Mismatched Gain 5.70 dBd 6.18 dBd 6.30 dBd
F/B 19.83 dB 26.18 dB 26.23 dB
96.000 MHz: Mismatched Gain 5.62 dBd 6.27 dBd 6.46 dBd
F/B 18.66 dB 28.35 dB 28.40 dB
98.000 MHz: Mismatched Gain 5.52 dBd 6.37 dBd 6.66 dBd
F/B 17.35 dB 30.33 dB 30.40 dB
100.000 MHz: Mismatched Gain 5.37 dBd 6.48 dBd 6.89 dBd
F/B 15.93 dB 30.77 dB 30.89 dB
102.000 MHz: Mismatched Gain 5.24 dBd 6.54 dBd 7.11 dBd
F/B 14.42 dB 27.52 dB 27.64 dB
104.000 MHz: Mismatched Gain 5.23 dBd 6.53 dBd 7.25 dBd
F/B 12.88 dB 22.85 dB 22.91 dB
106.000 MHz: Mismatched Gain 5.60 dBd 6.56 dBd 7.33 dBd
F/B 11.35 dB 18.51 dB 18.55 dB
108.000 MHz: Mismatched Gain 6.64 dBd 7.17 dBd 7.66 dBd
F/B 9.91 dB 14.78 dB 14.78 dB
Front-to-worst-backlobe ratio better characterizes pattern quality than conventional front-to-back ratio. F/B may mislead because it considers unwanted response only directly to the rear. The following plot overlays the augmented response (yellow) with the stock response (red) at 100 MHz, where the difference in backlobe shape is most pronounced. The backlobe improvement, while substantial, is not quite as dramatic as the F/B figures might suggest.
This shows Edward Swaim's augmented FM6 with feedpoint inductor and current balun. Boom weight and wind loading become unbalanced unless the mast attachment point is moved.
Circular Polarization
A pair of orthogonal Yagis offset a quarter wavelength with outputs combined in phase or out of phase yields circular polarization. Over ground this arrangement can increase response for signals with matching polarization about 2.7 dB, while rejecting cross-polarized signals, including multipath reflections, about 20 dB. This method of arraying two antennas is more compact than conventional horizontal or vertical stacking and may provide more benefit.
The simplest feed arrangement uses a 75:300-ohm balun at each Yagi and two 75-ohm cables of equal length to a 75-ohm power splitter. Coil each cable into a current balun at the feedpoints. From the splitter run a single 75-ohm feedline to the rear. Extend the boom one foot past the last reflector, coil the feedline into another current balun there, and drop it vertically. Keep the feedline away from the elements. Use a nonconductive mast. Swap the 300-ohm connections at one antenna to reverse the circularity sense.
Subtracting 0.5 dB for the loss of the power splitter, array gain is about 2.2 dB over ground for a signal with matching circularly. For a linearly polarized signal, the array delivers about 3.5 dB less power than a polarization-aligned Yagi. To eliminate power-splitter and balun losses (1.35 dB total), connect two equal lengths of 300-ohm twinlead from the Yagis in parallel and transform to 75 ohms with a series-section transformer. At the junction of the 300-ohm lines connect 23-1/4" of 75-ohm RG-59. To the RG-59 connect 12-7/8" of 50-ohm RG-58. Connect the RG-58 to 75-ohm feedline. For these lengths the RG-58 and RG-59 in the matching section must have a velocity factor of 0.66. Coil the matching section into a current balun at the 300-ohm junction. This scheme yields about 3.5 dB gain over a single Yagi with balun.
Hardwired as described, the array will receive signals with horizontal, vertical, and either right- or left-circular polarization. It will reject signals of opposite circularity. Most signals on the air today are right-circular, some are left-circular, and a few are horizontal or vertical. Check that the signals you want to receive have the same circularity sense or are linearly polarized. A more complex installation uses two feedlines and a switching arrangement to receive all polarizations and eliminate the penalty for linear polarization. An even more complex arrangement combines the signals from two feedlines with adjustable amplitude and phase. This method can exactly cancel an interfering signal by compensating for transmit circularity errors, attenuation of vertical polarization due to propagation over hills or mountains, variation in ground quality or antenna height, or off-axis signal arrival.
The first three patterns below show the response of a pair of
FM6s with outputs summed. The antennas are tilted 45 degrees to the ground as
illustrated above to maximize off-axis rejection of crosspolarized signals,
offset 31-3/8" in the forward direction for phase shift, and offset 2"
horizontally to separate the booms. The yellow traces are for matching
polarization, while red is for orthogonal polarization. Height and ground
characteristics affect circularity. These patterns are for a boom height of 30
feet over average-quality ground.
The gain figures for these antennas over ground are not comparable with the
figures for free-space antennas given earlier, but you can use the last plot
for a single horizontal Yagi at 30 feet as a gain reference. Not counting
power-splitter losses, in a right-circular field the circularly polarized
array has 2.7 dB gain over the single antenna.
More Updated October 12, 2007
Antenna File
Radio Shack FM Yagi
Free Space
98.000 MHz
39 6063-T832 wires, inches
d = .11 ; phasing-line diameter
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 kink
y2 = r * (1 - 3.875 / h) ; y at second kink
y3 = -y2 ; y at third kink
y4 = -y1 ; y at fourth kink
x1 = -17.75 + 3.375 ; x at first phasing-line kink, rear line
x2 = -17.75 + 3.875 ; x at second kink
x3 = -5.375 - 3.875 ; x at third kink
x4 = -5.375 - 3.375 ; x at fourth kink
x5 = -5.375 + 3.375 ; x at first phasing-line kink, forward line
x6 = -5.375 + 3.875 ; x at second kink
x7 = 7 - 3.875 ; x at third kink
x8 = 7 - 3.375 ; x at fourth kink
s1 = .375 / 2 ; half of rear phasing-line crossover spacing
s2 = .5625 / 2 ; half of front phasing-line crossover spacing
1 -33.7500 -33.0000 0.0000 -33.7500 -2.0000 0.0000 0.3750
1 -33.7500 -2.0000 0.0000 -33.7500 2.0000 0.0000 1.1
1 -33.7500 2.0000 0.0000 -33.7500 33.0000 0.0000 0.3750
1 -17.7500 -29.2500 0.0000 -17.7500 -r 0.0000 0.3750
1 -17.7500 r 0.0000 -17.7500 29.2500 0.0000 0.3750
1 -5.3750 -28.1250 0.0000 -5.3750 -r 0.0000 0.3750
1 -5.3750 r 0.0000 -5.3750 28.1250 0.0000 0.3750
1 7.0000 -21.6875 0.0000 7.0000 -r 0.0000 0.3750
1 7.0000 r 0.0000 7.0000 21.6875 0.0000 0.3750
1 10.2500 -25.0000 0.0000 10.2500 -2.0000 0.0000 0.3750
1 10.2500 -2.0000 0.0000 10.2500 2.0000 0.0000 1.1
1 10.2500 2.0000 0.0000 10.2500 25.0000 0.0000 0.3750
1 33.7500 -25.0000 0.0000 33.7500 -2.0000 0.0000 0.3750
1 33.7500 -2.0000 0.0000 33.7500 2.0000 0.0000 1.1
1 33.7500 2.0000 0.0000 33.7500 25.0000 0.0000 0.3750
1 -17.7500 r 0 x1 y1 0 d aluminum
1 x1 y1 0 x2 y2 -s1 d aluminum
1 x2 y2 -s1 x3 y3 -s1 d aluminum
1 x3 y3 -s1 x4 y4 0 d aluminum
1 x4 y4 0 -5.375 -r 0 d aluminum
1 -17.7500 -r 0 x1 -y1 0 d aluminum
1 x1 -y1 0 x2 -y2 s1 d aluminum
1 x2 -y2 s1 x3 -y3 s1 d aluminum
1 x3 -y3 s1 x4 -y4 0 d aluminum
1 x4 -y4 0 -5.375 r 0 d aluminum
1 -5.3750 r 0 x5 y1 0 d aluminum
1 x5 y1 0 x6 y2 -s2 d aluminum
1 x6 y2 -s2 x7 y3 -s2 d aluminum
1 x7 y3 -s2 x8 y4 0 d aluminum
1 x8 y4 0 7.0000 -r 0 d aluminum
1 -5.3750 -r 0 x5 -y1 0 d aluminum
1 x5 -y1 0 x6 -y2 s2 d aluminum
1 x6 -y2 s2 x7 -y3 s2 d aluminum
1 x7 -y3 s2 x8 -y4 0 d aluminum
1 x8 -y4 0 7.0000 r 0 d aluminum
1 7.0000 -r 0.0000 7.0000 -r -0.7500 .125 steel
1 7.0000 r 0.0000 7.0000 r -0.7500 .125 steel
1 7.0000 -r -0.7500 7.0000 0 -3.6250 #22 copper
1 7.0000 r -0.7500 7.0000 0 -3.6250 #22 copper
1 source
Wire 39, end2
The 4" x 1.625" x 1/32" bracket was modeled as a U channel by YO to get the
1.1" equivalent diameter of the inner taper section for the parasitic elements.
The 0.75" wires near the feedpoint model the terminal bolts.
The #22 wires model split 300-ohm twinlead. -3.625 is for the Radio Shack balun
with long leads. Use -1.875 for other baluns.
Disable bent-wire correction.
is here.
