This antenna has three reflectors, three driven elements, and four directors on a 134″ boom. A shorted stub adds inductance across the rear driven element. A matching line extends below the front driven-element. I designed the antenna with the AO 9.67 Antenna Optimizer, trading forward gain for very low backlobes.
This shows phasing line and stub detail. Blue dots mark analysis segments. The red dot marks the feedpoint.
The phasing lines cross midway between driven elements.
Calculated performance is for 28 analysis segments per element halfwave and parallel line segment length equal to the line spacing. 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 55.8-j12.8 1.43 0.14 0.04 7.06 38.42
89 51.9-j8.0 1.48 0.16 0.04 7.03 38.93
90 50.4-j2.7 1.49 0.17 0.04 7.02 38.81
91 50.6+j2.4 1.48 0.17 0.04 7.04 38.52
92 51.9+j7.2 1.47 0.16 0.03 7.08 38.42
93 54.1+j11.5 1.45 0.15 0.03 7.13 38.56
94 57.2+j15.2 1.43 0.14 0.03 7.19 39.00
95 61.0+j18.2 1.40 0.12 0.03 7.26 38.90
96 65.3+j20.3 1.38 0.11 0.03 7.34 38.66
97 69.9+j21.4 1.35 0.10 0.03 7.43 38.54
98 75.0+j21.1 1.32 0.09 0.03 7.54 38.46
99 80.3+j19.0 1.29 0.07 0.03 7.66 38.45
100 85.4+j15.0 1.25 0.06 0.03 7.78 38.44
101 89.1+j8.1 1.22 0.04 0.03 7.91 38.45
102 89.9-j1.3 1.20 0.04 0.03 8.04 38.47
103 85.7-j12.1 1.22 0.04 0.04 8.15 38.43
104 76.5-j20.8 1.32 0.08 0.04 8.23 38.42
105 64.1-j24.5 1.47 0.16 0.05 8.24 38.36
106 53.3-j22.4 1.63 0.26 0.06 8.17 38.25
107 48.2-j20.8 1.75 0.33 0.08 8.03 38.43
108 37.2-j31.9 2.47 0.86 0.13 7.21 40.10
Space ⅛″ aluminum wire ½″ center-to-center for the phasing lines. Orient them symmetrically with respect to the boom. Extend the phasing lines past the rear driven element to form the shorted stub. Extend them beyond the front driven-element to form the matching line. (They are modeled in the vertical plane, but any orientation that keeps them away from the first director is fine.) Cross the phasing lines midway between elements over a 1″ span with the wire surfaces separated ⅛″. To minimize boom coupling, connect the top wire at each crossover to the same side of the middle driven element. Support and space the wires with low-loss dielectric, such as polystyrene.
For the elements use ⅜″ aluminum tubing supported by insulated mounts. Drill a screw hole through the top of each driven element inner end. Use a lockwasher and nut inside the tubing. Bend one edge of two aluminum washers and secure the phasing line between them on top. Use galvanized or cad-plated hardware and apply an antioxidation compound. Measure driven element half-length from the center of the boom to the element tips. The last driven element and first director are very close. Make sure the elements are exactly parallel.
At crossover the phasing lines are not equidistant from the boom. The proximity imbalance can couple stray signals. Modeling suggests that elevating the phasing line plane ¾″ above the boom surface causes negligible pattern degradation. Use Stauff clamps with the mounting method described above to do this. Position the mast next to the forward phasing line crossover to approximately cancel any coupling.
Connect 75Ω coax to the end of the matching line. Keep the stripped coax leads as short as possible. Waterproof the connections and coax. Use a current choke at the feedpoint. With backlobes this low, even small feedline pickup may degrade the pattern. Use a coiled-coax choke for best feedline current suppression. Keep everything away from the phasing lines.
Read these notes before building anything.
10-Element Log-Yagi Free Space 88 92 98 101 103 107 108 MHz 35 6063-T832 wires, inches z = 19.8879 ; outer reflector height rp = 0 ; element positions de1p = 20.47588 de2p = 38.68067 de3p = 51.72175 d1p = 54.01671 d2p = 71.4959 d3p = 98.88953 d4p = 133.4537 r0 = 36.64897 ; inner reflector half-length r1 = 36.19538 ; outer reflector half-length de1 = 30.41594 ; boom center to driven-element tips de2 = 25.84483 de3 = 25.26526 d1 = 25.82585 ; director half-lengths d2 = 25.19954 d3 = 23.94531 d4 = 20.168 p = .5 ; parallel line spacing f = p / 2 s = 6.272795 ; stub length sp = de1p - s m = 2.789245 ; matching line length c1 = (de1p + de2p) / 2 ; position of crossover centers c2 = (de2p + de3p) / 2 c1a = c1 - f ; position of crossover ends c1b = c1 + f c2a = c2 - f c2b = c2 + f n0 = s / p ; number of phasing line segments n1 = (c1a - de1p) / p n2 = (c2a - de2p) / p n3 = m / p 1 rp -r1 z rp r1 z .375 ; reflectors 1 rp -r0 0 rp r0 0 .375 1 rp -r1 -z rp r1 -z .375 1 sp -f 0 sp f 0 .125 ; stub short n0 sp -f 0 de1p -f 0 .1315 ; stub lines n0 sp f 0 de1p f 0 .1315 1 de1p f 0 de1p de1 0 .375 ; driven elements 1 de1p -f 0 de1p -de1 0 .375 1 de2p f 0 de2p de2 0 .375 1 de2p -f 0 de2p -de2 0 .375 1 de3p f 0 de3p de3 0 .375 1 de3p -f 0 de3p -de3 0 .375 1 d1p -d1 0 d1p d1 0 .375 ; directors 1 d2p -d2 0 d2p d2 0 .375 1 d3p -d3 0 d3p d3 0 .375 1 d4p -d4 0 d4p d4 0 .375 n1 de1p f 0 c1a f 0 .1315 ; phasing lines 1 c1a f 0 c1 0 f .1315 1 c1 0 f c1b -f 0 .1315 n1 c1b -f 0 de2p -f 0 .1315 n1 de1p -f 0 c1a -f 0 .1315 1 c1a -f 0 c1 0 -f .1315 1 c1 0 -f c1b f 0 .1315 n1 c1b f 0 de2p f 0 .1315 ; 0.1315" in AO yields n2 de2p f 0 c2a f 0 .1315 ; correct line Z for 1 c2a f 0 c2 0 -f .1315 ; 0.125" wires spaced 0.5" 1 c2 0 -f c2b -f 0 .1315 n2 c2b -f 0 de3p -f 0 .1315 n2 de2p -f 0 c2a -f 0 .1315 1 c2a -f 0 c2 0 f .1315 1 c2 0 f c2b f 0 .1315 n2 c2b f 0 de3p f 0 .1315 n3 de3p f 0 de3p f -m .1315 ; matching lines n3 de3p -f 0 de3p -f -m .1315 2 de3p -f -m de3p f -m .125 ; feedpoint jumper 1 source Wire 35, center
The following table shows the largest performance degradation over 88, 93, 98, 103, and 108 MHz in dB when altering a symbol value by Tol. Note the high F/R sensitivity to the first two director lengths.
Symbol Tol Gain F/R
z 0.0394 0.00 0.15
rp 0.0394 0.00 0.11
de1p 0.0394 0.00 0.14
de2p 0.0394 0.01 0.05
de3p 0.0394 0.02 0.08
d1p 0.0394 0.03 0.22
d2p 0.0394 0.02 0.29
d3p 0.0394 0.01 0.18
d4p 0.0394 0.00 0.06
r0 0.0197 0.00 0.10
r1 0.0197 0.00 0.07
de1 0.0394 0.02 0.29
de2 0.0394 0.01 0.72
de3 0.0394 0.03 0.52
d1 0.0197 0.16 1.30
d2 0.0197 0.05 2.33
d3 0.0197 0.03 0.79
d4 0.0197 0.00 0.10
p 0.0394 0.03 1.10
s 0.0394 0.00 0.12
m 0.0394 0.02 0.12
Mark erected an earlier version of the antenna in New Jersey. He used a lockwasher between McMaster-Carr 2993T83 single-bolt clamps and the boom to help keep the elements parallel. Mark consistently gets 35 dB F/B despite using a short middle reflector and deviant phasing lines.
88–108 MHz