With ten feet of copper tubing you can make a vertically polarized omnidirectional antenna that covers the whole FM band. The design is a two-conductor dipole that approximates a solid-surface bow-tie dipole. The antenna provides a broad impedance bandwidth, low mismatch loss, and convenient 75Ω feed.
It's hard to beat a twin-lead folded dipole for simplicity, versatility, and low cost. But it can be awkward to mount one vertically for omnidirectional response. To avoid unwanted coupling, the feedline should run perpendicular to the antenna for at least several feet. In addition, though more broadband than a single-wire dipole, a folded dipole is still down 1.7 dB at the band edges. Subtract another 0.75 dB if you transform 300Ω to 75Ω with a ferrite balun. In contrast, the wideband omni is down just 0.5 dB at the band edges and uses a virtually lossless current choke.
The antenna consists of two copper tubes, each about four feet long, separated about one foot at the ends and connected in parallel at the center.
Mount a small, flat, nonconducting, rectangular plate to a ten-foot 1½″ ABS pipe (1.9″ OD) using PVC conduit clamps. Drill a hole in each clamp and secure it to the mast with a sheet metal screw. Cut four 25⅛″ tubes from one ten-foot length of ½″ Type M (0.028″ wall) or Type L (0.04″ wall) copper pipe (0.625″ OD). The upper pairs connect together as do the lower. Flatten one end of each tube. Drill a small hole as close as possible to the end of each flattened section and join a pair with a screw, washers, and nut. This is also where the feedline connects. Place the tubes on the plate with one pair extending above the feedpoint and the other below, as shown above. Spread the tubes until the far ends are 12¾″ apart center-to-center. Position the inner ends 1″ apart. Fasten the tubes to the plate with ½″ plastic pipe clamps. Solder the pairs together to avoid a bad connection due to corrosion. Attach 75Ω coax between the pairs. Weatherproof the coax and connections. Use a current choke at the feedpoint. Route the feedline down the mast and install a second balun 30″ below the first.
Instead of expensive copper pipe, you can use 6063-T5 aluminum angle. The 0.5″ × 0.5″ right-angle shape is electrically equivalent to a 0.4″ round conductor. Cut one 96″ piece into four 24″ lengths. Separate the outer ends 10¼″ and position the inner ends 2″ apart.
This test antenna made of #14 wire uses wider end separation. It has a coiled-coax choke formed from a short length of RG-59 followed by a large ferrite choke at the feedline junction below. It's best to place the first choke closer to the feedpoint than shown. A horizontal line helps stabilize the flimsy test mast.
I optimized the design with the AO 9.61 Antenna Optimizer using 28 analysis segments per conductor halfwave. Gain includes mismatch and conductor losses. ΔG is peak-to-peak gain variation over 360° azimuth.
Frequency Impedance SWR Mismatch Conductor Avg Omni ΔG
MHz ohms Loss dB Loss dB Gain dBd dB
88 44.3-j32.2 2.13 0.60 0.00 -0.82 0.08
89 45.8-j27.0 1.95 0.47 0.00 -0.69 0.08
90 47.3-j21.9 1.79 0.37 0.00 -0.57 0.08
91 48.9-j16.8 1.66 0.28 0.00 -0.48 0.08
92 50.5-j11.8 1.55 0.21 0.00 -0.40 0.08
93 52.1-j6.7 1.46 0.15 0.00 -0.34 0.08
94 53.9-j1.8 1.39 0.12 0.00 -0.30 0.09
95 55.6+j3.2 1.35 0.10 0.00 -0.28 0.09
96 57.4+j8.1 1.34 0.09 0.00 -0.27 0.09
97 59.6+j14.6 1.37 0.11 0.00 -0.27 0.09
98 61.6+j19.5 1.41 0.13 0.00 -0.29 0.10
99 63.6+j24.4 1.47 0.16 0.00 -0.32 0.10
100 65.6+j29.2 1.54 0.20 0.00 -0.35 0.10
101 67.8+j34.1 1.62 0.25 0.00 -0.39 0.10
102 69.9+j38.9 1.71 0.31 0.00 -0.44 0.11
103 72.2+j43.6 1.80 0.37 0.00 -0.50 0.11
104 74.5+j48.4 1.89 0.43 0.00 -0.55 0.11
105 76.9+j53.1 1.99 0.50 0.00 -0.62 0.11
106 79.4+j57.8 2.09 0.57 0.00 -0.68 0.12
107 81.9+j62.5 2.19 0.65 0.00 -0.75 0.12
108 84.5+j67.2 2.29 0.72 0.00 -0.82 0.12
This graph compares the omni, a dipole made of #12 wire, and a twin-lead folded dipole in free space. The omni curve is the average azimuth response. The folded dipole curve includes −0.75 dB for the loss of a 300Ω push-on ferrite balun.
4% of FM broadcast signals in the U.S. today are horizontally polarized. A vertically polarized antenna will not receive these signals well.
Two-Conductor Broadband Dipole Free Space 88 98 108 MHz 5 copper wires, inches f = .5 ang = 14.64813 l = 25.14813 z = l + f 2 0 0 f 0 0 -f #14 rotate end1 x ang 1 0 0 f 0 0 z .625 rotate end1 x -ang 1 0 0 f 0 0 z .625 rotate end1 x ang 1 0 0 -f 0 0 -z .625 rotate end1 x -ang 1 0 0 -f 0 0 -z .625 1 source Wire 1, center
With four folded dipoles you can make a Lindenblad array. It will receive horizontal, vertical, and right-circular signals with remarkably uniform azimuth response. The dipoles are tilted 31° and connected in phase.
With the center of both antennas 20 feet above ground with permittivity = 13 and conductivity = 5 mS/m (average quality at 1 MHz but something else at 98 MHz), the circularly polarized antenna has about 5 dB gain over the wideband vertical for right-circular signals near the horizon. The figure drops to 3 dB at 108 MHz.
Calculated performance is for 28 analysis segments per conductor halfwave. Values are over 360° azimuth. Gain includes mismatch and conductor losses. Axial ratio is the ratio of maximum to minimum linearly polarized power. H/V is the ratio of horizontal to vertical power. ΔG is peak-to-peak gain variation. Results are for the right-circular field at 1° elevation angle with the center of the antenna 20 feet above ground with permittivity = 13 and conductivity = 5 mS/m.
Frequency Impedance SWR Mismatch Conductor Avg Omni Avg Axial Avg H/V ΔG
MHz ohms Loss dB Loss dB Gain dBic Ratio dB dB dB
88 375-j241 2.09 0.58 0.01 -7.72 3.24 -1.89 0.03
89 356-j201 1.87 0.42 0.01 -7.46 3.14 -1.76 0.03
90 340-j164 1.68 0.29 0.01 -7.24 3.04 -1.63 0.03
91 321-j138 1.56 0.21 0.00 -7.01 2.94 -1.49 0.03
92 313-j106 1.41 0.13 0.00 -6.83 2.85 -1.36 0.03
93 308-j76 1.28 0.07 0.00 -6.68 2.77 -1.23 0.03
94 305-j47 1.17 0.03 0.00 -6.55 2.69 -1.10 0.03
95 303-j19 1.07 0.00 0.00 -6.44 2.61 -0.98 0.03
96 303-j2 1.01 0.00 0.00 -6.41 2.54 -0.84 0.03
97 306+j23 1.08 0.01 0.00 -6.34 2.47 -0.71 0.03
98 310+j48 1.17 0.03 0.00 -6.28 2.42 -0.58 0.03
99 316+j72 1.27 0.06 0.00 -6.24 2.36 -0.46 0.03
100 324+j95 1.37 0.11 0.00 -6.20 2.32 -0.33 0.03
101 332+j118 1.47 0.16 0.00 -6.18 2.28 -0.20 0.03
102 342+j140 1.57 0.22 0.00 -6.17 2.25 -0.07 0.03
103 354+j162 1.68 0.29 0.00 -6.17 2.23 0.06 0.03
104 367+j184 1.79 0.36 0.00 -6.17 2.22 0.20 0.03
105 382+j205 1.90 0.44 0.00 -6.18 2.21 0.33 0.03
106 398+j227 2.02 0.52 0.00 -6.19 2.22 0.47 0.03
107 417+j248 2.13 0.61 0.00 -6.21 2.23 0.61 0.03
108 438+j270 2.26 0.70 0.01 -6.23 2.26 0.76 0.03
The folded dipoles are 52⅛″ long. Make them from ⅜″ tubing spaced 1″. Orient the two conductors in the vertical plane and tilt them 31° with the right end higher than the left. Space opposite dipoles 351⁄16″. Connect the dipoles with equal lengths of 300Ω twin-lead. Join the four leads that go to the upper dipole ends, join the leads that go to the lower ends, and attach 75Ω coax to the two joints. Use a current choke at the junction and add another to the coax 30″ below. The arms that support the dipoles can be metallic, but for best performance the mast should be nonconductive in the vicinity of the antenna. Read these notes before building anything.
Circularly Polarized Omni 20' High 88 98 108 MHz 16 6063-T832 wires, inches ang = -30.64113 a = 26.06819 s = 17.5441 shift z 240 rotate x ang 1 s a 0 s -a 0 .375 1 s a 1 s -a 1 .375 1 s -a 0 s -a 1 .375 1 s a 0 s a 1 .375 rotate x -ang 1 -s -a 0 -s a 0 .375 1 -s -a 1 -s a 1 .375 1 -s -a 0 -s -a 1 .375 1 -s a 0 -s a 1 .375 rotate x end y ang 1 -a s 0 a s 0 .375 1 -a s 1 a s 1 .375 1 -a s 0 -a s 1 .375 1 a s 0 a s 1 .375 rotate y -ang 1 a -s 0 -a -s 0 .375 1 a -s 1 -a -s 1 .375 1 -a -s 0 -a -s 1 .375 1 a -s 0 a -s 1 .375 4 sources Wire 1, center Wire 5, center Wire 9, center Wire 13, center
88–108 MHz