Wideband Vertical Omni

With ten feet of aluminum 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 a convenient 75Ω feedpoint.

It's hard to beat a twin-lead folded dipole for simplicity, versatility, and low cost. However, it can be awkward to mount one vertically for omnidirectional response. To avoid unwanted coupling, the feedline should run perpendicular to the antenna for several feet. In addition, though more broadband than a dipole, a folded dipole still is down 1.7 dB at the band edges. Subtract another 0.75 dB for balun loss if you must transform 300Ω to 75Ω. In contrast, the wideband omni is down only 0.5 dB at the band edges and uses a lossless current balun.

Design

The antenna consist of two ⅜″ conductors, each about 4 feet long, separated about two feet at the ends and connected in parallel at the centers.

Construction

Mount a small, flat, rectangular, insulated plate (wood or plastic) to a nonconductive mast such as PVC. Cut four 27″ lengths of ⅜″ aluminum tubing. Flatten one end of each tube and drill a hole as close to the end as possible. Bolt pairs of tubes together, securing a couple inches of #14 copper wire under a washer at each bolt. Spread the tubes until the far ends are 21″ apart center-to-center. Place the tubes on the plate with one pair of connected tubes extending above and the other below, as shown above. Position the flattened ends ″ apart. Fasten the tubes to the plate using any convenient method, such as tie wraps through holes or small bolts. Attach the wire from the lower tubes to the shell of a panel-mount F-connector positioned within the ″ gap. Use two washers and a nut to secure the wire to the shell. Crimp the wire from the upper tubes around the F-connector center pin and solder it. Use the shortest possible length for both wires. Weatherproof the entire feedpoint, including the F-connector and the wire/bolt junctions.

To reduce signal pickup on the coax shield, use a current balun at the feedpoint. Route the feedline down the mast. To even better isolate it, install a second current balun 30″ below the first.

This is a test antenna made of #14 wire. It uses a coiled-coax balun formed from a short length of RG-59 followed by a large ferrite sleeve at the feedline junction below. It's best to place the first balun closer to the feedpoint than shown. A horizontal line helps stabilize the flimsy test mast.

Performance

I optimized the design with the AO 8.50 Antenna Optimizer using 40 analysis segments per halfwave. Forward gain includes mismatch and conductor losses.

 Frequency  Impedance    SWR   Mismatch  Conductor   Forward
    MHz        ohms             Loss dB   Loss dB   Gain dBd
    88   43.0 - j29.7   2.12     0.60      0.01      -0.75
    89   44.5 - j24.6   1.94     0.47      0.01      -0.62
    90   46.0 - j19.5   1.80     0.37      0.00      -0.51
    91   47.7 - j14.4   1.67     0.28      0.00      -0.42
    92   49.3 - j9.3    1.56     0.21      0.00      -0.34
    93   51.0 - j4.3    1.48     0.17      0.00      -0.29
    94   52.7 + j0.6    1.42     0.13      0.00      -0.25
    95   54.5 + j5.6    1.39     0.12      0.00      -0.22
    96   56.3 + j10.5   1.39     0.12      0.00      -0.21
    97   58.2 + j15.4   1.41     0.13      0.00      -0.22
    98   60.2 + j20.3   1.45     0.15      0.00      -0.23
    99   62.2 + j25.2   1.51     0.18      0.00      -0.25
   100   64.3 + j30.0   1.58     0.22      0.00      -0.29
   101   66.5 + j34.9   1.65     0.27      0.00      -0.33
   102   68.8 + j39.8   1.74     0.33      0.00      -0.38
   103   71.1 + j44.6   1.83     0.39      0.00      -0.43
   104   73.5 + j49.4   1.92     0.46      0.00      -0.49
   105   75.9 + j54.1   2.02     0.53      0.00      -0.55
   106   78.5 + j58.9   2.12     0.60      0.00      -0.61
   107   81.2 + j63.8   2.22     0.68      0.00      -0.68
   108   83.9 + j68.5   2.33     0.75      0.00      -0.75

Patterns

Antenna Comparison

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 75:300Ω push-on ferrite balun.

6% of FM broadcast signals in the U.S. today are horizontally polarized. A vertically polarized antenna will not receive these signals well.

Antenna File

Two-Conductor Broadband Dipole
Free Space
88 98 108 MHz
5 6063-T832 wires, inches
f = .25
l = 26.90757
s = 21.69137
y = s / 2
z = SQR(l * l - y * y) + f
1   0  y  z   0  0  f   .375
1   0 -y  z   0  0  f   .375
1   0  y -z   0  0 -f   .375
1   0 -y -z   0  0 -f   .375
1   0  0  f   0  0 -f    #14
1 source
Wire 5, center

August 15, 201388108 MHz