Wideband Vertical Omni

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 still is 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 only 0.5 dB at the band edges and uses a lossless current balun.

Design

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.

Construction

Mount a small, flat, rectangular, plate made of wood or plastic to a ten-foot 1″ ABS pipe (1.9″ OD) using PVC conduit clamps. Use a sheet metal screw to secure each clamp to the mast. Cut four 25⅛″ tubes from one ten-foot length of ″ Type M (0.028″ wall) or Type L (0.04″ wall) copper pipe (0.625″ OD). Flatten one end of each tube and solder pairs together. Drill a small hole at the soldered end of each pair and install a screw, two washers, and two nuts for the feedline connection. Spread the tubes until the far ends are 12″ apart center-to-center. Place the tubes on the plate with one pair extending above the feedpoint and the other below, as shown above. Position the inner ends 1″ apart. Fasten the tubes to the plate with ″ pipe clamps (replace the nail with a screw and nut). Attach 75Ω coax. Weatherproof the coax and connections.

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

This test antenna made of #14 wire uses wider end separation. It has a coiled-coax balun 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 balun closer to the feedpoint than shown. A horizontal line helps stabilize the flimsy test mast.

Performance

I optimized the design with the AO 9.61 Antenna Optimizer using 28 analysis segments per conductor halfwave. Average omnidirectional 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

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.

5% 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 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

September 17, 201588108 MHz