Wideband Vertical Omni
A circularly polarized antenna that rejects multipath reflections is often best, but you may want to receive signals from all directions or cover the whole FM broadcast band. With ten feet of aluminum tubing you can make a vertically polarized omnidirectional antenna that does this. The design is a two-conductor dipole with the conductors spread to act as a single wide conductor. This provides a broad impedance bandwidth, low mismatch loss, and a convenient 75-ohm 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 about 1.7 dB at the band edges. Subtract another 0.75 dB for balun loss if you must convert to 75 ohms. In contrast, the wideband omni described here is down only 0.5 dB at the band edges and needs no balun other than a lossless coiling of the coaxial feedline.
Design
The antenna consist of two conductors, each 55" long and 3/8" in diameter, separated 21-7/8" at the ends and connected in parallel at the centers.
Construction
Mount a small wooden board to a mast made of nonconductive material, such as wood or PVC. Cut four 27-1/4" lengths of 3/8"-diameter aluminum tubing. Flatten one end of each tube and drill a hole. 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 21-7/8" apart center-to-center. Place the tubes on the board with one pair of connected tubes extending above and the other below, as shown above. Position the flattened ends 1/2" apart. Fasten the tubes to the board using any convenient method, such as tie wraps through holes or small wood screws. Using two washers and a nut, attach the wire from the lower tubes to the shell of a panel-mount F-connector positioned within the 1/2" gap. Crimp the wire from the upper tubes to 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 outside of the coax, coil it into a current balun at the feedpoint. Route the feedline down the mast. To even better isolate it, coil the feedline into a second balun 30" below the first.
This is a test antenna made of #14 wire. SWR bandwidth is narrower and gain somewhat lower than for an antenna made of 3/8" tubing. It's best to form the coiled-feedline balun closer to the feedpoint than shown. A horizontal line helps stabilize the flimsy test mast.
Performance
I optimized the antenna with the AO 8.00 Antenna Optimizer. Below are results in free space for a segmentation density of 40 segments per halfwave. Mismatch loss is due to SWR. Ohmic loss is due to conductor resistance. Mismatched gain is the average azimuth response including ohmic and mismatch losses. Balun loss is not included. The SWR reference impedance is 75 ohms.
Frequency 88 93 98 103 108 MHz Mismatched Gain -0.80 -0.38 -0.34 -0.55 -0.87 dBd Impedance 44-j29 53-j4 63+j20 74+j44 88+j67 ohms SWR 2.05 1.43 1.41 1.79 2.27 Mismatch Loss 0.54 0.14 0.13 0.36 0.71 dB Ohmic Loss 0.00 0.00 0.00 0.00 0.00 dB
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 omni and dipole curves assume that the feedline need not be lengthened to coil it and include no balun loss. The folded dipole curve includes -0.75 dB for the loss of a 75:300-ohm balun.
Antenna Model
Two-Conductor Broadband Dipole Free Space 98 MHz 4 6063-T832 wires, inches l = 27.49985 y = 10.93285 z = SQR(l * l - y * y) 1 0 y z 0 0 0 .375 1 0 -y z 0 0 0 .375 1 0 y -z 0 0 0 .375 1 0 -y -z 0 0 0 .375 2 sources Wire 1, end2 Wire 2, end2
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Updated March 11, 2008
