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 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 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, nonconducting, rectangular plate to a ten-foot 1″ 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 ″ 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 screws, washers, and 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, replacing the nail with a screw and nut. Attach 75Ω coax. Weatherproof the coax and connections. Use a current balun at the feedpoint. Route the feedline down the mast and install a second 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. 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.

4% 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

Circularly Polarized Omni

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 average-quality ground, 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.

Modeling Results

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 average-quality ground.

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

Construction

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 35116″. 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 balun 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.

Antenna File

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

September 11, 201888108 MHz