The great majority of FM broadcast signals in the U.S. are circularly polarized. You can take advantage of them with an end-fire array of square loops. Parasitic loops respond to circular fields without modification. The addition of a diagonal conductor to a split driven loop promotes a circular response. Power recovered from the orthogonal field increases forward gain and helps cancel rear signals. The antennas also respond to horizontal and vertical linear fields, used mostly by translator and booster stations. I designed the antennas for right-circular polarization, which seems far more common than left-circular. They will attenuate right-circular signals that become left-circular upon reflection, which can reduce multipath interference. Flip the driven loop in the horizontal plane to reverse the circularity sense.
Free-space models are inadequate for circularly polarized designs because antenna height and ground quality affect circularity. To represent a typical installation, I optimized all designs at a boom height of 20 feet over ground with dielectric constant 13, conductivity 5 mS/m. This is average ground at 1 MHz, but it is something else at 98 MHz.
Calculated performance is for a perfectly circular transmit signal with a single ground reflection. But transmit antennas may exhibit an axial ratio of several dB, especially when the tower structure is not properly accounted for. In addition, scattering may occur multiple times during propagation over irregular terrain. Each instance differentially alters the orthogonal fields, which degrades circularity. Finally, antenna height and ground characteristics may differ from those modeled. Because of these factors, rear rejection may be substantially lower than calculated for many signals. Although it is less sensitive, forward gain also may decline. Axial ratio measurements in irregular terrain showed wide variation among broadcast signals.
Given the likely axial ratio variability, I used the AO 9.61 Antenna Optimizer to maximize forward gain without regard for the pattern. The two- and four-element designs provide high gain in small spaces. These compact antennas have turning radii of just 25″ and 32″. The larger antennas provide even more gain. None of the designs has decent rear-signal rejection over the entire band unless the signal is right-circular with favorable axial ratio.
Calculated performance is for 17 analysis segments per conductor halfwave with bent-wire correction. The gain reference is a circularly polarized isotropic antenna in free space. All results are at 1° elevation angle.
Blue dots mark analysis segments. The red dot is the 300Ω feedpoint. The boom length is 25″.
Forward gain includes mismatch and conductor losses. Axial ratio is the ratio of maximum to minimum linearly polarized forward power. H/V is the ratio of horizontal to vertical forward power. F/R is the ratio of forward power to that of the worst backlobe in the rear half-plane.
Frequency Impedance SWR Mismatch Conductor Forward Axial H/V F/R
MHz ohms Loss dB Loss dB Gain dBic Ratio dB dB dB
88 318+j141 1.58 0.23 0.06 -1.72 7.98 0.13 12.07
89 424+j116 1.60 0.24 0.05 -1.41 6.68 -0.95 11.38
90 476+j45 1.61 0.25 0.04 -1.30 5.43 -1.55 11.03
91 479-j15 1.60 0.24 0.04 -1.27 4.36 -1.74 10.91
92 463-j50 1.57 0.22 0.03 -1.30 3.52 -1.68 10.96
93 445-j68 1.54 0.20 0.03 -1.34 2.90 -1.53 11.10
94 428-j78 1.52 0.19 0.03 -1.38 2.52 -1.44 11.31
95 414-j80 1.48 0.17 0.02 -1.41 2.19 -1.30 11.56
96 402-j80 1.45 0.15 0.02 -1.42 1.96 -1.25 11.86
97 391-j78 1.42 0.13 0.02 -1.42 1.80 -1.25 12.20
98 380-j74 1.38 0.11 0.02 -1.41 1.70 -1.30 12.48
99 370-j68 1.34 0.09 0.02 -1.39 1.64 -1.40 12.03
100 359-j62 1.30 0.07 0.02 -1.35 1.64 -1.52 11.61
101 349-j54 1.25 0.05 0.02 -1.31 1.69 -1.66 11.19
102 340-j46 1.21 0.04 0.02 -1.28 1.84 -1.84 10.77
103 329-j34 1.15 0.02 0.02 -1.24 2.01 -1.96 10.39
104 318-j21 1.09 0.01 0.02 -1.22 2.23 -2.04 10.01
105 307-j5 1.03 0.00 0.03 -1.20 2.51 -2.08 9.66
106 297+j13 1.04 0.00 0.03 -1.19 2.83 -2.08 9.32
107 288+j34 1.13 0.02 0.03 -1.21 3.18 -2.01 8.98
108 280+j57 1.23 0.05 0.03 -1.25 3.56 -1.90 8.65
These patterns show the response to interfering signals with worst-case polarization.
2-el CP Quad 20' High 88 93 98 103 108 MHz 9 copper wires, inches ang = -11.58096 ; skew compensation a = 12.12322 ; driven element lower wire b = 21.4524 ; driven element half-side c = 16.14408 ; driven element diagonal wire r = 17.62746 ; reflector half-side s = 24.92588 ; loop spacing q = .5 * s ; driven element position p = -q ; reflector position shift z 20' rotate z ang 1 p -r -r p r -r #14 ; reflector 1 p r -r p r r #14 1 p r r p -r r #14 1 p -r r p -r -r #14 1 q -a -b q b -b #14 ; driven element 1 q b -b q b b #14 1 q b b q -b b #14 1 q -b b q -b -b #14 1 q -b -b q c c #14 1 source Wire 5, end2
Use #14 bare copper wire supported by nonconductive X-shaped spreaders. The driven loop is 42⅞″ on three sides. The bottom wire is 339⁄16″ long. The diagonal wire slanted 45° is 533⁄16″ long. The reflector loop is 35¼″ on each side and spaced 2415⁄16″ from the driven loop. Use a compact, low-loss L-network balun. To decouple the feedline, install current chokes at 30″ intervals. The last one should be several feet from the antenna. For best performance the mast section near the antenna should be nonconductive.
The construction method described below for the five-element design will work. But to reduce visibility in a residential setting, use thin, tapered fiberglass spreaders mounted on a small-diameter boom, both with muted color. You can eliminate the boom by using sloping spreaders and a custom central hub.
The model includes an azimuth rotation to compensate for pattern skew. When aligning a rotor indicator or installing the antenna in a fixed direction, point the boom 12° to the left of the intended bearing.
The following table shows the largest performance degradation over 88, 93, 98, 103, and 108 MHz in dB when altering a symbol value by Tol.
Symbol Tol Gain F/R
ang 1.0000 0.04 0.29
a 0.0394 0.00 0.00
b 0.0197 0.04 0.11
c 0.0394 0.02 0.04
r 0.0197 0.04 0.00
s 0.0394 0.00 0.00
This design uses four elements on three spreaders. The additional reflector loop improves forward gain 0.4 dB over much of the band. The boom length is 51″.
Forward gain includes mismatch and conductor losses. Axial ratio is the ratio of maximum to minimum linearly polarized forward power. H/V is the ratio of horizontal to vertical forward power. F/R is the ratio of forward power to that of the worst backlobe in the rear half-plane.
Frequency Impedance SWR Mismatch Conductor Forward Axial H/V F/R
MHz ohms Loss dB Loss dB Gain dBic Ratio dB dB dB
88 243-j13 1.24 0.05 0.05 -1.09 7.96 1.85 16.86
89 290-j1 1.03 0.00 0.04 -0.79 6.73 1.30 16.57
90 324-j3 1.08 0.01 0.03 -0.68 5.77 1.02 16.13
91 346-j9 1.16 0.02 0.03 -0.63 5.07 0.90 15.96
92 359-j15 1.20 0.04 0.03 -0.60 4.58 0.86 15.84
93 366-j17 1.23 0.05 0.03 -0.57 4.23 0.80 15.35
94 371-j18 1.25 0.05 0.02 -0.53 4.00 0.69 14.99
95 374-j17 1.25 0.06 0.02 -0.47 3.79 0.57 14.66
96 376-j13 1.26 0.06 0.02 -0.40 3.60 0.38 14.38
97 379-j9 1.27 0.06 0.02 -0.32 3.41 0.13 14.14
98 382-j5 1.27 0.06 0.02 -0.22 3.20 -0.19 13.94
99 385-j1 1.28 0.07 0.03 -0.12 3.02 -0.52 13.80
100 389+j1 1.30 0.07 0.03 0.00 2.82 -0.96 13.70
101 393+j0 1.31 0.08 0.03 0.12 2.65 -1.44 13.65
102 394-j4 1.31 0.08 0.03 0.25 2.59 -1.96 13.66
103 389-j11 1.30 0.07 0.03 0.37 2.69 -2.50 13.73
104 377-j20 1.26 0.06 0.03 0.49 3.02 -3.02 13.86
105 353-j24 1.20 0.04 0.04 0.57 3.61 -3.47 14.02
106 321-j19 1.10 0.01 0.04 0.59 4.48 -3.75 14.16
107 283-j0 1.06 0.00 0.05 0.49 5.63 -3.75 14.17
108 249+j35 1.26 0.06 0.05 0.19 7.04 -3.37 13.13
These patterns show the response to interfering signals with worst-case polarization.
4-el CP Quad 20' High 88 93 98 103 108 MHz 17 copper wires, inches r = 17.68269 ; inner reflector half-side s = 20.01408 ; outer reflector half-side a = 14.82155 ; driven element lower wire b = 19.63566 ; driven element half-side c = 21.63432 ; driven element diagonal wire d1 = 13.58508 ; director half-side dep = 26.51343 ; driven element position dex = dep + 1 ; diagonal wire tip position d1p = 50.84081 ; director position shift z 20' 1 0 -r -r 0 r -r #14 ; inner reflector 1 0 r -r 0 r r #14 1 0 r r 0 -r r #14 1 0 -r r 0 -r -r #14 1 0 -s -s 0 s -s #14 ; outer reflector 1 0 s -s 0 s s #14 1 0 s s 0 -s s #14 1 0 -s s 0 -s -s #14 1 dep -a -b dep b -b #14 ; driven element 1 dep b -b dep b b #14 1 dep b b dep -b b #14 1 dep -b b dep -b -b #14 1 dep -b -b dex c c #14 1 d1p -d1 -d1 d1p d1 -d1 #14 ; director 1 d1p d1 -d1 d1p d1 d1 #14 1 d1p d1 d1 d1p -d1 d1 #14 1 d1p -d1 d1 d1p -d1 -d1 #14 1 source Wire 9, end2
Construct like the five-element design described below. The length of the driven element lower wire is 347⁄16″. The length of the diagonal wire is 58⅜″.
If necessary, span the reflector wires with thin polystyrene rods to stabilize the spacing. A simple attachment method is to heat the wires and melt them into the plastic.
The following table shows the largest performance degradation over 88, 93, 98, 103, and 108 MHz in dB when altering a symbol value by Tol.
Symbol Tol Gain F/R
r 0.0197 0.01 0.12
s 0.0197 0.00 0.01
a 0.0394 0.00 0.00
b 0.0197 0.03 0.08
c 0.0394 0.00 0.02
d1 0.0197 0.04 0.19
dep 0.0394 0.00 0.02
d1p 0.0394 0.00 0.01
This design uses five elements on a 115″ boom.
Forward gain includes mismatch and conductor losses. Axial ratio is the ratio of maximum to minimum linearly polarized forward power. H/V is the ratio of horizontal to vertical forward power. F/R is the ratio of forward power to that of the worst backlobe in the rear half-plane.
Frequency Impedance SWR Mismatch Conductor Forward Axial H/V F/R
MHz ohms Loss dB Loss dB Gain dBic Ratio dB dB dB
88 292+j41 1.15 0.02 0.05 -0.04 7.11 0.39 17.61
89 344+j31 1.18 0.03 0.04 0.22 5.81 -0.09 16.65
90 372+j9 1.24 0.05 0.04 0.33 4.75 -0.21 16.17
91 384-j8 1.28 0.07 0.03 0.39 3.99 -0.22 15.98
92 387-j18 1.30 0.07 0.03 0.44 3.50 -0.11 15.96
93 387-j23 1.30 0.08 0.03 0.50 3.18 -0.04 15.41
94 388-j25 1.31 0.08 0.03 0.58 2.99 -0.12 14.98
95 388-j24 1.31 0.08 0.03 0.70 2.80 -0.23 14.58
96 389-j23 1.31 0.08 0.03 0.84 2.61 -0.44 14.21
97 390-j23 1.31 0.08 0.03 1.00 2.42 -0.73 13.87
98 390-j25 1.31 0.08 0.03 1.17 2.22 -1.08 13.54
99 388-j29 1.31 0.08 0.03 1.36 2.09 -1.47 13.24
100 380-j34 1.29 0.07 0.03 1.55 2.02 -1.84 12.95
101 364-j37 1.25 0.05 0.04 1.73 2.11 -2.11 12.68
102 342-j33 1.18 0.03 0.04 1.90 2.40 -2.20 12.46
103 319-j20 1.09 0.01 0.04 2.03 2.82 -2.00 12.29
104 299+j2 1.01 0.00 0.05 2.14 3.28 -1.49 12.24
105 287+j27 1.11 0.01 0.06 2.29 3.54 -0.74 12.40
106 281+j49 1.20 0.04 0.07 2.63 3.21 0.02 12.91
107 269+j66 1.29 0.07 0.12 3.36 1.61 -0.38 13.77
108 255+j21 1.20 0.04 0.27 3.35 6.41 -6.15 14.82
These patterns show the response to interfering signals with worst-case polarization.
5-el CP Quad 20' High 88 90 98 106 108 MHz 21 copper wires, inches r = 17.71214 ; reflector half-side a = 14.49214 ; driven element lower wire b = 20.20037 ; driven element half-side c = 21.99771 ; driven element diagonal wire d1 = 13.7816 ; director half-sides d2 = 13.88908 d3 = 14.00479 dep = 27.06915 ; driven element position dex = dep + 1 ; diagonal wire tip position d1p = 51.54301 ; director positions d2p = 82.69584 d3p = 114.5893 shift z 20' 1 0 -r -r 0 r -r #14 ; reflector 1 0 r -r 0 r r #14 1 0 r r 0 -r r #14 1 0 -r r 0 -r -r #14 1 dep -a -b dep b -b #14 ; driven element 1 dep b -b dep b b #14 1 dep b b dep -b b #14 1 dep -b b dep -b -b #14 1 dep -b -b dex c c #14 1 d1p -d1 -d1 d1p d1 -d1 #14 ; director 1 1 d1p d1 -d1 d1p d1 d1 #14 1 d1p d1 d1 d1p -d1 d1 #14 1 d1p -d1 d1 d1p -d1 -d1 #14 1 d2p -d2 -d2 d2p d2 -d2 #14 ; director 2 1 d2p d2 -d2 d2p d2 d2 #14 1 d2p d2 d2 d2p -d2 d2 #14 1 d2p -d2 d2 d2p -d2 -d2 #14 1 d3p -d3 -d3 d3p d3 -d3 #14 ; director 3 1 d3p d3 -d3 d3p d3 d3 #14 1 d3p d3 d3 d3p -d3 d3 #14 1 d3p -d3 d3 d3p -d3 -d3 #14 1 source Wire 5, end2
Use #14 bare copper wire supported by ½″ PVC pipe (0.84″ OD). The length of the driven element lower wire is 3411⁄16″. The length of the diagonal wire is 5911⁄16″. It extends a couple inches past the corner so place them on opposite sides of the spreader. Use a 10-foot piece of 1½″ ABS pipe (1.9″ OD) for the boom. Use nonconductive upper and lower boom guys. In windy areas, add side guys. Mount the spreaders with PVC conduit clamps. Secure the clamps with a sheet metal screw. Use a compact, low-loss L-network balun. To decouple the feedline, install current chokes at 30″ intervals. The last one should be several feet from the antenna. The mast section near the antenna should be nonconductive. Read these notes before building anything.
The following table shows the largest performance degradation over 88, 93, 98, 103, and 108 MHz in dB when altering a symbol value by Tol.
Symbol Tol Gain F/R
r 0.0197 0.02 0.12
a 0.0394 0.00 0.00
b 0.0197 0.04 0.07
c 0.0394 0.02 0.02
d1 0.0197 0.23 0.43
d2 0.0197 0.31 0.40
d3 0.0197 0.23 0.58
dep 0.0394 0.01 0.02
d1p 0.0394 0.01 0.01
d2p 0.0394 0.01 0.01
d3p 0.0394 0.01 0.02
This design uses seven elements on a 186″ boom.
Forward gain includes mismatch and conductor losses. Axial ratio is the ratio of maximum to minimum linearly polarized forward power. H/V is the ratio of horizontal to vertical forward power. F/R is the ratio of forward power to that of the worst backlobe in the rear half-plane.
Frequency Impedance SWR Mismatch Conductor Forward Axial H/V F/R
MHz ohms Loss dB Loss dB Gain dBic Ratio dB dB dB
88 293+j35 1.13 0.02 0.05 0.60 7.03 0.37 18.96
89 342+j30 1.17 0.03 0.04 0.91 5.90 -0.11 18.59
90 372+j12 1.24 0.05 0.04 1.07 4.97 -0.31 18.25
91 385-j3 1.28 0.07 0.03 1.18 4.27 -0.37 18.21
92 391-j14 1.31 0.08 0.03 1.27 3.75 -0.39 17.72
93 395-j20 1.32 0.09 0.03 1.37 3.36 -0.43 17.32
94 398-j25 1.34 0.09 0.03 1.49 3.07 -0.59 17.09
95 400-j29 1.35 0.10 0.03 1.63 2.75 -0.73 16.92
96 400-j33 1.35 0.10 0.03 1.79 2.41 -0.90 16.82
97 397-j39 1.35 0.10 0.03 1.97 2.05 -1.07 16.77
98 389-j45 1.34 0.09 0.03 2.16 1.69 -1.19 16.74
99 377-j49 1.31 0.08 0.03 2.37 1.38 -1.24 16.71
100 361-j47 1.26 0.06 0.04 2.59 1.15 -1.15 16.66
101 343-j39 1.20 0.04 0.04 2.83 1.05 -0.92 16.51
102 326-j27 1.13 0.02 0.04 3.11 1.04 -0.64 16.20
103 312-j10 1.05 0.00 0.05 3.47 0.99 -0.44 15.92
104 300+j8 1.03 0.00 0.06 3.93 0.91 -0.58 15.64
105 295+j25 1.09 0.01 0.08 4.50 1.55 -1.46 15.23
106 283+j25 1.11 0.01 0.12 4.93 3.48 -3.14 14.44
107 232+j48 1.37 0.11 0.17 4.60 5.62 -3.37 13.06
108 244+j70 1.39 0.12 0.38 4.10 3.66 -2.89 12.51
These patterns show the response to interfering signals with worst-case polarization.
7-el CP Quad 20' High 88 89 90 98 106 107 108 MHz 29 copper wires, inches r = 17.68041 ; reflector half-side a = 14.47533 ; driven element lower wier b = 20.19889 ; driven element half-side c = 22.44163 ; driven element diagonal wire d1 = 13.8335 ; director half-sides d2 = 14.07744 d3 = 13.784 d4 = 13.74143 d5 = 13.91304 dep = 29.19122 ; driven element position dex = dep + 1 ; diagonal wire tip position d1p = 50.496 ; director positions d2p = 78.3315 d3p = 116.5934 d4p = 151.7725 d5p = 186.4434 shift z 20' 1 0 -r -r 0 r -r #14 ; reflector 1 0 r -r 0 r r #14 1 0 r r 0 -r r #14 1 0 -r r 0 -r -r #14 1 dep -a -b dep b -b #14 ; driven element 1 dep b -b dep b b #14 1 dep b b dep -b b #14 1 dep -b b dep -b -b #14 1 dep -b -b dex c c #14 1 d1p -d1 -d1 d1p d1 -d1 #14 ; director 1 1 d1p d1 -d1 d1p d1 d1 #14 1 d1p d1 d1 d1p -d1 d1 #14 1 d1p -d1 d1 d1p -d1 -d1 #14 1 d2p -d2 -d2 d2p d2 -d2 #14 ; director 2 1 d2p d2 -d2 d2p d2 d2 #14 1 d2p d2 d2 d2p -d2 d2 #14 1 d2p -d2 d2 d2p -d2 -d2 #14 1 d3p -d3 -d3 d3p d3 -d3 #14 ; director 3 1 d3p d3 -d3 d3p d3 d3 #14 1 d3p d3 d3 d3p -d3 d3 #14 1 d3p -d3 d3 d3p -d3 -d3 #14 1 d4p -d4 -d4 d4p d4 -d4 #14 ; director 4 1 d4p d4 -d4 d4p d4 d4 #14 1 d4p d4 d4 d4p -d4 d4 #14 1 d4p -d4 d4 d4p -d4 -d4 #14 1 d5p -d5 -d5 d5p d5 -d5 #14 ; director 5 1 d5p d5 -d5 d5p d5 d5 #14 1 d5p d5 d5 d5p -d5 d5 #14 1 d5p -d5 d5 d5p -d5 -d5 #14 1 source Wire 5, end2
Use elements and spreaders as described for the five-element design. A spliced ABS boom might work if fully guyed, but an aluminum boom would be better. The length of the driven element lower wire is 3411⁄16″. The length of the diagonal wire is 605⁄16″.
The following table shows the largest performance degradation over 88, 93, 98, 103, and 108 MHz in dB when altering a symbol value by Tol.
Symbol Tol Gain F/R
r 0.0197 0.04 0.15
a 0.0394 0.00 0.01
b 0.0197 0.04 0.03
c 0.0394 0.03 0.03
d1 0.0197 0.24 0.03
d2 0.0197 0.37 0.42
d3 0.0197 0.24 0.09
d4 0.0197 0.14 0.23
d5 0.0197 0.04 0.03
dep 0.0394 0.00 0.00
d1p 0.0394 0.00 0.00
d2p 0.0394 0.00 0.00
d3p 0.0394 0.00 0.01
d4p 0.0394 0.01 0.01
d5p 0.0394 0.00 0.00
This compares the quads, circularly polarized crossed Yagis, Antennacraft FM6, small 5-element Yagi, Antenna Performance Specialties APS-13, 10-element Home Depot Yagi, and Körner 9.2, 15.12, and 19.3 for a right-circular field with the booms 20 feet above 13/5 ground. Boom length precedes the antenna name.
C Circular Hpwr = Vpwr H Horizontal Vpwr = 0 V Vertical Hpwr = 0 h Mostly horizontal Hpwr > Vpwr > 0 v Mostly vertical Vpwr > Hpwr > 0 Class Percent C H V h v All 100 85 4 9 1 1 Full service 51 91 2 4 2 1 Translator 37 76 7 17 0 0 LPFM 10 98 1 1 0 0 Booster 2 57 8 30 1 3
This table lists antenna polarization by service class for U.S. FM broadcast stations as of December 2020.
If you build a right-circular antenna and a favorite station is left-circular, you'll be disappointed. To prevent this, look up important stations in the FCC database. Check the horizontal and vertical transmit power to determine polarization. To determine circularity sense, check the specifications for the antenna make and model at the manufacturer's website. Some manufacturers do not list circularity sense. As best I can tell, current antenna models from the following are right-circular: ERI, Jampro, Micronetixx, PSI nonpanel, SWR nonpanel except the FM1, Progressive Concepts except the CIRPA, Nicom except the BKG 88, and Shively Labs except the 6832, 6842, and Versa2une. Exceptions are left-circular. Some interleaved Dielectric antennas are right-circular for analog and left-circular for HD Radio. Harris FMH and Bext antennas are left-circular.
If you're unable to identify a station's antenna, try to find an image of its tower, perhaps with Google Street View.
These antennas are right-circular.
These antennas are left-circular.
If you can receive a station with a tilted dipole, you can determine its circularity sense by finding whether a left or right tilt maximizes signal strength. When all else fails, contact the station chief engineer.
88–108 MHz