A four-foot wire loop can improve reception of circularly polarized FM broadcast signals that the great majority of U.S. stations transmit. In addition to several dB of forward gain, the antenna can attenuate right-circular signals to the rear to reduce interference and left-circular signals in the forward direction to lower multipath distortion. The loop also responds to horizontal and vertical linear fields, used mostly by translator and booster stations.
This image shows the antenna geometry. All wires are in the same plane. Maximum response is along the X axis when the plane is oriented 25° from broadside to it as shown. Polarization is right-circular toward X and left-circular in the opposite direction. Blue dots mark analysis segments. The red dot is the 300Ω feedpoint. I optimized the design for maximum forward gain with the AO 8.51 Antenna Optimizer.
I modeled the top wire 102″ above ground with dielectric constant 13, conductivity 5 mS/m. This is average ground quality at 1 MHz but something else at 98 MHz. The model used 17 analysis segments per conductor halfwave with bent-wire correction. Forward gain includes mismatch and conductor losses. The gain reference for the plot above is a linearly polarized isotropic antenna in free space. For the results below it is a 58¼″ folded dipole at a peak height of 102″ for two cases: HFD is with it horizontal, while TFD is with it tilted 45°. F/B is the ratio of forward power to that directly to the rear. Axial ratio is the ratio of maximum to minimum linearly polarized forward power. H/V is the ratio of horizontal to vertical forward power.
Frequency Impedance SWR Mismatch Conductor Gain Gain F/B Axial H/V
MHz ohms Loss dB Loss dB HFD dB TFD dB dB Ratio dB dB
88 271-j1 1.11 0.01 0.03 5.55 3.97 12.91 2.03 -1.92
89 299+j16 1.06 0.00 0.02 5.32 3.65 12.85 2.47 -2.38
90 331+j29 1.14 0.02 0.02 5.15 3.40 12.75 2.89 -2.83
91 366+j34 1.25 0.05 0.02 5.01 3.17 12.62 3.30 -3.26
92 404+j30 1.36 0.10 0.02 4.89 2.98 12.45 3.72 -3.70
93 438+j16 1.47 0.16 0.02 4.80 2.84 12.28 4.11 -4.10
94 467-j8 1.56 0.21 0.02 4.78 2.79 12.11 4.49 -4.49
95 486-j41 1.64 0.26 0.02 4.77 2.76 11.93 4.86 -4.86
96 492-j79 1.71 0.31 0.02 4.79 2.79 11.71 5.24 -5.23
97 483-j116 1.75 0.34 0.02 4.83 2.87 11.51 5.60 -5.56
98 461-j145 1.78 0.35 0.03 4.90 3.00 11.31 5.95 -5.86
99 429-j166 1.78 0.36 0.03 4.98 3.16 11.07 6.30 -6.15
100 395-j173 1.76 0.34 0.03 5.10 3.38 10.82 6.63 -6.38
101 360-j170 1.72 0.31 0.03 5.24 3.63 10.56 6.96 -6.58
102 328-j157 1.65 0.27 0.03 5.39 3.90 10.27 7.27 -6.72
103 301-j138 1.58 0.22 0.03 5.53 4.16 9.94 7.57 -6.82
104 278-j113 1.48 0.17 0.03 5.68 4.44 9.61 7.84 -6.86
105 261-j83 1.39 0.12 0.03 5.81 4.70 9.25 8.10 -6.84
106 250-j52 1.30 0.08 0.03 5.91 4.92 8.87 8.33 -6.78
107 243-j18 1.25 0.05 0.03 5.97 5.11 8.49 8.54 -6.67
108 242+j18 1.25 0.05 0.03 5.99 5.25 8.10 8.73 -6.52
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. Finally, antenna height and ground characteristics may differ from those modeled. Because of these factors, rear rejection is likely to 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.
This shows how the gain and pattern at 98 MHz vary with height of the top wire. The gain reference is a circularly polarized isotropic antenna in free space.
This shows the antenna current distribution for constant source voltage. The trace is perpendicular to the wire plane and projected to the left.
Circularly Polarized Indoor Loop Ceiling Height 88 98 108 MHz 5 copper wires, inches ang = 25.24347 a = 6.368921 b = 23.81123 c = 11.06614 h = 102 - b shift z h rotate z -ang 1 0 -a -b 0 b -b #14 1 0 b -b 0 b b #14 1 0 b b 0 -b b #14 1 0 -b b 0 -b -b #14 1 0 -b -b 0 c c #14 1 source Wire 1, end2
Use #14 bare copper wire supported by nonconductive spreaders. The loop is 47⅝″ on three sides. The diagonal wire is 495⁄16″ long. The bottom wire is 303⁄16″ long. In the bottom corner install a 300Ω ferrite or L-network balun. To improve feedline decoupling, add a current choke 30″ beyond the feedpoint.
To aim the antenna, align its plane perpendicular to the target direction and then rotate it 25° to the left. For right-circular polarization, the feedpoint should be on the left as you look through the loop in the target direction.
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/B
ang 1.0000 0.04 0.13
a 0.0394 0.00 0.01
b 0.0394 0.02 0.07
c 0.0394 0.01 0.02
88–108 MHz