Circularly Polarized Loop

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. It can also attenuate right-circular signals that become left-circular upon ionospheric reflection, which may be good or bad. The loop receives both horizontally polarized and vertically polarized signals, used mostly by translators and boosters.

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.

Modeling Results

I modeled the top wire 102″ above average-quality ground (dielectric constant 13, conductivity 5 mS/m). This represents a ceiling eight feet from a floor 6″ above ground level. 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 58″ 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.

Antenna File

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 49516″ long. The bottom wire is 30316″ long. In the bottom corner install a 75:300Ω balun and follow it with a 75Ω current balun. To improve feedline decoupling, install another current balun 30″ beyond the first.

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.

Sensitivity Analysis

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

May 28, 201788108 MHz