Circularly Polarized Indoor Loop

A circularly polarized loop makes a compact indoor antenna for the FM broadcast band with higher gain than any other simple design. Its rejection of right-circular signals to the rear can reduce interference, while rejection of left-circular signals in the forward direction can reduce multipath distortion. The design works best at the low end of the band but is usable over the entire band.

This image shows the antenna geometry. The forward lobe is along the X axis for right-circular signals.

This shows details of the inner loop and analysis segmentation. Blue dots indicate segment boundaries. The red dot locates the 75-ohm feedpoint, which is across the parallel wires. A 300-ohm loop design attributed to Ethan Funk inspired the geometry.

I optimized the design with the AO-Pro 8.05 Antenna Optimizer for installation at ceiling height over average-quality ground. I optimized for a combination of maximum gain and minimum backlobes at the low end of the band. The antenna has considerably more gain than an ordinary folded dipole. Unlike the dipole, it rejects right-circular signals to the rear. Although the pattern degrades higher in frequency, the gain holds up well and the antenna is usable over the entire FM band.

A circularly polarized loop for the attic is described here.

Modeling Results

Below are calculated performance figures for the top wire 102" above ground, representing a ceiling 8 feet from a floor 6 inches above ground level. The model used 54 segments per halfwave. Mismatch loss is due to SWR. Wire loss is due to conductor resistance. Mismatched gain is forward gain, including wire and mismatch losses. The gain reference is a 58-1/4" folded dipole with 0.75 dB of balun loss at a height of 102" in a right-circular field. (Note that the reference antenna is 2.5 dB down from a resonant dipole at the band edges due to mismatch and balun losses.) F/B is the ratio of forward power to that directly to the rear. The SWR reference impedance was 75 ohms for the loop and 300 ohms for the folded dipole. The models used average-quality ground.

88.000 MHz:   Impedance          72.4 - j17.3 ohms
              SWR                 1.27
              Mismatch Loss       0.06 dB
              Wire Loss           0.05 dB
              Mismatched Gain     5.77 dB
              F/B                21.48 dB

90.000 MHz:   Impedance          72.1 + j1.0 ohms
              SWR                 1.04
              Mismatch Loss       0.00 dB
              Wire Loss           0.05 dB
              Mismatched Gain     5.44 dB
              F/B                27.47 dB

92.000 MHz:   Impedance          79.6 + j17.2 ohms
              SWR                 1.26
              Mismatch Loss       0.06 dB
              Wire Loss           0.05 dB
              Mismatched Gain     5.07 dB
              F/B                17.39 dB

94.000 MHz:   Impedance          94.7 + j29.4 ohms
              SWR                 1.52
              Mismatch Loss       0.19 dB
              Wire Loss           0.04 dB
              Mismatched Gain     4.72 dB
              F/B                13.34 dB

96.000 MHz:   Impedance         117 + j34 ohms
              SWR                 1.76
              Mismatch Loss       0.35 dB
              Wire Loss           0.04 dB
              Mismatched Gain     4.45 dB
              F/B                10.93 dB

98.000 MHz:   Impedance         141 + j24 ohms
              SWR                 1.96
              Mismatch Loss       0.48 dB
              Wire Loss           0.04 dB
              Mismatched Gain     4.28 dB
              F/B                 9.26 dB

100.000 MHz:  Impedance         157 - j2 ohms
              SWR                 2.10
              Mismatch Loss       0.58 dB
              Wire Loss           0.04 dB
              Mismatched Gain     4.20 dB
              F/B                 8.02 dB

102.000 MHz:  Impedance         153 - j34 ohms
              SWR                 2.18
              Mismatch Loss       0.64 dB
              Wire Loss           0.04 dB
              Mismatched Gain     4.19 dB
              F/B                 7.04 dB

104.000 MHz:  Impedance         131 - j57 ohms
              SWR                 2.19
              Mismatch Loss       0.65 dB
              Wire Loss           0.04 dB
              Mismatched Gain     4.26 dB
              F/B                 6.23 dB

106.000 MHz:  Impedance         105 - j63 ohms
              SWR                 2.16
              Mismatch Loss       0.63 dB
              Wire Loss           0.04 dB
              Mismatched Gain     4.37 dB
              F/B                 5.54 dB

108.000 MHz:  Impedance          81.7 - j58.9 ohms
              SWR                 2.10
              Mismatch Loss       0.58 dB
              Wire Loss           0.04 dB
              Mismatched Gain     4.51 dB
              F/B                 4.93 dB

Patterns

Antenna File

CP Loop
Ceiling Height
90 MHz
13 copper wires, inches
s = .5
f = 9.753424
x = 19.3115
h = 102 - x							; Keep top wire at ceiling
shift z h
1	0  0 -x		0  f -x		#14
1	0  f -x		0  x -x		#14
1	0  x -x		0  x  0		#14
1	s  0 -x		s  f -x		#14
1	s  f -x		s  x -x		#14
1	s  x -x		s  x  x		#14
1	0  x  0		s  0  0		#14
1	s  0  0		s  0 -x		#14
1	s  x  x		s -x  x		#14
1	s -x  x		s -x -x		#14
1	s -x -x		s  0 -x		#14
1	s  0 -x		0  0 -x		#14
1	0  f -x		s  f -x		#14	; Feedpoint
1 source
Wire 13, end2
1 load
c = 17.97695
Wire 13, end2 c pF

Optimized at 88, 90, and 92 MHz
50% gain, 50% F/B
F/B region = 135 deg
54 segments per halfwave
No bent-wire correction

Use #14 bare copper wire. The outer loop is 38-5/8" on a side. The inner loop is exactly half that size and spaced 1/2" away. The feedpoint is 9-3/4" from the centerline of the loop. Across the two wires at this point solder 75-ohm coax with 18 pF of capacitance in series with the center conductor. Coil the coax into a current balun at the feedpoint.

More is here.

Updated January 28, 2008