Circularly Polarized Attic Loop

A circularly polarized loop makes a compact attic antenna for the FM broadcast band with higher gain than any other simple antenna. This design was optimized for a height of 12 feet above ground, typical of a single-story attic. 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 triangular loop and analysis segmentation. Blue dots indicate segment boundaries. The red dot locates the 75-ohm feedpoint, which is across the parallel wires at the corner.

I optimized the design with the AO 8.05 Antenna Optimizer at a center height of 12 feet over average-quality ground 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 antenna still has several dB gain over a folded dipole over the entire FM band.

A circularly polarized loop for indoor use is described here.

Modeling Results

Below are calculated performance figures for 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 12 feet high 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.

88.000 MHz:   Impedance         101 - j11 ohms
              SWR                 1.38
              Mismatch Loss       0.11 dB
              Wire Loss           0.05 dB
              Mismatched Gain     6.95 dB
              F/B                19.98 dB

90.000 MHz:   Impedance          92.8 + j9.5 ohms
              SWR                 1.27
              Mismatch Loss       0.06 dB
              Wire Loss           0.05 dB
              Mismatched Gain     6.13 dB
              F/B                29.24 dB

92.000 MHz:   Impedance          96.2 + j25.5 ohms
              SWR                 1.47
              Mismatch Loss       0.16 dB
              Wire Loss           0.04 dB
              Mismatched Gain     5.29 dB
              F/B                16.15 dB

94.000 MHz:   Impedance         109 + j33 ohms
              SWR                 1.67
              Mismatch Loss       0.29 dB
              Wire Loss           0.04 dB
              Mismatched Gain     4.72 dB
              F/B                11.42 dB

96.000 MHz:   Impedance         122 + j36 ohms
              SWR                 1.85
              Mismatch Loss       0.40 dB
              Wire Loss           0.04 dB
              Mismatched Gain     4.24 dB
              F/B                 9.01 dB

98.000 MHz:   Impedance         134 + j23 ohms
              SWR                 1.86
              Mismatch Loss       0.41 dB
              Wire Loss           0.04 dB
              Mismatched Gain     4.30 dB
              F/B                 7.18 dB

100.000 MHz:  Impedance         134 + j7 ohms
              SWR                 1.79
              Mismatch Loss       0.36 dB
              Wire Loss           0.04 dB
              Mismatched Gain     4.48 dB
              F/B                 5.93 dB

102.000 MHz:  Impedance         123 - j4 ohms
              SWR                 1.64
              Mismatch Loss       0.26 dB
              Wire Loss           0.04 dB
              Mismatched Gain     4.73 dB
              F/B                 5.03 dB

104.000 MHz:  Impedance         104 - j11 ohms
              SWR                 1.42
              Mismatch Loss       0.13 dB
              Wire Loss           0.04 dB
              Mismatched Gain     5.22 dB
              F/B                 4.22 dB

106.000 MHz:  Impedance          87.8 - j5.7 ohms
              SWR                 1.19
              Mismatch Loss       0.03 dB
              Wire Loss           0.04 dB
              Mismatched Gain     5.50 dB
              F/B                 3.65 dB

108.000 MHz:  Impedance          72.5 + j2.8 ohms
              SWR                 1.05
              Mismatch Loss       0.00 dB
              Wire Loss           0.04 dB
              Mismatched Gain     5.79 dB
              F/B                 3.13 dB

Patterns

Antenna File

RCP Attic Loop
12' High
90 MHz
10 copper wires, inches
s = .5
x = 19.01295
y = 5.245943
z = 13.76963
shift z 144
1	0  y -x		0  x -x		#14
1	0  x -x		0  x  z		#14
1	s  y -x		s  x -x		#14
1	s  x -x		s  x  x		#14
1	s  y -x		0  x  z		#14
1	s  x  x		s -x  x		#14
1	s -x  x		s -x -x		#14
1	s -x -x		s  y -x		#14
1	s  y -x		0  y -x		#14
1	0  x -x		s  x -x		#14
1 source
Wire 10, end2
1 load
c = 23.59173
Wire 10, end2 c pF

Optimized at 88, 90, and 92 MHz
25% gain, 75% F/B
F/B OK > 20 dB
F/B region = 135 deg
54 segments per halfwave
No bent-wire correction

Use #14 bare copper wire. The outer loop is 38" on a side. The triangular inner loop is spaced 1/2" from the outer loop. The parallel wire and the slanted wire begin 13-3/4" from the right-hand wires and join 5-1/4" below the upper wire. Across the two wires at the lower right corner solder 75-ohm coax with a 24-pF capacitor in series with the center conductor. Coil the coax into a current balun at the feedpoint.

More is here.

Updated February 7, 2008