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. For right-circular signals the forward lobe is toward X.
This shows details of the inner loop and analysis segmentation. Blue dots mark segment boundaries. The red dots mark the two terminals of the 75Ω feedpoint. The inner and outer loops connect to the left of the feedpoint. A 300Ω 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 a horizontally polarized 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. However, at 90 MHz the gain over a resonant tilted dipole is only 1.9 dB. If you don't need the loop's directivity and bandwidth, its small gain advantage may not be worth the trouble if you're willing to cut a dipole to resonance.
Renee looks pleased with her acrylic CP loop. She uses it to broadcast music within the Nicholas Building in Melbourne, Australia.
Below are calculated performance figures at 1° elevation angle for the top wire 102″ above ground, representing a ceiling 8′ feet from a floor 6″ 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 horizontal 58¼″ folded dipole with 0.75 dB of balun loss at a height of 102″. (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. Ellipticity is the ratio of minimum to maximum linearly polarized forward response expressed in dB. The SWR reference impedance was 75Ω for the loop and 300Ω 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
Ellipticity -1.43 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
Ellipticity -0.81 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
Ellipticity -2.43 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
Ellipticity -6.31 dB
103.000 MHz: Impedance 144 - j48 ohms
SWR 2.19
Mismatch Loss 0.65 dB
Wire Loss 0.04 dB
Mismatched Gain 4.21 dB
F/B 6.62 dB
Ellipticity -8.86 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
Ellipticity -11.21 dB
This is the pattern at a center height of 12′, typical of a single-story attic. Gain increases 3.8 dB due to the extra height, but the backlobe is larger. Unlike a linearly polarized antenna, the azimuth pattern of a circularly polarized antenna varies somewhat with height. For best performance at low heights, a circularly polarized antenna should be optimized at the installation height, as done here.
CP Loop Ceiling Height 88 90 92 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 Trade-offs: 50% gain, 50% F/B F/B region = 135-225 deg 54 segments per halfwave No bent-wire correction
Use #14 bare copper wire supported by nonconductive spreaders. The outer loop is 385⁄8″ on a side. The inner loop is exactly half that size and spaced ½″ away. The corner of the inner loop connects to the center of the lower wire of the outer loop. The feedpoint is 9¾″ from the junction. Across the two wires at this point solder 75Ω coax with 18 pF of capacitance in series with the center conductor. The shield can connect to either wire. Use a current balun at the feedpoint.
88–108 MHz