This two-element cubical quad is configured for right-circular polarization and optimized for the low end of the FM broadcast band.
This image shows the antenna geometry. The forward lobe is along the X axis. For left-circular polarization, rotate the driven element 180° in the horizontal plane.
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 driven element.
I optimized the design with the AO-Pro 8.05 Antenna Optimizer for good gain and small backlobes at the low end of the band. Because the antenna captures orthogonal power unavailable to a linearly polarized antenna, forward gain from 88–92 MHz is similar to that of a ten-foot Yagi. Although performance degrades higher in frequency, the antenna remains surprisingly usable over the entire band. This is remarkable for a compact directive antenna with a boom length of two feet.
Below are calculated performance figures for a segmentation density of 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 linearly polarized dipole. F/R 120°-240° is the ratio of forward power to that of the worst backlobe from 120° to 240° to the rear. Ellipticity is the ratio of minimum to maximum linearly polarized forward response expressed in dB. The SWR reference impedance is 75Ω.
88.000 MHz: Impedance 79.5 + j1.1 ohms
SWR 1.06
Mismatch Loss 0.00 dB
Wire Loss 0.08 dB
Mismatched Gain 7.80 dBd
F/R 120°-240° 30.00 dB
Ellipticity -5.28 dB
90.000 MHz: Impedance 73.2 + j0.1 ohms
SWR 1.02
Mismatch Loss 0.00 dB
Wire Loss 0.06 dB
Mismatched Gain 7.41 dBd
F/R 120°-240° 32.59 dB
Ellipticity -3.16 dB
92.000 MHz: Impedance 72.2 - j0.2 ohms
SWR 1.04
Mismatch Loss 0.00 dB
Wire Loss 0.05 dB
Mismatched Gain 6.94 dBd
F/R 120°-240° 29.20 dB
Ellipticity -2.55 dB
98.000 MHz: Impedance 67.4 + j4.4 ohms
SWR 1.13
Mismatch Loss 0.02 dB
Wire Loss 0.04 dB
Mismatched Gain 5.92 dBd
F/R 120°-240° 19.75 dB
Ellipticity -3.67 dB
103.000 MHz: Impedance 71.3 + j16.2 ohms
SWR 1.25
Mismatch Loss 0.06 dB
Wire Loss 0.04 dB
Mismatched Gain 5.28 dBd
F/R 120°-240° 15.05 dB
Ellipticity -5.57 dB
108.000 MHz: Impedance 87.0 + j20.1 ohms
SWR 1.34
Mismatch Loss 0.09 dB
Wire Loss 0.04 dB
Mismatched Gain 4.77 dBd
F/R 120°-240° 12.33 dB
Ellipticity -7.34 dB
CP Cubical Quad Free Space 88 90 92 MHz 17 copper wires, inches s = .5 f = 7.426954 x = 17.17732 r = 18.0908 v = -23.70532 1 0 0 -x 0 f -x #14 ; Driven element 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 v -r -r v r -r #14 ; Reflector 1 v r -r v r r #14 1 v r r v -r r #14 1 v -r r v -r -r #14 1 source Wire 13, end2 1 load c = 19.40773 Wire 13, end2 c pF Trade-offs: 50% gain, 50% F/B F/B OK > 30 dB F/B region = 120-240 deg 54 segments per halfwave No bent-wire correction
Use #14 bare copper wire supported by nonconductive spreaders. The reflector loop is 36 3⁄16″ on a side, while the driven loop is 343⁄8″. The loops are 243⁄16″ apart. The inner driven loop is exactly half the size of the outer and is spaced ½″ toward the reflector. The corner of the inner loop connects to the center of the lower wire of the outer loop. The feedpoint is 7 7⁄16″ from the junction. Across the two wires at this point solder 75Ω coax with a 20-pF capacitor in series with the center conductor. The shield can connect to either wire. Use a current balun at the feedpoint.
88–108 MHz