Circularly polarized cubical quads provide a simple and inexpensive way to extract orthogonal power from circularly polarized FM signals that linearly polarized antennas ignore. However, the patterns degrade for signals with nonideal circularity. Rear-signal rejection over most of the band is mainly due to cancellation of orthogonal signal components, not to inherent antenna directivity. You can obtain good patterns for nonideal signals by phasing a crossed pair of Yagis with decent wideband patterns.
This array uses crossed five-element Yagis. The boom length is 64″. Tilting the Yagis 45° equalizes ground interaction and any residual coupling to the feedline (the mast section near the antenna should be nonconductive). Blue dots mark analysis segments. The red dot marks the 75Ω feedpoint.
I used the AO 9.67 Antenna Optimizer to optimize both forward gain and the pattern. F/R during optimization used the right-circular field in the forward direction and the total field in the rear quarter-plane. This models interference from the rear with worst-case polarization. I optimized the array at a boom height of 20 feet over ground with dielectric constant 13, conductivity 5 mS/m. This is average ground at 1 MHz, but it is something else at 98 MHz. All results are at 1° elevation angle.
Coincident crossed Yagis must be phased 90°. A simple way to do this is to lengthen one driven element until its phase is +45°, shorten the other to -45°, and connect them in parallel. No phasing line is needed. The resulting impedance is close to that of a single Yagi. The image shows the driven elements from the front.
Crossed Yagis offset a quarter wavelength require no phasing. But in addition to lengthening the boom, this scheme has a major drawback: rear response is right-circular, not left-circular. Rear rejection comes only from inherent directivity, not crosspolarization. This will increase interference from most unwanted signals.
Calculated performance is for the right-circular field using 28 analysis segments per conductor halfwave. The gain reference is a circularly polarized isotropic antenna in free space. Forward gain includes mismatch and conductor losses. Axial ratio is the ratio of maximum to minimum linearly polarized forward power. H/V is the ratio of horizontal to vertical forward power. F/R is the ratio of forward power to that of the worst backlobe in the rear half-plane.
Frequency Impedance SWR Mismatch Conductor Forward Axial H/V F/R
MHz ohms Loss dB Loss dB Gain dBic Ratio dB dB dB
88 101+j8 1.37 0.11 0.01 -1.10 2.66 0.33 14.49
89 99.8+j2.7 1.33 0.09 0.01 -0.99 2.32 0.90 14.84
90 97.9-j0.6 1.31 0.08 0.01 -0.89 2.30 1.37 15.26
91 95.9-j2.7 1.28 0.07 0.01 -0.78 2.38 1.71 15.73
92 93.7-j3.8 1.26 0.06 0.01 -0.65 2.47 1.93 16.27
93 91.5-j4.1 1.23 0.05 0.01 -0.49 2.52 2.08 16.90
94 89.5-j3.9 1.20 0.04 0.01 -0.31 2.51 2.13 17.62
95 87.5-j3.2 1.17 0.03 0.01 -0.12 2.47 2.14 18.47
96 85.6-j2.1 1.14 0.02 0.01 0.09 2.38 2.09 19.48
97 84.0-j0.8 1.12 0.01 0.01 0.32 2.24 1.98 20.74
98 82.4+j0.6 1.10 0.01 0.01 0.57 2.07 1.82 22.35
99 80.9+j2.0 1.08 0.01 0.01 0.83 1.80 1.58 23.36
100 79.6+j3.5 1.08 0.01 0.01 1.10 1.44 1.23 23.24
101 78.3+j4.6 1.08 0.01 0.02 1.39 0.84 0.71 23.06
102 77.2+j5.6 1.08 0.01 0.02 1.67 0.16 0.10 22.64
103 75.9+j5.8 1.08 0.01 0.02 1.92 0.78 -0.60 21.64
104 73.5+j5.0 1.07 0.01 0.02 2.13 1.94 -1.22 20.68
105 69.0+j4.8 1.11 0.01 0.03 2.26 3.28 -1.36 19.59
106 64.7+j6.9 1.20 0.03 0.04 2.30 4.35 -0.54 17.89
107 64.0+j6.9 1.21 0.04 0.05 2.38 4.02 1.25 16.51
108 56.2+j8.3 1.37 0.11 0.11 1.90 1.26 -0.36 13.55
These patterns show the response to interfering signals with worst-case polarization.
CP Crossed Yagis 20' High 88 90 92 94 96 99 102 105 107 108 MHz 14 6063-T832 wires, inches h = 240 ; boom height s = 1 ; feedpoint half-gap ang = 28.14672 ; driven element angle r = 32.46452 ; element half-lengths de1 = 26.29803 de2 = 32.58456 d1 = 26.47305 d2 = 25.62802 d3 = 23.29429 dep = 21.7494 ; elements positions d1p = 27.04338 d2p = 37.49581 d3p = 63.35833 e = h + s g = h - s f = SQR(2) * s shift z h 1 dep -s 0 dep 0 0 #18 ; RG-6 center conductor 1 dep 0 0 dep s 0 .23 ; RG-6 shield rotate x 45 1 0 -r 0 0 r 0 .375 shift y f z e rotate end1 z -ang 1 dep 0 0 dep de1 0 .375 shift y -f z g rotate end1 z ang 1 dep 0 0 dep -de1 0 .375 rotate z end shift y 0 z h 1 d1p -d1 0 d1p d1 0 .375 1 d2p -d2 0 d2p d2 0 .375 1 d3p -d3 0 d3p d3 0 .375 rotate x -45 1 -1 -r 0 -1 r 0 .375 shift y f z g rotate end1 z -ang 1 dep 0 0 dep de2 0 .375 shift y -f z e rotate end1 z ang 1 dep 0 0 dep -de2 0 .375 rotate z end shift x -1 y 0 z h 1 d1p -d1 0 d1p d1 0 .375 1 d2p -d2 0 d2p d2 0 .375 1 d3p -d3 0 d3p d3 0 .375 1 source Wire 1, end2
Use ⅜″ elements supported by insulated mounting brackets. Symbols r, de1, de2, d1, d2, and d3 are element half-lengths (center to tip), dep, d1p, d2p, and d3p are element positions relative to the reflector (center to center), and ang is the driven element angle. Offset the parasitic elements of the two Yagis 1″ along the boom and orient all elements 45° to the ground. The rear-Yagi elements should go from upper right to lower left as viewed from the front. A 2″ gap separates the paired driven element halves. Route 75Ω coax leads straight across the gap, as modeled, with no excess lead length. Use a current choke at the feedpoint. Read these notes before building anything.
The following table shows the largest performance degradation over the antenna file frequencies in dB when altering a symbol value by Tol.
Symbol Tol Gain F/R
h 0.3937 0.01 0.01
s 0.0394 0.02 0.06
ang 1.0000 0.01 0.37
r 0.0197 0.01 0.02
de1 0.0394 0.02 0.05
de2 0.0394 0.01 0.02
d1 0.0197 0.08 0.29
d2 0.0197 0.10 0.27
d3 0.0197 0.01 0.05
dep 0.0394 0.01 0.06
d1p 0.0394 0.02 0.06
d2p 0.0394 0.02 0.08
d3p 0.0394 0.00 0.02
This compares the crossed Yagis, four circularly polarized cubical quads, Antennacraft FM6, small 5-element Yagi, Antenna Performance Specialties APS-13, 10-element Home Depot Yagi, and Körner 9.2, 15.12, and 19.3 for a right-circular field with the booms 20 feet above 13/5 ground. Boom length precedes the antenna name.
C Circular Hpwr = Vpwr H Horizontal Vpwr = 0 V Vertical Hpwr = 0 h Mostly horizontal Hpwr > Vpwr > 0 v Mostly vertical Vpwr > Hpwr > 0 Class Percent C H V h v All 100 85 4 9 1 1 Full service 51 91 2 4 2 1 Translator 37 76 7 17 0 0 LPFM 10 98 1 1 0 0 Booster 2 57 8 30 1 3
This table lists antenna polarization by service class for U.S. FM broadcast stations as of December 2020.
If you build a right-circular antenna and a favorite station is left-circular, you'll be disappointed. To prevent this, look up important stations in the FCC database. Check the horizontal and vertical transmit power to determine polarization. To determine circularity sense, check the specifications for the antenna make and model at the manufacturer's website. Some manufacturers do not list circularity sense. As best I can tell, current antenna models from the following are right-circular: ERI, Jampro, Micronetixx, PSI nonpanel, SWR nonpanel except the FM1, Progressive Concepts except the CIRPA, Nicom except the BKG 88, and Shively Labs except the 6832, 6842, and Versa2une. Exceptions are left-circular. Some interleaved Dielectric antennas are right-circular for analog and left-circular for HD Radio. Harris FMH and Bext antennas are left-circular.
If you're unable to identify a station's antenna, try to find an image of its tower, perhaps with Google Street View.
These antennas are right-circular.
These antennas are left-circular.
If you can receive a station with a tilted dipole, you can determine its circularity sense by finding whether a left or right tilt maximizes signal strength. When all else fails, contact the station chief engineer.
88–108 MHz