Circularly Polarized Cubical Quads

Circular polarization is widely used for FM broadcast signals in the U.S. It's easy to take advantage of it with an end-fire array of square loops. Parasitic loops respond to circular fields without modification, while the addition of a diagonal conductor to the driven loop promotes a traveling wave. Power recovered from the orthogonal field increases forward gain and helps cancel rear signals. The antennas also respond to horizontal and vertical linear fields, used mostly by translator and booster stations. I designed the antennas for right-circular polarization, which seems far more common than left-circular. They will attenuate right-circular signals that become left-circular upon reflection. This includes multipath reflections, which is desirable, and ionospheric reflections, which may or may not be. Flip the driven element in the horizontal plane to reverse the circularity sense.

Free-space models are inadequate for circularly polarized designs because antenna height and ground quality affect circularity. To represent a typical installation, I optimize designs at a boom height of 20 feet over average-quality ground (dielectric constant 13, conductivity 5 mS/m).

Calculated performance is for a perfectly circular transmit signal with a single ground reflection. But transmit antennas may exhibit an axial ratio of several dB, especially when the tower structure is not properly accounted for. In addition, scattering may occur multiple times during propagation over irregular terrain. Each instance differentially alters the orthogonal fields. Finally, antenna height and ground characteristics may differ from those modeled. Because of these factors, rear rejection is likely to be substantially lower than calculated for many signals. Although it is less sensitive, forward gain also may decline. Axial ratio measurements in irregular terrain showed wide variation among broadcast signals.

Given the likely axial ratio variability, I used the AO 9.61 Antenna Optimizer to maximize forward gain without regard for the pattern. The two- and four-element designs are suitable for urban or suburban reception in small spaces. The turning radius of these compact antennas is 25″ and 32″ respectively. The larger designs are suitable for weak-signal reception in rural locations with few interfering signals.

Two Elements

Blue dots mark analysis segments. The red dot is the 300Ω feedpoint. The boom length is 25″.

Modeling Results

Calculated performance is for a boom height of 20 feet over average-quality ground at 1° elevation angle. The model used 17 analysis segments per halfwave with bent-wire correction. 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 response. H/V is the ratio of horizontal to vertical forward response. F/R is the ratio of forward response 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     318 + j141   1.58     0.23      0.06     -1.72      7.98       0.13     12.07
    89     424 + j116   1.60     0.24      0.05     -1.41      6.68      -0.95     11.38
    90     476 + j45    1.61     0.25      0.04     -1.30      5.43      -1.55     11.03
    91     479 - j15    1.60     0.24      0.04     -1.27      4.36      -1.74     10.91
    92     463 - j50    1.57     0.22      0.03     -1.30      3.52      -1.68     10.96
    93     445 - j68    1.54     0.20      0.03     -1.34      2.90      -1.53     11.10
    94     428 - j78    1.52     0.19      0.03     -1.38      2.52      -1.44     11.31
    95     414 - j80    1.48     0.17      0.02     -1.41      2.19      -1.30     11.56
    96     402 - j80    1.45     0.15      0.02     -1.42      1.96      -1.25     11.86
    97     391 - j78    1.42     0.13      0.02     -1.42      1.80      -1.25     12.20
    98     380 - j74    1.38     0.11      0.02     -1.41      1.70      -1.30     12.48
    99     370 - j68    1.34     0.09      0.02     -1.39      1.64      -1.40     12.03
   100     359 - j62    1.30     0.07      0.02     -1.35      1.64      -1.52     11.61
   101     349 - j54    1.25     0.05      0.02     -1.31      1.69      -1.66     11.19
   102     340 - j46    1.21     0.04      0.02     -1.28      1.84      -1.84     10.77
   103     329 - j34    1.15     0.02      0.02     -1.24      2.01      -1.96     10.39
   104     318 - j21    1.09     0.01      0.02     -1.22      2.23      -2.04     10.01
   105     307 - j5     1.03     0.00      0.03     -1.20      2.51      -2.08      9.66
   106     297 + j13    1.04     0.00      0.03     -1.19      2.83      -2.08      9.32
   107     288 + j34    1.13     0.02      0.03     -1.21      3.18      -2.01      8.98
   108     280 + j57    1.23     0.05      0.03     -1.25      3.56      -1.90      8.65

Antenna File

2-el CP Quad
20' High
88 93 98 103 108 MHz
9 copper wires, inches
ang = -11.58096			; skew compensation
a = 12.12322			; driven element lower wire
b = 21.4524			; driven element half-side
c = 16.14408			; driven element diagonal wire
r = 17.62746			; reflector half-side
s = 24.92588			; loop spacing
q = .5 * s			; driven element position
p = -q				; reflector position
shift z 20'
rotate z ang
1  p -r -r  p  r -r  #14	; reflector
1  p  r -r  p  r  r  #14
1  p  r  r  p -r  r  #14
1  p -r  r  p -r -r  #14
1  q -a -b  q  b -b  #14	; driven element
1  q  b -b  q  b  b  #14
1  q  b  b  q -b  b  #14
1  q -b  b  q -b -b  #14
1  q -b -b  q  c  c  #14
1 source
Wire 5, end2

Construction

Use #14 bare copper wire supported by nonconductive X-shaped spreaders. The driven loop is 42⅞″ on three sides. The bottom wire is 33916″ long. The diagonal wire slanted 45° is 53316″ long. The reflector loop is 35¼″ on each side and spaced 241516″ from the driven loop. Use a 75:300Ω balun. To decouple the feedline, starting at the feedpoint install current baluns at 30″ intervals. The last one should be several feet from the antenna. The mast section near the antenna should be nonconductive.

The PVC construction described below for the larger designs will make the small antenna rather conspicuous in a residential setting. Thin, tapered fiberglass spreaders mounted on a small-diameter boom, both with muted color, will reduce visibility. You can eliminate the boom by using sloping spreaders and a complex central hub. A 75:300Ω ferrite balun with twinlead and spade lugs is much smaller and lighter than a coiled halfwave coaxial balun. It will increase loss about 0.4 dB. Although they are less effective, ferrite chokes are lighter and less visible than coiled-coax current baluns.

The model includes an azimuth rotation to compensate for the slight pattern skew characteristic of low antennas in a circular field. For a fixed direction or when aligning a rotor indicator, point the boom 12° to the left of the intended bearing.

Sensitivity Analysis

The following table shows the change in average performance over 88, 93, 98, 103, and 108 MHz in dB when altering a single dimension by ⅛″ (116″ for symbols b and r, which represent half of a loop side, and 5° for ang).

Symbol    Gain    F/R
   ang    0.04   0.29
     a    0.00   0.01
     b    0.00   0.04
     c    0.00   0.02
     r    0.01   0.05
     s    0.00   0.00

Four Elements

This design uses four elements on three spreaders. The additional reflector loop improves forward gain 0.4 dB over much of the band. The boom length is 51″.

Modeling Results

Calculated performance is for a boom height of 20 feet over average-quality ground at 1° elevation angle. The model used 17 analysis segments per halfwave with bent-wire correction. 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 response. H/V is the ratio of horizontal to vertical forward response. F/R is the ratio of forward response 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     243 - j13    1.24     0.05      0.05     -1.09      7.96       1.85     16.86
    89     290 - j1     1.03     0.00      0.04     -0.79      6.73       1.30     16.57
    90     324 - j3     1.08     0.01      0.03     -0.68      5.77       1.02     16.13
    91     346 - j9     1.16     0.02      0.03     -0.63      5.07       0.90     15.96
    92     359 - j15    1.20     0.04      0.03     -0.60      4.58       0.86     15.84
    93     366 - j17    1.23     0.05      0.03     -0.57      4.23       0.80     15.35
    94     371 - j18    1.25     0.05      0.02     -0.53      4.00       0.69     14.99
    95     374 - j17    1.25     0.06      0.02     -0.47      3.79       0.57     14.66
    96     376 - j13    1.26     0.06      0.02     -0.40      3.60       0.38     14.38
    97     379 - j9     1.27     0.06      0.02     -0.32      3.41       0.13     14.14
    98     382 - j5     1.27     0.06      0.02     -0.22      3.20      -0.19     13.94
    99     385 - j1     1.28     0.07      0.03     -0.12      3.02      -0.52     13.80
   100     389 + j1     1.30     0.07      0.03      0.00      2.82      -0.96     13.70
   101     393 + j0     1.31     0.08      0.03      0.12      2.65      -1.44     13.65
   102     394 - j4     1.31     0.08      0.03      0.25      2.59      -1.96     13.66
   103     389 - j11    1.30     0.07      0.03      0.37      2.69      -2.50     13.73
   104     377 - j20    1.26     0.06      0.03      0.49      3.02      -3.02     13.86
   105     353 - j24    1.20     0.04      0.04      0.57      3.61      -3.47     14.02
   106     321 - j19    1.10     0.01      0.04      0.59      4.48      -3.75     14.16
   107     283 - j0     1.06     0.00      0.05      0.49      5.63      -3.75     14.17
   108     249 + j35    1.26     0.06      0.05      0.19      7.04      -3.37     13.13

Antenna File

4-el CP Quad
20' High
88 93 98 103 108 MHz
17 copper wires, inches
r = 17.68269		; inner reflector half-side
s = 20.01408		; outer reflector half-side
a = 14.82155		; driven element lower wire
b = 19.63566		; driven element half-side
c = 21.63432		; driven element diagonal wire
d1 = 13.58508		; director half-side
dep = 26.51343		; driven element position
dex = dep + 1		; diagonal wire tip position
d1p = 50.84081		; director position
shift z 20'
1    0  -r  -r    0   r  -r  #14	; inner reflector
1    0   r  -r    0   r   r  #14
1    0   r   r    0  -r   r  #14
1    0  -r   r    0  -r  -r  #14
1    0  -s  -s    0   s  -s  #14	; outer reflector
1    0   s  -s    0   s   s  #14
1    0   s   s    0  -s   s  #14
1    0  -s   s    0  -s  -s  #14
1  dep  -a  -b  dep   b  -b  #14	; driven element
1  dep   b  -b  dep   b   b  #14
1  dep   b   b  dep  -b   b  #14
1  dep  -b   b  dep  -b  -b  #14
1  dep  -b  -b  dex   c   c  #14
1  d1p -d1 -d1  d1p  d1 -d1  #14	; director
1  d1p  d1 -d1  d1p  d1  d1  #14
1  d1p  d1  d1  d1p -d1  d1  #14
1  d1p -d1  d1  d1p -d1 -d1  #14
1 source
Wire 9, end2

Construction

Construct like the five-element design below. The length of the driven element lower wire is 34716″. The length of the diagonal wire is 58⅜″.

If necessary, span the reflector wires with thin polystyrene rods to stabilize the spacing. A simple attachment method is to heat the wires and melt them into the plastic.

Sensitivity Analysis

The following table shows the change in average performance over 88, 93, 98, 103, and 108 MHz in dB when altering a single dimension by ⅛″ (116″ for symbols r, s, b, and d1, which represent half of a loop side).

Symbol    Gain    F/R
     r    0.01   0.13
     s    0.00   0.00
     a    0.00   0.01
     b    0.00   0.07
     c    0.00   0.03
    d1    0.00   0.04
   dep    0.00   0.01
   d1p    0.00   0.00

Five Elements

You can build this five-element design with inexpensive parts from a hardware store. The boom length is 115″.

Modeling Results

Calculated performance is for a boom height of 20 feet over average-quality ground at 1° elevation angle. The model used 17 analysis segments per halfwave with bent-wire correction. 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 response. H/V is the ratio of horizontal to vertical forward response. F/R is the ratio of forward response 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     292 + j41    1.15     0.02      0.05     -0.04      7.11       0.39     17.61
    89     344 + j31    1.18     0.03      0.04      0.22      5.81      -0.09     16.65
    90     372 + j9     1.24     0.05      0.04      0.33      4.75      -0.21     16.17
    91     384 - j8     1.28     0.07      0.03      0.39      3.99      -0.22     15.98
    92     387 - j18    1.30     0.07      0.03      0.44      3.50      -0.11     15.96
    93     387 - j23    1.30     0.08      0.03      0.50      3.18      -0.04     15.41
    94     388 - j25    1.31     0.08      0.03      0.58      2.99      -0.12     14.98
    95     388 - j24    1.31     0.08      0.03      0.70      2.80      -0.23     14.58
    96     389 - j23    1.31     0.08      0.03      0.84      2.61      -0.44     14.21
    97     390 - j23    1.31     0.08      0.03      1.00      2.42      -0.73     13.87
    98     390 - j25    1.31     0.08      0.03      1.17      2.22      -1.08     13.54
    99     388 - j29    1.31     0.08      0.03      1.36      2.09      -1.47     13.24
   100     380 - j34    1.29     0.07      0.03      1.55      2.02      -1.84     12.95
   101     364 - j37    1.25     0.05      0.04      1.73      2.11      -2.11     12.68
   102     342 - j33    1.18     0.03      0.04      1.90      2.40      -2.20     12.46
   103     319 - j20    1.09     0.01      0.04      2.03      2.82      -2.00     12.29
   104     299 + j2     1.01     0.00      0.05      2.14      3.28      -1.49     12.24
   105     287 + j27    1.11     0.01      0.06      2.29      3.54      -0.74     12.40
   106     281 + j49    1.20     0.04      0.07      2.63      3.21       0.02     12.91
   107     269 + j66    1.29     0.07      0.12      3.36      1.61      -0.38     13.77
   108     255 + j21    1.20     0.04      0.27      3.35      6.41      -6.15     14.82

These patterns are for the total field. They show the response to interfering signals with worst-case polarization. Actual antenna patterns are unlikely to be this poor for many signals, but they're also unlikely to be as good as the right-circular patterns shown above. The other quad designs show a similar level of pattern degradation.

Antenna File

5-el CP Quad
20' High
88 90 98 106 108 MHz
21 copper wires, inches
r = 17.71214		; reflector half-side
a = 14.49214		; driven element lower wire
b = 20.20037		; driven element half-side
c = 21.99771		; driven element diagonal wire
d1 = 13.7816		; director half-sides
d2 = 13.88908
d3 = 14.00479
dep = 27.06915		; driven element position
dex = dep + 1		; diagonal wire tip position
d1p = 51.54301		; director positions
d2p = 82.69584
d3p = 114.5893
shift z 20'
1    0  -r  -r    0   r  -r  #14	; reflector
1    0   r  -r    0   r   r  #14
1    0   r   r    0  -r   r  #14
1    0  -r   r    0  -r  -r  #14
1  dep  -a  -b  dep   b  -b  #14	; driven element
1  dep   b  -b  dep   b   b  #14
1  dep   b   b  dep  -b   b  #14
1  dep  -b   b  dep  -b  -b  #14
1  dep  -b  -b  dex   c   c  #14
1  d1p -d1 -d1  d1p  d1 -d1  #14	; director 1
1  d1p  d1 -d1  d1p  d1  d1  #14
1  d1p  d1  d1  d1p -d1  d1  #14
1  d1p -d1  d1  d1p -d1 -d1  #14
1  d2p -d2 -d2  d2p  d2 -d2  #14	; director 2
1  d2p  d2 -d2  d2p  d2  d2  #14
1  d2p  d2  d2  d2p -d2  d2  #14
1  d2p -d2  d2  d2p -d2 -d2  #14
1  d3p -d3 -d3  d3p  d3 -d3  #14	; director 3
1  d3p  d3 -d3  d3p  d3  d3  #14
1  d3p  d3  d3  d3p -d3  d3  #14
1  d3p -d3  d3  d3p -d3 -d3  #14
1 source
Wire 5, end2

Construction

Use #14 bare copper wire supported by ½″ PVC pipe (0.84″ OD). The length of the driven element lower wire is 341116″. The length of the diagonal wire is 591116″. Place the diagonal wire and the corner on opposite sides of the spreader. For the boom use a 10-foot piece of 1½″ PVC pipe (1.9″ OD). Drill two offset holes through it for each spreader. Secure with PVC cement or bolts. Use a halfwave coaxial balun. Form the balun into a small-diameter coil and mount it perpendicular to the driven loop. To decouple the feedline, starting at the feedpoint install current baluns at 30″ intervals. The last one should be several feet from the antenna. The mast section near the antenna should be nonconductive.

Sensitivity Analysis

The following table shows the change in average performance over 88, 93, 98, 103, and 108 MHz in dB when altering a single dimension by ⅛″ (116″ for symbols r, b, and d1-d3, which represent half of a loop side).

Symbol    Gain    F/R
     r    0.01   0.09
     a    0.00   0.01
     b    0.02   0.08
     c    0.01   0.04
    d1    0.06   0.18
    d2    0.09   0.14
    d3    0.03   0.09
   dep    0.00   0.01
   d1p    0.00   0.01
   d2p    0.00   0.01
   d3p    0.00   0.01

Seven Elements

This design uses seven elements on a 186″ boom.

Modeling Results

Calculated performance is for a boom height of 20 feet over average-quality ground at 1° elevation angle. The model used 17 analysis segments per halfwave with bent-wire correction. 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 response. H/V is the ratio of horizontal to vertical forward response. F/R is the ratio of forward response 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     293 + j35    1.13     0.02      0.05      0.60      7.03       0.37     18.96
    89     342 + j30    1.17     0.03      0.04      0.91      5.90      -0.11     18.59
    90     372 + j12    1.24     0.05      0.04      1.07      4.97      -0.31     18.25
    91     385 - j3     1.28     0.07      0.03      1.18      4.27      -0.37     18.21
    92     391 - j14    1.31     0.08      0.03      1.27      3.75      -0.39     17.72
    93     395 - j20    1.32     0.09      0.03      1.37      3.36      -0.43     17.32
    94     398 - j25    1.34     0.09      0.03      1.49      3.07      -0.59     17.09
    95     400 - j29    1.35     0.10      0.03      1.63      2.75      -0.73     16.92
    96     400 - j33    1.35     0.10      0.03      1.79      2.41      -0.90     16.82
    97     397 - j39    1.35     0.10      0.03      1.97      2.05      -1.07     16.77
    98     389 - j45    1.34     0.09      0.03      2.16      1.69      -1.19     16.74
    99     377 - j49    1.31     0.08      0.03      2.37      1.38      -1.24     16.71
   100     361 - j47    1.26     0.06      0.04      2.59      1.15      -1.15     16.66
   101     343 - j39    1.20     0.04      0.04      2.83      1.05      -0.92     16.51
   102     326 - j27    1.13     0.02      0.04      3.11      1.04      -0.64     16.20
   103     312 - j10    1.05     0.00      0.05      3.47      0.99      -0.44     15.92
   104     300 + j8     1.03     0.00      0.06      3.93      0.91      -0.58     15.64
   105     295 + j25    1.09     0.01      0.08      4.50      1.55      -1.46     15.23
   106     283 + j25    1.11     0.01      0.12      4.93      3.48      -3.14     14.44
   107     232 + j48    1.37     0.11      0.17      4.60      5.62      -3.37     13.06
   108     244 + j70    1.39     0.12      0.38      4.10      3.66      -2.89     12.51

Antenna File

7-el CP Quad
20' High
88 89 90 98 106 107 108 MHz
29 copper wires, inches
r = 17.68041		; reflector half-side
a = 14.47533		; driven element lower wier
b = 20.19889		; driven element half-side
c = 22.44163		; driven element diagonal wire
d1 = 13.8335		; director half-sides
d2 = 14.07744
d3 = 13.784
d4 = 13.74143
d5 = 13.91304
dep = 29.19122		; driven element position
dex = dep + 1		; diagonal wire tip position
d1p = 50.496		; director positions
d2p = 78.3315
d3p = 116.5934
d4p = 151.7725
d5p = 186.4434
shift z 20'
1    0  -r  -r    0   r  -r  #14	; reflector
1    0   r  -r    0   r   r  #14
1    0   r   r    0  -r   r  #14
1    0  -r   r    0  -r  -r  #14
1  dep  -a  -b  dep   b  -b  #14	; driven element
1  dep   b  -b  dep   b   b  #14
1  dep   b   b  dep  -b   b  #14
1  dep  -b   b  dep  -b  -b  #14
1  dep  -b  -b  dex   c   c  #14
1  d1p -d1 -d1  d1p  d1 -d1  #14	; director 1
1  d1p  d1 -d1  d1p  d1  d1  #14
1  d1p  d1  d1  d1p -d1  d1  #14
1  d1p -d1  d1  d1p -d1 -d1  #14
1  d2p -d2 -d2  d2p  d2 -d2  #14	; director 2
1  d2p  d2 -d2  d2p  d2  d2  #14
1  d2p  d2  d2  d2p -d2  d2  #14
1  d2p -d2  d2  d2p -d2 -d2  #14
1  d3p -d3 -d3  d3p  d3 -d3  #14	; director 3
1  d3p  d3 -d3  d3p  d3  d3  #14
1  d3p  d3  d3  d3p -d3  d3  #14
1  d3p -d3  d3  d3p -d3 -d3  #14
1  d4p -d4 -d4  d4p  d4 -d4  #14	; director 4
1  d4p  d4 -d4  d4p  d4  d4  #14
1  d4p  d4  d4  d4p -d4  d4  #14
1  d4p -d4  d4  d4p -d4 -d4  #14
1  d5p -d5 -d5  d5p  d5 -d5  #14	; director 5
1  d5p  d5 -d5  d5p  d5  d5  #14
1  d5p  d5  d5  d5p -d5  d5  #14
1  d5p -d5  d5  d5p -d5 -d5  #14
1 source
Wire 5, end2

Construction

Construct like the five-element design, but use a spliced boom, perhaps telescoped, and nonconductive boom guys. The length of the driven element lower wire is 341116″. The length of the diagonal wire is 60516″.

Sensitivity Analysis

The following table shows the change in average performance over 88, 93, 98, 103, and 108 MHz in dB when altering a single dimension by ⅛″ (116″ for symbols r, b, and d1-d5, which represent half of a loop side).

Symbol    Gain    F/R
     r    0.01   0.14
     a    0.00   0.01
     b    0.01   0.05
     c    0.01   0.01
    d1    0.03   0.10
    d2    0.09   0.16
    d3    0.04   0.02
    d4    0.02   0.04
    d5    0.00   0.03
   dep    0.00   0.00
   d1p    0.00   0.00
   d2p    0.00   0.00
   d3p    0.00   0.00
   d4p    0.00   0.00
   d5p    0.00   0.00

Quads vs Crossed Yagis

Antenna Comparison

This compares the quads and the Antennacraft FM6, Antenna Performance Specialties APS-13, Körner 9.2, 15.12, and 19.3, and crossed Yagis for a right-circular field at 1° elevation angle with the booms 20 feet above average-quality ground.

Transmit Polarization

The following table lists antenna polarization percentages by service class for U.S. FM broadcast stations.

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  82   5  10   1   1 
Full service       59  90   3   4   2   1
LPFM                7  98   2   0   0   0  
Translator         33  67  11  22   0   0 
Booster             2  57  10  32   1   0 

To determine its transmit polarization, look up a station in the FCC database. To determine circularity sense, check the antenna specifications at the manufacturer's website. If the FCC antenna data is missing, contact the station chief engineer. Some manufacturers do not list circularity sense. As far as I can tell, current antenna models from the following are right-circular: ERI, Jampro, Micronetixx, PSI nonpanel, SWR nonpanel except the FM1, 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. Older Harris FMH 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.


July 27, 201588–108 MHz