Circularly Polarized Rabbit Ears

You can make a simple but effective indoor antenna for the FM broadcast band with television rabbit ears. The antenna takes advantage of the widespread use of circular polarization for FM signals. It is easy to rotate to exploit deep response nulls for right-circular, left-circular, horizontal, and vertical fields. This lets you reject co-channel or adjacent-channel interference regardless of transmit polarization. It also lets you minimize multipath distortion of circularly polarized signals by nulling reflections of opposite circularity. The antenna delivers maximum signal at the low end of the band. You can optimize the response higher in frequency by shortening the telescoping rods.

Construction

Any set of rabbit ears will work, but the dimensions given here are for Sony models AN-14 or AN-16. Because rod tapering affects antenna resonance, rabbit ears with different taper-section lengths or diameters may not resonate at the intended frequency when adjusted as described.

To form the recommended geometry, extend the Sony rods to full length (43″ as measured from the swivel joint). Then angle the rods so that the tips are 40″ apart, forming an angle of 54 between the rods. This geometry resonates the antenna at 90 MHz when matched as described below. Mismatch loss is less than 0.7 dB from 88 to 92 MHz.

To shift the system resonance and maximum response higher in the band, shorten the rods ″ per MHz.

The antenna impedance is about 75 + j300 Ω at the target frequency. A series capacitance of 6 pF will match this impedance to 75Ω coax. Remove the original 300Ω feedline and solder a 12 pF capacitor to each of the solder lugs. Solder the shield of the coax to one capacitor and the center conductor to the other. If you like you can form the feedline into a current balun, but usually this is not needed.

You can try unmodified rabbit ears, arranged in the suggested geometry, to experiment with their interference-rejection properties. The matching capacitance and 75Ω feedline increase signal levels but do not affect the pattern. Mismatch loss for unmodified rabbit ears in the suggested geometry is about 4 dB for a tuner input impedance of 300Ω. This is low enough that you may want to use unmodified rabbit ears for multipath rejection on local signals.

When using rabbit ears other than the Sony models, start with the recommended geometry. Find a weak signal near the target center frequency and adjust the rod lengths equally to maximize signal strength. Resonance is sensitive to the angle between rods but the pattern isn't. It may be easier to adjust rod angle than rod length for maximum signal.

Sony rabbit ears snap into a slot at the back of a TV set. Disassemble the antenna, unbolt the plastic snap, and mount the antenna on a base you can place on a flat surface. A way to tilt the rods without changing the angle between them is useful because tilting may yield a deeper null. You can build a tilt mechanism into the base or you can simply join the rods with a string of the proper length. This will let you tilt the rods with the swivels and then easily restore the correct geometry. A stiff insulated rod works even better than string.

Patterns

This is the azimuth response for circular polarization. The antenna rods are in the 90270 plane. Maximum response occurs at an oblique angle to the rods with bidirectional lobes. You can use the sharp, deep nulls to reject interference. The gain reference is a linearly polarized halfwave dipole.

When a circularly polarized signal reflects from a surface, the circularity sense reverses. Thus right-circular signals yield left-circular reflections. If you orient a circularly polarized antenna so that reflections arrive at opposite-circularity nulls, you can reduce multipath distortion on FM signals. Unlike the simple pattern nulls characteristic of linearly polarized antennas, opposite-circularity nulls of circularly polarized antennas occur within the desired-circularity main lobe. Thus you can maximize the desired signal while rejecting multipath reflections from the same general direction. The bidirectional nulls of rabbit ears let you simultaneously reject reflections from the front and rear.

Rabbit ears reduce opposite-circularity reflections by at least 10 dB over nearly 40 in azimuth. This wide range allows the antenna to reject diffuse forward diffraction components that may accompany a signal scattered over a range of hills or mountains. For a single specular reflection, orient the antenna to place a null at the reflection angle. The broad main lobe permits good direct-path pickup even when nulling an off-axis reflection.

This overlays the response for horizontal and vertical polarization. Maximum horizontal response is perpendicular to the rod plane, while maximum vertical response is in line with the rods. The sharp nulls for both polarizations provide interference rejection for linearly polarized stations.

Unlike for the other fields, the response for vertically polarized signals is not maximum in the horizontal plane. The maximum occurs directly above the antenna at −1.80 dBd. For a weak vertically polarized signal, tilt the antenna toward the horizon to pick up a dB or two.

Wideband Version

If you spread the rods until the angle between them is 90 with the tips 62″ apart, antenna Q drops, SWR bandwidth widens, and mismatch loss decreases higher in the band. Match this wideband version using two 10 pF capacitors, retaining the original 300Ω feedline. The wideband and narrowband patterns are similar. The primary advantage of the wideband design is greater signal strength higher in the band without adjusting rod length. The main disadvantage is that the wider wingspan makes the antenna harder to rotate without hitting nearby objects. Another disadvantage is weaker response to vertical polarization.

Performance

The following figures compare the response of rabbit ears and a horizontal folded dipole to circularly polarized signals. The folded dipole is the common kind made of 300Ω twinlead with a 58″ length that minimizes worst-case SWR across the FM band. The figures are maximum azimuth gain plus mismatch loss. Add −0.75 dB to the wideband rabbit ears and folded dipole figures if you must use a push-on balun with these antennas.

  88     90     92     98     108    MHz
-3.03  -2.32  -2.78  -6.44  -10.48   dBdc   Narrowband rabbit ears
-3.11  -2.82  -2.70  -3.15   -4.80   dBdc   Wideband rabbit ears
-4.79  -4.09  -3.56  -3.13   -4.71   dBdc   Horizontal folded dipole
 1.76   1.77   0.78  -3.31   -5.77   dB     NRE advantage over FD
 1.68   1.27   0.86  -0.02   -0.09   dB     WRE advantage over FD

These should be regarded as reference figures. While rabbit ears can always be rotated for maximum signal, a folded dipole in a fixed orientation may or may not be broadside to a given signal. In addition, signal quality may be limited by co-channel or adjacent-channel interference, or by multipath distortion. Signal quality can always be optimized by rotating rabbit ears, but this isn't possible with a fixed antenna.

The following figures compare maximum azimuth gain plus mismatch loss at 90 MHz for the two rabbit ears versions.

 Circular   Horizontal    Vertical
-2.32 dBdc   -0.99 dBd   -4.01 dBd     Narrowband rabbit ears
-2.82 dBdc   -0.74 dBd   -6.43 dBd     Wideband rabbit ears
 0.50 dB     -0.25 dB     2.42 dB      NRE advantage over WRE

A circularly polarized loop offers more gain than rabbit ears and nonzero F/B. Its turning radius is nearly identical.

Model Limitations

The models are for antennas in free space. Ground affects the patterns, particularly for circularly polarized signals. Rejection of opposite-circularity signals varies with ground quality and antenna height.

The models assume that the transmit antenna has perfect circularity. Some stations may have circularity errors of up to several dB in certain directions. These errors can affect null depth and multipath suppression, and to a lesser extent, signal strength.

Rotating and tilting the antenna can compensate for most ground and height effects, and for transmit circularity errors. Such adjustment matches the axial ratio of the antenna to that of the incoming signal in the local environment.

Antenna File

I optimized the designs with the AO 7.00 Antenna Optimizer program using 80 segments/halfwave. The models account for conductor and mismatch losses. The antenna file for narrowband rabbit ears follows.

Sony AN-14 Rabbit Ears
Free Space
90 MHz
19 chromium wires, inches		; chrome-plated rods
angle = 63
1  0 -.5      0   0  .5      0      .1	; solder lugs & leads
1  0  .5      0   0  .5      1.25   .5	; swivel mount
1  0 -.5      0   0 -.5      1.25   .5	; swivel mount
shift y .5 z 1.25
rotate x -angle
1  0 0        0   0  6.65    0   .3125	; telescoping rod
1  0 6.65     0   0 12       0   .25
1  0 12       0   0 17.375   0   .21875
1  0 17.375   0   0 22.65    0   .1875
1  0 22.65    0   0 27.8125  0   .15625
1  0 27.8125  0   0 32.9     0   .125
1  0 32.9     0   0 38       0   .09375
1  0 38       0   0 43       0   .0625
shift y -.5
rotate x angle
1  0 0        0   0  -6.65   0   .3125	; telescoping rod
1  0 -6.65    0   0 -12      0   .25
1  0 -12      0   0 -17.375  0   .21875
1  0 -17.375  0   0 -22.65   0   .1875
1  0 -22.65   0   0 -27.8125 0   .15625
1  0 -27.8125 0   0 -32.9    0   .125
1  0 -32.9    0   0 -38      0   .09375
1  0 -38      0   0 -43      0   .0625
1 source
Wire 1, center
1 load
Wire 1, center 6 pF			; 12 pF cap in each feeder

August 29, 201388108 MHz