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 nearly universal use of circular polarization today on FM. It offers easy rotation to exploit deep response nulls for all polarizations (right-circular, left-circular, horizontal, and vertical). 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 FM band where many interesting stations are found. You can optimize the response higher in the band 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. Rod tapering affects resonance, so rabbit ears with different taper-section lengths or diameters may not resonate at the intended frequency when adjusted to the recommended length.
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 degrees 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 0.5" per MHz.
The antenna impedance is about 75 + j300 ohms at the target frequency. A series capacitance of 6 pF will match this impedance to 75-ohm coax. Remove the original 300-ohm 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.
You can try unmodified rabbit ears, arranged in the suggested geometry, to experiment with their interference-rejection properties. The matching capacitance and 75-ohm 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 300-ohm tuner input. 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 suggested 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. If you alter it, do so carefully.
Sony rabbit ears are designed to snap into a slot at the back of a TV set. Disassemble the antenna, unbolt the plastic snap, and mount the antenna to a base you can place on a flat surface. A way to tilt the rods without changing the angle between them is useful. Tilting often yields 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 nonconductive rod works even better.
Patterns
This plot shows the azimuth response of the antenna to circular polarization. The right- and left-circular responses are mirror images. The antenna rods are in the 90-270 degree plane. Maximum response occurs at an oblique angle to the rod plane with a bidirectional lobe. Note the sharp, deep nulls. The gain reference is a circularly polarized dipole in free space (a pair of linear halfwave dipoles at right angles phased 90 degrees). The gain of a linear halfwave dipole is -3.01 dBdc. Most FM stations transmit right-circular polarization, but some transmit left.
When a circularly polarized signal reflects from a surface, the circularity reverses. Thus right-circular signals produce 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 lobes. Thus you can maximize the desired signal while rejecting multipath reflections from the same general direction. With the bidirectional nulls of rabbit ears, you can simultaneously reject reflections from the front and rear.
Rabbit ears reduce opposite-circularity reflections by at least 10 dB over nearly 40 degrees 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 a wide-angle reflection.
This shows ellipticity, which is the inverse of axial ratio expressed in dB. Axial ratio is the magnitude of the larger linear polarization component divided by the magnitude of the smaller. Ellipticity is very close to 0 dB at the response nulls, indicating nearly perfect circularity. The axial ratio of an undesired signal may not be unity due to imperfect circularity of the transmit antenna or to reflection from a surface with a complex reflection coefficient. Rotating rabbit ears until the antenna circularity is opposite that of the undesired signal will cancel the signal.
This overlays the responses to horizontal and vertical polarization. Maximum horizontal response is perpendicular to the rod plane, while maximum vertical response is in line with the rods. Note the sharp nulls for both polarizations. These nulls can be used to reject interference from stations that transmit linear polarization. The gain reference for these plots is a linear halfwave dipole.
Unlike the other fields, vertical polarization 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.
This overlays right-circular patterns at 88 and 108 MHz using rectangular coordinates. The patterns are remarkably similar. More surprising still is how well the circularity, indicated by null depth, holds up across the band. Rabbit ears exhibit narrowband impedance characteristics but wideband directivity.
Wideband Version
If you spread the rods until the angle between them is 90 degrees, with the tips 62" apart, the antenna Q drops, SWR bandwidth widens, and mismatch loss decreases higher in the band. This wideband version should be matched using two 10 pf capacitors, retaining the original 300-ohm 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.Numbers
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 300-ohm twinlead with a 58.25" 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 balun with these antennas.88 90 92 98 108 MHz -3.03 -2.32 -2.78 -6.44 -10.48 Narrowband rabbit ears dBdc -3.11 -2.82 -2.70 -3.15 -4.80 Wideband rabbit ears dBdc -4.79 -4.09 -3.56 -3.13 -4.71 Horizontal folded dipole dBdc 1.76 1.77 0.78 -3.31 -5.77 NRE advantage over FD dB 1.68 1.27 0.86 -0.02 -0.09 WRE advantage over FD dB
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.
Although its orientation is fixed, a circularly polarized loop offers more gain than rabbit ears, as well as nonzero F/B.
The following figures compare maximum azimuth gain plus mismatch loss at 90 MHz for the two rabbit ears versions.
Circular Horizontal Vertical dBdc dBd dBd -2.32 -0.99 -4.01 Narrowband rabbit ears -2.82 -0.74 -6.43 Wideband rabbit ears 0.50 -0.25 2.42 NRE advantage dB
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 ellipticity 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 resistivity 1.29E-7 wires, inches ; Chrome-plated parts 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
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Updated January 17, 2008
