You can make a simple but effective antenna for the attic with 12′ of #12 copper wire. The antenna takes advantage of the nearly universal use today of circular polarization for FM broadcast in the U.S. The antenna can recover twice as much power from a circularly polarized signal as can a folded dipole. It also can reduce multipath distortion in a way not possible with ordinary antennas.
The antenna is planar, with no physically distinct front or back, but it exhibits a directive pattern with a rear null. One design is optimized for best performance at the low end of the FM band. Two others are optimized at higher frequencies.
When a circularly polarized signal reflects from a surface, the circularity sense reverses. Thus a right-circular signal yields left-circular reflections. In the forward direction a circularly polarized antenna receives only one circularity sense well. Such an antenna can maximize a desired signal while minimizing multipath interference from the same general direction. A linearly polarized antenna can't do this.
This antenna is somewhat larger than a circularly polarized loop and doesn't perform quite as well. But it is simpler and easier to construct.
The antenna consist of two wires, each about a half-wavelength long, crossed at right angles and connected in parallel. One wire is longer than resonance and the other is shorter. The specific lengths cause the wire-current phases to differ by 90°. Orthogonal dipoles with quadrature currents yield circular polarization. No phasing line is needed, and the antenna can be directly connected to 75-ohm coax. The dipole reactances tend to cancel, keeping SWR and mismatch loss remarkably low across the entire FM band. The antenna exhibits right-circular polarization broadside to one face and left-circular broadside to the other.
To construct the antenna, you need two 6′ lengths of #12 electrical wire. With a razor blade strip the insulation from each wire. (Place the edge of the blade, not the tip, against the wire and peel back the insulation as you cut.) Fold each wire to locate the center. Wrap the center of one wire around the center pin of a female F-connector. Arrange the two halves of the wire at a right angle, crimp the wire to the pin, and solder the connection. Take care not to melt the insulation within the connector. Bend the center of the remaining 6′ wire at a right angle and solder it to a large washer that fits the shell of the F-connector. Fasten the washer to the shell with the F-connector nut. Arrange all wires at right angles. Cut one wire so that the distance from the wire tip to the F-connector pin is length A, as given below. Cut the opposite wire to the same length. Cut each of the remaining wires to length B. The reference point for all measurements is the center pin of the F-connector. After assembly handle the antenna carefully to avoid breaking the fragile center pin.
Dimensions for best rear null depend on frequency, antenna height, and ground conductivity. The following dimensions yield best performance over average ground. Use the design frequency closest to that of the stations you want to receive best.
90 MHz 98 MHz 104 MHz
Height A B A B A B
12' 32-3/4" 28-3/8" 30-1/4" 26-1/8" 28-1/2" 24-5/8"
15 33-1/8 28-5/8 30-3/8 26-1/4 28-3/4 24-3/4
20 33-3/8 28-3/4 30-5/8 26-3/8 29 24-7/8
25 33-1/2 28-7/8 30-7/8 26-1/2 29-1/8 25
30 33-5/8 28-7/8 30-7/8 26-5/8 29-1/4 25
35 33-3/4 29 31 26-5/8 29-1/4 25-1/8
40 33-3/4 29-1/8 31-1/8 26-3/4 29-3/8 25-1/8
45 33-7/8 29-1/8 31-1/8 26-3/4 29-3/8 25-1/4
50 33-7/8 29-1/4 31-1/8 26-7/8 29-3/8 25-1/4
For flat terrain, use the height of the center of the antenna above ground. For an elevated site with a distant horizon in the direction of reception, use the dimensions for 50′ regardless of the antenna height above local ground.
Mount the antenna wires in the vertical plane at an angle of 45° to the ground. Staple them to the rafters (insulated staples are best), support them with string, or mount them on a wooden X-frame for easy rotation. Keep the wires away from anything conductive. Run both wires connected to the center pin to the right or left, not up or down. For right-circular polarization, the longer dipole slants from the upper left to the lower right as you look through the antenna toward the distant station, as in the image above.
To reduce signal pickup on the coax shield, use a current balun at the feedpoint. It's best to run the feedline perpendicular to the antenna plane for a few feet before letting it drop. If you can't do that, install another current balun 30″ below the first.
You can tilt the plane of the antenna to adjust circularity. This might be worth doing to better null interference to a single station.
This is a test antenna made of #14 wire. Although the dimensions are for #12 wire, #10 or #14 will work just as well. The construction method shown won't keep the wires straight outdoors for long.
This is the azimuth pattern at an elevation angle of 1° above the horizon for the 90-MHz antenna at a height of 12′, typical of a single-story attic. The antenna wires are in the 90°-270° plane. Maximum response occurs perpendicular to the wires with a broad forward lobe. The slight pattern skew is typical of low antennas in a circular field. The red trace is for a 58¼″ twin-lead folded dipole 12′ high. In the forward direction its response is 4.1 dB lower than that of the attic antenna. It is 0.75 dB lower still if you use a 75:300Ω balun. The gain reference is a circularly polarized isotropic antenna in free space.
The left-circular pattern is the mirror image of the right. The antenna suppresses left-circular polarization, including multipath reflections, over a wide angle in the forward direction.
These are the patterns at 88 and 92 MHz. The rear null has degraded, but rejection is still at least 10 dB over a wide angle.
This overlays the patterns for horizontal and vertical polarization. Response is bidirectional for horizontal polarization, and within 3 dB of omnidirectional for vertical. The gain reference for this plot is a linearly polarized isotropic antenna in free space.
This is the 25′ design. The azimuth pattern is more symmetrical and the signal level nearly 6 dB greater than at 12′.
I optimized all designs with the AO 7.03 Antenna Optimizer. The model accounted for conductor and mismatch losses and used average-quality ground (dielectric constant 13, conductivity 5 mS/m).
Most FM stations today use right-circular polarization, but some use left. For best reception the signals of interest from a given direction should all have the same circularity sense, or they should be linearly polarized.
For a linearly polarized signal, the attic antenna response is 2-3 dB weaker than that from a polarization-aligned dipole. Few stations today use pure horizontal or vertical polarization. Most that do are low-power translator or booster stations that extend a station's service area, or stations at the low end of the band that use vertical polarization to avoid interfering with a horizontally polarized NTSC-TV channel 6 sound subcarrier at 87.75 MHz.
You can determine the polarization of any U.S. FM station here. Enter the station callsign. If the data listing makes no mention of horizontal or vertical, polarization is circular. Contact the station to determine the circularity sense. It's easy to determine once you've built the antenna by rotating it for maximum signal and then checking the wire orientation, as described earlier.
The antenna 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 impair rear rejection and multipath suppression, and to a lesser extent, signal strength.
Circularity can be compromised by propagation over hills or mountains. Such paths cause multiple ground reflections, each of which seems to attenuate the vertical component more than the horizontal and degrade circularity. I optimized the listed antenna dimensions for a single ground reflection from a transmit location within line of site. I obtained about 10 dB F/B from an experimental antenna that I had optimized in free space (without accounting for ground reflection) on a signal that originated 35 miles away at 2600′ elevation and diffracted over a nearby ridge. I was able to increase F/B for this signal by tilting the plane of the antenna.
Attic Antenna 12' High 90 MHz 5 copper wires, inches h = 32.75 v = 28.375 s = .375 shift z 12' rotate x -45 1 0 0 0 0 -h 0 #12 1 0 0 0 0 0 v #12 1 s 0 0 s h 0 #12 1 s 0 0 s 0 -v #12 1 0 0 0 s 0 0 #12 1 source Wire 5, end1 30 segments/halfwave. Disable bent-wire correction.
88–108 MHz