Tilted Dipole

If you use an indoor dipole for FM reception, you can increase signal strength a couple dB, decrease sensitivity to forward multipath reflections several dB, and reject signals from the opposite direction a similar amount by simply tilting the dipole 45°.

A tilted dipole and its image over lossy earth form an elliptically polarized antenna array. Ground-reflection coefficients differ enough for horizontal and vertical waves to provide ellipticity from phase rotation. To benefit from tilting a dipole, the stations you want to receive must be in the same general direction, signals must be circularly polarized and match the circularity sense of the antenna, and the antenna must not be too high.

Most FM stations use right-circular polarization. To enhance their reception, orient a horizontal dipole broadside to the desired station. Then looking through the dipole toward the station, lower the right end. For the odd left-circular signal, lower the left end.

The yellow curve is for a tilted dipole with one end at the ceiling (8' above a floor 6" above ground level) and the other end 1 meter lower. The red curve is for a horizontal dipole at the ceiling. The slight azimuth pattern skew is typical of low antennas in a circular field. The response to left-circular fields, typical of multipath reflections, is the yellow curve mirror-imaged about the 90°-270° line. These are NEC results for average-quality ground using the Sommerfeld-Norton ground model.

This shows how gain enhancement at 1° elevation varies with peak antenna height (highest point of the antenna) for four kinds of ground at 98 MHz. The curves are for gain enhancement, not absolute gain. Higher antennas always deliver a stronger signal, so don't lower a tilted dipole intending to hit the peak of a curve. Absolute gain for horizontal, vertical, and tilted dipoles at heights between 5 and 50 feet is shown here.

 Ground    Diel   Cond
 Quality   Const  mS/m
Very good   20     30    Pastoral, low hills, rich soil; Dallas TX to Lincoln NE
Average     13      5    Pastoral, medium hills and forestation, heavy clay soil; central VA
Sandy       10      2    Sandy, dry, flat, coastal
Very poor    5      1    Cities, industrial areas

This shows how the relative gain at 1° elevation varies with tilt angle for a peak height of 8.5' over average ground at 98 MHz. The optimum tilt angle depends on antenna height, ground quality, and elevation angle.

Attach one end of the dipole as high as you can and adjust the height of the other end for the strongest signal from the station you're having the most trouble receiving. An LED signal-strength indicator won't show small changes in level, but an analog meter should reveal them. To avoid detuning the antenna, keep it away from other objects, including walls.

Nearly all FM broadcast signals in the U.S. today are circularly polarized, but a few stations use horizontal or vertical polarization. Tilting a horizontal dipole will greatly increase a vertically polarized signal. It will drop a horizontally polarized signal about 3 dB.

More is here.

Updated March 31, 2008