Tilted Dipole

If you use an indoor dipole for FM reception, you can increase signal strength in a favored direction, reduce it in the opposite direction, and decrease forward multipath reflections simply by tilting the antenna.

A tilted dipole and its electromagnetic image over ground form an elliptically polarized antenna array. Ground-reflection coefficients that differ for horizontal and vertical waves plus the reflected-wave delay provide ellipticity. 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, ground quality must not be too poor, 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 until the angle is about 45°. For a left-circular signal, lower the left end.

The yellow curve is for a dipole tilted 45° with one end at the ceiling (8′ above a floor 6″ above ground level). 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 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.

 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 graph shows how gain enhancement at 1° elevation varies with peak antenna height for a dipole tilted 45° 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.

This shows how the relative gain at 1° elevation varies with tilt angle for a peak height of 8.5′ at 98 MHz. The optimum tilt angle falls somewhat for lower-quality ground.

If you simply tilt a dipole 45°, you'll be very close to optimum. But if you want to experiment, 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 if possible. Keep the feedline perpendicular to the dipole for as great a distance as possible.

Arraying two tilted dipoles side by side improves directivity. With the centers 92″ apart, gain is 3.3 dB over a single antenna, assuming 0.3 dB loss in a power combiner. The pattern nulls can reduce co-channel or multipath interference. The dipoles and feedlines should be identical. For full benefit the array must be accurately aimed, with the dipoles broadside to the desired direction. If the array reduces signal strength, flip one dipole end for end.

93% of full-service FM broadcast signals in the U.S. today are circularly or elliptically polarized (81% including translators). A circularly polarized antenna can either help or hurt reception of the remaining linearly polarized signals. Tilting a horizontal dipole will greatly increase the strength of a vertically polarized signal. It will drop a horizontally polarized signal about 3 dB.