Elliptically Polarized 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 below ground form an elliptically polarized antenna array. The orthogonal image and the reflected-wave propagation delay provide ellipticity. To benefit from tilting a dipole, desired stations 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 seem to 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 left-circular signals, lower the left end.

The yellow curve is for a dipole tilted 45. The red curve is for a horizontal or vertical dipole. The highest point of each antenna is at the ceiling (eight feet above a floor 6″ above ground level). The slight pattern skew is characteristic of low antennas in a circular field. The response to orthogonal multipath reflections is the yellow curve mirror-imaged about the 90270 line. These are NEC results for average ground quality using the Sommerfeld-Norton ground model. Tilted dipole axial ratio is 9.4 dB so polarization is more elliptical than circular.

 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 the variation in gain enhancement with peak antenna height for a dipole tilted 45 for four types of ground. The curves are for a 98-MHz signal arriving at a 1 elevation angle. They show gain enhancement, not absolute gain. Higher antennas 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 versus height is shown here (these curves are for center height, not peak height).

This shows gain variation with tilt angle for a peak height of 8 feet at 98 MHz and 1 elevation angle. The optimum angle falls somewhat as ground quality worsens.

Tilting a dipole 45 is very close to optimum for the ideal case. But coupling to nearby conductors, indoor reflections, terrain irregularity, and imperfect transmit antenna circularity can make it worthwhile to experiment. Place 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. Avoid stray coupling by positioning the antenna away from anything conductive. 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 and assuming 0.3 dB loss in a power combiner, gain is 3.3 dB over a single antenna. 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.

11% of FM broadcast signals in the U.S. today are vertically polarized. Tilting a horizontal dipole will greatly increase their strength. 6% are horizontally polarized. Tilting a dipole 45 will weaken these signals 3 dB.


August 15, 201388108 MHz