If you use an indoor dipole or folded dipole for FM reception, you can increase signal strength in a favored direction, reduce it in the opposite direction, and decrease forward multipath reflections by simply tilting the antenna. The improvement is small but free.
A tilted dipole and its electromagnetic image below ground form an elliptically polarized antenna array. The orthogonal image and its propagation delay provide the elliptical response. The arrangement enables the dipole to capture orthogonal power in a circularly polarized signal that a horizontal or vertical antenna ignores. To benefit, 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 extremely poor or extremely good, and the antenna must not be too high.
Most stations seem to use right-circular polarization. To enhance 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 roughly 45°. For left-circular signals, lower the left end.
The yellow curve is for a dipole tilted 41°. The red curve is for a horizontal or vertical dipole. The highest point of each antenna is 9′ above ground. The response to orthogonal multipath reflections is the yellow curve mirror-imaged about the 90°–270° line. These are NEC-2 results for pastoral land (permittivity = 11, conductivity = 25) using the Sommerfeld-Norton ground model.
Ground Tilt Gain/H Gain/V Seawater 77° 10.9 0.2 dB Freshwater lake 61 4.9 0.9 Marsh, unflooded rice paddy 45 2.0 2.4 Rich agricultural land 41 1.3 2.9 Pastoral land, medium hills, forest 41 0.9 2.3 Mountains; rocky, steep hills 41 0.5 2.0 Flat desert, cities 36 0.2 1.7
The table shows tilted dipole gain over horizontal and vertical dipoles. Tilt angle is that which maximizes the signal. The peak height of each antenna is 9′. The tilt advantage decreases as height increases. Modeled is a 98 MHz circularly polarized signal arriving at a 1° elevation angle. The ground type hundreds of feet away where reflection occurs is what matters, not that under the antenna. The model assumes flat earth with no obstructions.
Coupling to nearby conductors, indoor reflections, terrain irregularity, and imperfect transmit antenna circularity can make it worthwhile to experiment with the tilt angle. 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. Use an analog signal-strength meter to reveal small changes in level. Position the antenna away from anything conductive. Keep the feedline perpendicular to the dipole for as great a distance as possible.
About 9% of U.S. FM broadcast signals today are vertically polarized. Tilting a horizontal dipole will greatly increase their strength. 4% are horizontally polarized. Tilting the dipole will weaken these signals.
88–108 MHz