Click graph to change ground type
Dielectric Conductivity Ground Constant mS/m Type 81 5000 Salt water 80 1 Fresh water 20 30 Pastoral, low hills, rich soil (Dallas TX to Lincoln NE) 13 5 Pastoral, medium hills and forestation, heavy clay soil (central VA) 10 2 Sandy, dry, flat, coastal 5 1 Cities, industrial areas 3 0.55 Desert (southwest ID)
These graphs show forward gain for horizontal, vertical, and tilted orientation of a halfwave dipole for center heights between 3 and 50 feet. These are NEC results using the Sommerfeld-Norton ground model for a 98-MHz circularly polarized signal arriving at a 1° elevation angle. The gain reference is a circularly polarized isotropic antenna in free space. The difference between the solid and dotted green curves is F/B for the tilted dipole.
Apply the curves to any antenna by adding its free-space gain in dBd to the Y axis. The model is for flat ground with a single specular reflection and no obstructions. Only general trends hold for irregular terrain where scattering at multiple points causes complex wave interference. The curves do not apply when local obstructions block the direct path.
This FCC map is intended for AM. It may be less applicable to FM with its much shallower ground penetration.
Elevating an antenna may be the easiest and cheapest way to increase signal strength. For example, raising a dipole from 15 to 30 feet increases signal strength about 6 dB (5.6 dB if RG-6 feedline must be lengthened). This is similar to the gain a small Yagi would provide over the dipole with both antennas at 15 feet.