Practical_Antenna_Handbook_0071639586

38 p a r t I I : F u n d a m e n t a l s
Ground-Wave Propagation Frequency Dependence
The frequency of a radio signal in large measure determines its surface-wave behavior.
In the low-frequency (LF) band (30 to 300 kHz), ground losses are small for vertically
polarized signals, so medium-distance communication (up to several hundred miles) is
possible. In the medium-wave (MW) band (300 to 3000 kHz, which includes both the
AM broadcast band and the 160-m amateur band), distances of a few hundred miles are
common. In the high-frequency (HF) band, ground losses are typically greater, so the
surface-wave distance reduces drastically. It is possible, in the upper end of the HF
band (3000 to 30,000 kHz), for surface-wave signals to die out within a few dozen miles.
This phenomenon is often seen in the 15- and 10-m amateur radio bands, as well as the
11-m (27-MHz) citizens band. Stations only 20 mi apart may not be able to communicate,
but both can talk to a third station across the continent via ionospheric skip!
Tropospheric Propagation
The troposphere is the portion of the atmosphere between the surface of the earth and
the stratosphere (or about 4 to 7 mi up). Some older texts group tropospheric propaga-
Direction of
propagation
Low dielectric constant
(low moisture content)
Surface of earth
High dielectric constant
(high moisture content)
A
High dielectric constant
Direction of
propagation
Low dielectric constant
Surface of earth
B
Figure 2.18 Refraction in the troposphere.