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Frequency Range Abbreviations) in the HF and MF range,
the space wave is still important but another phenomenon
becomes significant: ground-wave propagation.
Radio waves can travel along any surface or interface
between two media in which the wave travels with
different velocities. The surface of the Earth is a good
example and waves do travel along it. Known by its
technical name of “dirt,” the Earth is actually quite lossy at
almost all frequencies, converting the electromagnetic
energy to heat, or “worm burning” as hams call it. So, as
the wave travels along the ground, it is rapidly attenuated.
Ground loss gets higher with frequency, so the
ground wave is most effective at and below HF. The
typical range of a 10 meter ( 28 MHz) or CB ( 27 MHz)
ground wave signal is about 5-10 miles depending on the
terrain. At AM broadcast frequencies in the MF range (0.5
to 1.8 MHz), the ground wave can be received out to 100
miles or so. That’s why you can listen to the local AM
stations even well past the calculated radio horizon.
Tropospheric or Weather-Related
Remember that statement about changes in wave
velocity causing the wave to bend? In the May column,
you learned that wave velocity is determined by the
permittivity and permeability of whatever medium the
wave travels through. Permittivity is what determines a
material’s dielectric constant — the same dielectric
constant used to determine capacitance.
So, wherever dielectric constant changes, the wave’s
velocity and thus its direction also change. Does this
happen in the atmosphere? You bet!
There are a lot of instances in which the dielectric
constant changes — often abruptly — in the lower
atmosphere, known as the troposphere. These are caused
by weather-related events: storms, cold and warm fronts,
even temperature inversion layers.
Anywhere there is an abrupt change in the
atmosphere, the effect on radio waves is almost as if they
were reflected from the air having a different dielectric
constant. Propagation of radio waves between two points
by using this reflecting effect (even though it’s really a
sharp bend created by refraction) is called tropospheric
propagation or just “tropo” by hams.
Figure 2 shows a typical example of how tropo might
occur along the inversion layer that builds up whenever
there are stable conditions near the ground. A layer of
warm air — often humid and dusty — builds up but is
trapped under the cold dry air above.
You can see where the inversion layer is from a plane
when taking off or landing — it’s the darker colored air on
which the clouds seem to rest. If you can launch a radio
signal along this layer at a shallow angle, it can travel for
quite a distance! Tropo contacts from this and similar
effects can be made at VHF and UHF frequencies — even
microwaves — over hundreds of miles.
During microwave contests, hams put up portable
stations at high places (such as in Figure 3), hoping for the
best possible angle for tropo.
Since you can’t “see” tropo, how do you know when
it’s happening? A surprisingly easy way is available to
everyone by using an ordinary FM broadcast receiver and
a simple outdoor antenna, such as a folded dipole or
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Frequency Range Abbreviations
MF (Medium Frequency) 0.3 to 3 MHz
HF (High Frequency) 3 to 30 MHz
VHF (Very High Frequency) 30 to 300 MHz
UHF (Ultra High Frequency) 300 to 3,000 MHz
SHF (Super High Frequency) 3 to 30 GHz
EHF (Extremely High Frequency) 30 to 300 GHz
FIGURE 2. The inversion layer between warm air trapped near
the ground and colder air above it. The change in dielectric
constant between the two regions causes refraction of radio
waves and can conduct the waves along the inversion layer for
FIGURE 3. A duct can form between two layers of air with
different properties or (as shown here) along a weather front. In
this case, the warm moist air of a storm system has pushed over
the top of a cooler layer, creating two separate refracting
"walls." Signals can be trapped in the duct until one of the layers
disperses, allowing signals to reach the ground again.