15 Signal propagation (AM/FM DXing)
Description
This article is from the Shortwave radio FAQ, by Ralph Brandi rbrandi@lucent.com with numerous contributions by others.
15 Signal propagation (AM/FM DXing)
The distant stations you are able to receive will depend largely upon
signal propagation. This varies depending upon the time of day, the
season, and other factors. For mediumwave, the single most important
factor for good DX is the time of day. Mediumwave signals almost always
get absorbed by the D Layer of the ionosphere during daylight hours. As
a result, all mediumwave signals received during midday hours will
arrive by ground wave, making reception of signals over a few hundred
km/miles away unusual in daylight. At night, however, the ionosphere
reflects mediumwave signals, making it possible for signals to be heard
at much greater distances, up to a few thousand km/miles, via `skywave'.
To a lesser extent, the period up to two hours after local sunrise, and
two hours before local sunset, called "Critical Hours", have varying
levels of skywave, and also can provide some very unusual reception
opportunities for the mediumwave DXer. Reception also tends to be better
in winter than in summer, due to lower levels of atmospheric noise and
longer hours of darkness. In the United States, due to the large number
of stations, many smaller mediumwave stations are required to sign off
or reduce power sharply at sunset so as to reduce interference to
distant stations.
Whereas the mediumwave band can be counted on to provide distant
reception with much dependability, this is not the case at all on the
VHF-FM band. Under `normal' conditions, VHF-FM signals generally carry
no more than 150-250 km (100-150 miles), or `line of sight', since the
ionosphere generally does not reflect VHF-FM signals. VHF-FM
transmitting antennas are thus usually located as high as possible. Tall
towers, high buildings, and mountaintops are common VHF-FM transmitter
sites.
However, under certain rare conditions, the atmosphere will even reflect
VHF-FM signals, thus making it possible to receive these stations at
quite long distances. There are two major forms this distant reception
can take; the most common is Tropospheric Ducting, or tropo for short.
Typically, this occurs when a warm air mass forms on top of a cooler
mass closer to the ground. The area between these masses acts like a
pipe, `bending' the signals back to the earth well beyond the horizon.
This reception is most common in local late spring and summer months, in
the post-sunrise hours. It will enable the alert VHF-FM DXer to log
stations up to 800 km (500 miles) away in optimum conditions.
The other relatively widespread form of VHF-FM DX is called Sporadic E,
or E-skip, because it is the E Layer of the ionosphere which reflects
the signals. Like the name implies, this form of propogation is very
sporadic, yet very intense. When it's in, it is VERY strong. Stations
from a relatively limited geographic area 1300-2000 km (800 to 1200
miles) away will suddenly boom in, strong, often in stereo but quite
fadey, even overpowering semilocals in many cases. It will start at the
bottom of the VHF-FM Band (actually TV channels 2-6 first) and work its
way up in frequency. The highest frequency at which signals are
reflected by the ionosphere is called the Maximum Usable Freqiuency
(MUF), just as it is in shortwave, and it can occassionally surpass the
top of the VHF-FM dial in an unusually good opening.
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