 |
Guide to DXing |
|
Real Radio |
|
DX Archive |
|
|
|
|
Propagation / Fading ? @ DX Guide - DXing Info |
|
A Detailed literature can be
found here
If you've listened to short-wave long enough, you've probably noticed
changes in the way signals behave on each band as the day or night
progresses. For example, higher frequencies produce signals best in the
daytime, while no distant signals can be heard on lower frequencies. Then at
night the higher frequency stations weaken or disappear while the lower
frequencies come alive with signals. Why does short-wave reception generally
follow this pattern? The answer can be found in understanding propagation,
the study of how radio signals travel.
| Important Terms |
|
PROPAGATION:
The study of how radio signals
travel.
SKY WAVE SIGNALS:
Signals that travel
straight across the horizon and on into space.
SKIP SIGNAL / MULTIPLE SKIP:
Radio signals
which bounce off the ionosphere, one or more times.
D LAYER:
That layer of the ionosphere which
absorbs lower frequency signals during the day.
E LAYER:
The ionosphere layer located about
60 miles above the earth which has the ability, to reflect
shortwave radio signals back to earth.
SOLAR CYCLE:
The 11-year cycle of sunspot
activity. At cycle, peak the sun activates the ionosphere so
that the M.U.F. is reliably higher.
GRAY LINE:
The line of twilight that
stretches around the globe.
|
You already know that short-wave signals produce sky wave signals. These are
the signals that leave the antenna parallel to the ground and travel
straight across the horizon and into the sky. Of course, they have to bounce
back down to be heard, and that's done by the ionosphere.
Understand the ionosphere, and you'll understand short-wave propagation.
The ionosphere is a part of the atmosphere that surrounds the earth, and is
created by direct sunlight. It's why daytime and nighttime conditions on the
short-wave bands change over a twenty-four hour period. When the sun strikes
the earth's atmosphere, it creates different layers of ionosphere with each
layer having different characteristics that will affect lower to higher
radio frequencies on the short-wave band. The overall band is one point six
to thirty megahertz. The only ionosphere layers we are
concerned about now are called the D and E layers.
The E layer is the most important. Located about 60 miles above the earth,
the ionosphere E layer has the ability to refract, or reflect, short-wave
radio signals back to earth. This is called a skip signal. Often the signal
will again reflect back upward from the earth and bounce off the ionosphere
a second time, landing back on earth even further away. That's called
multiple skip. So now you see how short-wave radio can travel such long
distances. The ionosphere E layer is reflecting signals far beyond the
horizon, all day and even through the night.
The E layer is strongest during the day, because it receives it's daily
strengthening when the sun is directly overhead. It's this strengthening
that allows higher short-wave frequencies to become active during the day.
The specific point at the high end of the short-wave band that is active is
called the Maximum Usable Frequency, or M.U.F., and that varies from hour to
hour and day to day. It will peak during midday at about twenty-five
megahertz. At night, as the layer weakens, the Maximum Usable Frequency
comes downward and the higher bands will weaken. Radio signals above the
Maximum Usable Frequency will penetrate the E layer, and travel on into
space.
On the other hand, the lower frequencies come alive at night using the E
layer to reflect stations thousands of miles away. Why couldn't they do this
during the daytime? Because the daylight was also activating the D layer of
the ionosphere. The D layer is closer to earth at an altitude of about 50
miles. The D layer absorbs lower frequency signals, creating blackout
conditions for all but local stations on these lower frequencies.
Fortunately the D layer is quick to disappear as the sun sets. And it is
also fortunate that it allows higher frequency signals to penetrate up to
the E layer during the day. This is why one end of the shortwave band
behaves differently from the other end from night to daytime.
Let's look now at the fine points of propagation, and see how it all ties in
with the sun and and the ionosphere.
First, a quick summary, picture a D layer of the ionosphere above the earth
only in the daylight, and an E layer of the ionosphere even higher above the
earth existing 24 hours a day.
The E layer reflects shortwave signals. The D layer absorbs only the
lower frequency shortwave signals during the daylight hours. The more
sunlight the E layer receives, the higher the shortwave frequency it will
reflect.
At night, the D layer disappears, and even though the E layer is not at its
strongest, it's now allowed to reflect lower frequency signals. You can see
how your listening on different bands or frequencies is determined at
different times of the day by propagation. It is now obvious that the sun
has everything to do with shortwave propagation and changes in the sun will
affect shortwave reception.
The sun goes thru an 11-year cycle of sunspots, called the solar cycle.
During the peak of the solar cycle, the sun activates the ionosphere so that
the Maximum Usable Frequency is reliably higher. Many international
broadcasters adjust their frequency schedules higher at this time to take
advantage of this and get away from crowded lower frequency bands.
Also during the solar cycle peak, radio blackouts can occur when solar flares
erupt. The flares create the northern lights, which absorbs radio signals
passing thru the polar region. Since so many signals travel this way to
distant listeners, a near blackout on the bands will be noticed, with only
locals and stations from equatorial latitudes heard.
More common changes you'll experience with the sun are the seasonal changes.
With less sunshine in the winter, the paths of darkness between transmitter
and receiver are broad and last longer. For lower frequency signals
dependent on the path of darkness, reception will be best in the winter.
Reception also is quieter in the winter with fewer thunderstorms in the
temperate zones.
Some stations will be heard only in the winter as a result. With longer days,
sunrise or sunset comes earlier to the transmitter and receiver points,
cutting down the listening opportunities using the path of darkness.
Also note, that when wanting to hear a station on a higher frequency, the
best reception will be with a daylight path between you and that station.
Remember, it's the sun that strengthens the ionosphere.
The most unusual path of darkness that may bring you some DX is called the
Grayline. This is simply the line of twilight, either dawn or dusk, that
stretches around the globe at any given time. When this twilight point is
evident at your receiver site and the transmitter site at the same time, you
may be able to briefly hear the station's signals traveling that line, even
though it may NOT be the shortest path between you and that station. The
condition will exist only for several minutes. And as the sun's seasonal
position constantly changes each day, a specific reception condition from
your point to that station's will gradually be lost until that time again
next year.
A Detailed literature can be
found in Propagation
[ Back ] [ Next ]
|
|
|
MW Guide:
Introduction
|
|
|
|
Complimentary Industry Resources
DXinginfo.Com is trying to offer you a new, exciting, and entirely free professional resource.
Visit http://dxinginfo.tradepub.com
today to browse our selection of complimentary Industry magazines, white papers, webinars, podcasts, and more across 34 industry sectors. No credit cards, coupons, or promo codes required.
Try it today!
|
|
|
