When you pick up a handheld transceiver and communicate on
VHF or UHF across town or through a repeater you are generally
transmitting line of sight. Not so however with HF transmission.
When you are using 160m to 10m the signal reaches the earth's
ionosphere and bounces back to earth to be received at a greater
distance than line of sight.
The ionosphere is a layer in the Earth's atmosphere that lies
in a range of 80 to 300 miles above the Earth's surface that
reflects radio waves. As the sun shines on the ionosphere it
changes composition and height which affects the propagation
characteristics. In general signals below 30 MHz bounce off this
layer and return to Earth while signals above 30 MHz go through
the layer into outer space. So watch what you say on 2 meters,
someone on Alpha Centauri might be listening. Signals below 30
MHz can also travel by ground wave and be received a short
distance from the transmitter.
Radio signals that are bounced or refracted off the
ionosphere are also affected by the time of day and season of
the year. During the 24 hours cycle the ionosphere changes in
height above the Earth and bounces some signals while absorbing
others. During the day the higher frequencies (above 10Mhz) tend
to propagate while lower frequencies are absorbed. At night the
reverse happens. There are many exceptions to this but it is a
good general guideline.
Seasons also affect propagation. Summertime in the northern
hemisphere means that higher frequencies have better propagation
while in the winter the lower frequencies improve. An
interesting time of the year for propagation is when the seasons
change from fall to winter and from winter to spring. This is
often when the best DX can be found. Because the seasonal
change is occurring in both hemispheres but in the opposite
direction DX from North American to Australia or southern Africa
can be at its best.
The Sunspot Cycle
phenomenon that affects radio propagation is the 11 year
sunspot cycle. A peak occurred during the year 2000 and
the next peak will occur around 2011. A sunspot low occurs
at the midpoint of this cycle. When the sunspots are at
their maximum propagation is at its best. At this time the
higher shortwave frequencies exhibit the best propagation
extending to 6 meters which becomes quite popular during
this time of the cycle. 10 meters can easily work stations
worldwide with low power (even qrp) and a modest antenna.
Station WWV does more than
broadcast the time. In addition to the time they
broadcast propagation information at 18 minutes
past each hour. Check 2.5, 5, 10, 15, 20 MHz.
Also check their web site at WWv
Making Propagation Work for You
|Why should you be
interested in propagation? Well if you want to make those
DX contacts you will need to be aware of when propagation
is best for conditions to your target. One way to do this
is to listen. Another is to check propagation sources such
as WWV mentioned above. You can also get familiar with the
propagation tables presented monthly in QST and CQ
magazines. These charts will tell you when to expect the
best propagation on what bands and at what time for your
location to other areas of the world. An excellent source
is to check the DX clusters online or by packet radio.
This can give you current information about DX activity
that is happening right now.
VHF and UHF
Although these frequencies are not normally thought to be
bands where propagation occurs there are times when it signals
on VHF and UHF travel well beyond their expected distance. These
conditions generally occur erratically and are not as
predictable as HF conditions. Some of the conditions that can
affect VHF and UHF are:
- This condition refers to tropospheric propagation which is
primarily affected by weather systems. Look for significant
weather changes such as a cold front moving in or a major
storm system. Also early evening as the earth cools and
early mornings as the earth warms can cause these
- Sporadic E - occurs when small patches
of the ionosphere are ionized . Sporadic E occurs during
June-July and early August and again in December and
January. as the name suggests this condition is sporadic and
you have to be very patient to benefit from it.
- Aurora - The northern lights (aurora
borealis) occurs in the northern parts of Canada and can
also be seen from northern U.S. states. The
"lights" are caused by a solar storm and can have
an effect on radio propagation. If can affect VHF and UHF as
well as HF propagation. To use the aurora you may need to
point your antenna north to work a station to the south as
signals bounce back from the aurora.
Depending on who you want to work and where the operating
mode you choose can be almost as important as the propagation.
The section Operating Modes explains the
various modes that you can choose from as a radio amateur.
© 2001 - 2007 Don Cassel VE3XD