Propagation

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

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.

Daily Patterns

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.

Seasonal Patterns

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

Another 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.

WWV Information

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 web site

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.

DX Propagation Web Sites

These are a few web sites that can tell you right now what is happening for HF propagation.

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:

Tropo - 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 conditions.

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.