Radio Wave Propagation

Like light, radio waves can be refracted, reflected, scattered, or diffracted by objects along the path. This leads to a number of different propagation modes which can produce useable signal levels even when no line-of-sight exists. At HF much of this takes place in the ionosphere, where particles ionized by the action of sunspots bend or reflect light to quite distant areas. At VHF/UHF these ionized regions often can’t influence the radio wave, and much more often natural or man-made objects reflect or diffract the waves.

The direct path and reflected path can combine together to cause fading of the signal. This isn’t usually a big problem for reasonable ranges, as there are usually buildings, trees, and the natural variation in the land to break up the reflected wave. However, this can be a problem for short ranges over smooth terrain with low power radios (like walkie-talkies).

The level of ionization that occurs is based on the amount of solar radiation, most notably extreme UV and X-rays, reaching the atmosphere. It has been observed that there is a good correlation between the number of visible sunspots and the rate of ionization. The number of sunspots vary in an approximate 11 year cycle, with the spots “facing” earth being the ones most important to this effect. The sun rotates on its axis, so sunspots can rotate out of view and return about 27 days later if they haven’t faded away. The tilt and rotation of the earth also influence the amount of ionization, so seasonal as well as diurnal variations occur. The current cycle began in 1996, but surprised everyone by having a secondary peak near the end of 2001. Predictions are that the cycle will bottom out in 2007.

The D-layer does not refract signals well, and at frequencies below 5 MHz, tends to absorb most radio energy. The amount of absorption depends not only on frequency, but also on the path length traversed, so that signals up to 10 MHz only pass through at high angles. At night the D-layer disappears, allowing low-frequency long distance (DX) communications to take place.

The E-layer was the first layer recognized as supporting long-range communications. It too is present mostly in daylight hours but at times of high solar activity can extend well into the hours of darkness. The E-layer is responsible for much of the 6 meter, and sometimes 2 meter DX, most notably during sunspot peaks. A variant called sporadic E, or Es, is where the ionization in the E-layer is bunched together in “clouds” by upper atmospheric winds. These short-lived patches of extra ionization lead to clusters of DX contacts from opposite sides of the event.

The F-region is the responsible for most of the HF long-range paths. It splits into two regions, F1 and F2, during the daylight hours, merging into a single layer at night. While ionization is less at night, so are the losses in passing through the D and E layers, so long distance contacts persist on the lower HF bands.

Let’s not forget the Troposphere. While it is not an ionized region, the variations in atmospheric density can act as means of refracting and scattering VHF and UHF signals. This is the region where weather plays an important part, rather than the sun, and even rain can act as a scattering mechanism.

The highest frequency at which a signal aimed straight up at the ionosphere is reflected back to earth is called the maximum useable frequency (MUF) or critical frequency. Frequencies above this will pass through the ionosphere, except at shallow angles where the wavefront is bent back down towards the earth. This is the feature that gives us the ability to make long range contacts. However, in doing so there are closer regions, that are beyond the range of ground wave communication, that will not have any significant signal present - this is called the "skip zone". Thus, there are times on certain frequency bands where you may be able to talk to California from Maryland, while not being able to talk to Kentucky.

The distances covered can sometimes be much larger than expected, due to multiple-hop propagation, or due to a wave that gets "trapped" following the ionoshpere. When ionization conditions are good it is even possible to talk to stations in the skip zone, where there should be no signal present. This is due the the reflection, or backscatter, of the original signal from the target area in multiple directions - one of which way be into the skip zone. For backscatter propagation to work, both operators should point their antennas toward the region where there is good propagation. Backscatter signals can be extremely weak, and fluttery, but it is possible to make contacts that might otherwise not be predicted.

Other strange paths due to the ionozed layers are possible as well, one of the most interesting of these being grayline propagation, where the radio signals follow the sharply changing ionized area along the sunrise/sunset line. Losses at lower frequencies are low in the dark region, and less near this edge, than they are in the sunlit regions as the D-layer hasn't had a chance to become absorptive. Signals are enhanced along the boundary, so the thing to do is to point your antenna approximately along the boundary (in both directions) and listen for distant stations. The direction of the grayline varies throughout the year, due to the tilt of the earth's axis, and can cover different regions at your local sunrise and sunset. There used to be a set of sliderule like devices for doing grayline prediction, but there are now many computer programs that will display the location of the terminator on a map of the world.

In the VHF frequencies (especially 6 meters) sporadic E (Es) is a frequent summertime event, where small localized pockets of ionization are formed in the E-layer and act as scattering centers for stations on opposite sides of the formation. The map above shows the end points of a number of contacts taking place on 6 meters during an Es event. The ionized region can pretty well be located as the intersection of the crossing of those contacts. Disturbance to low TV channels is a clue to run for your 6 meter radio - especially if you don't get channel 2 in your area and suddenly see it!