Do you remember your band chart?
Common amateur activity falls into three major bands: HF, VHF, and UHF bands. HF includes frequencies from 160 meters all the way up to 10 meters. These bands will give you world wide communications. In the Novice and Technician Plus classes, most of this activity is restricted to data transmissions, but you can make voice transmissions on the 10 meter band.. The lower band waves are able to travel far into the atmosphere (almost into space) and then bounce off one of a few layers of ionized gases in the ionosphere. This property is what allows energy waves to travel so far across the earth - the energy is radiated, bounces off of a layer, then is reflected back to another point on the earth. Bouncing waves off of the ionosphere is called sky-wave propagation. Theoretically speaking, if you pretend that the earth is perfectly round, that there is no loss in radio wave energy during transmission / reflection, and that you could reflect a wave from an infinite distance away from the earth and then back to the earth, then one could reach any point on the globe. But, there are many interferences during transmission and the physical characteristics of the earth deviate, so this will hardly be possible most of the time. But you can probably envision this: the farther away that energy is reflected away from the earth, then the greater physical distance on the earth that can be covered with that energy.
Now check out the ionosphere. This region becomes ionized due to ultraviolet (UV) exposure from the sun. The ionosphere is layered so that depending on what time of day it is, you can propagate over great physical distances. Ionospheric charging over any given area is greatest at midday, and at its least before sunrise. Due to the rotation of the earth, any point receives varying doses of UV radiation throughout the day - the end effect is that the ionosphere stratifies. The layers formed are described below:
D Layer: This layer is close to the earth and is only present during the day hours when the ionosphere is highly charged by UV radiation. Without daylight, the D layer completely disappears. The D layer is not very useful or reliable in very long-range communications - intense ionization causes layer D to absorb most MF and HF signals. D is only good for reflecting some very low frequency signals over relatively short distances, compared to the other layers.
E Layer: The E-sporadic layer remains charged for some time after UV exposure is lost, so it will be a reliable source of reflection for just a little while after the sun goes down. It is weakest just after sundown.
The E layer is most effective for 6-meter propagation. Long range skip off of E-sporadic for the 10-meter band indicates that you can probably get the same results with 6-meter and 2-meters.
F Layer: The far layer that exist 24 hours a day, no matter where you are located. However, while receiving direct sunlight, it is most heavily charged and splits into two distinct layers - F1 the inner layer and F2 the outermost. At night, it combines back into one layer. This effect is illustrated in the figure below.
As the amount of ultraviolet radiation that enters the ionosphere increases, the amount of radiofrequency reflectivity increases. And the amount of UV is dependent on solar activity and the sunspot cycle. As solar activity increases, more sunspots erupt on the sun's surface, therefore increasing the dose of ultraviolet radiation that the earth receives. A typical sunspot cycle last about 11 years.
MF and HF frequencies can take full advantage of ionospheric bending. But these waves are highly dependent on angle of incidence. If the angle that the wave approaches the atmosphere is straight up (or not far off center), then it will not be bent back towards the earth, but instead into space. But as long as the wave angles more towards the horizon, then that wave will not overshoot and return to earth hundreds to thousands of miles away, depending on what layer it strikes and how dense the layer is with ionic particles.
MAXIMUM USABLE FREQUENCY
The maximum usable frequency (MUF), or critical frequency, is the highest frequency that can be reflected back to the earth from the ionosphere. The MUF is constantly changing due to the amount of UV radiation that the ionosphere is receiving. All frequencies above the MUF go on through the ionosphere into space. Just past this the boundary, waves can usually propagate over long distances through the scatter effect. Angle of incidence plays its part here, also. The MUF will only be reflected if is not "too vertical".
VHF AND UHF PROPAGATION
Lower frequency waves can be reflected in the ionosphere, while most VHF and all UHF frequencies can not. Commonly used amateur bands in the VHF-UHF bands are 6 meters (50 MHz), 2 meters (144 MHz), and 70 centimeters (440 MHz). The energy carried by these frequencies is so high that they can penetrate the ionosphere and just keep going! So don't count on being able to communicate with someone on the VHF and UHF bands by sky-wave propagation. The only way that these signals can travel and be adequately received is by line-of-sight propagation: directly from point A to point B. Some signals can even travel on and just below the ground. This is called ground-wave propagation. The effective range is only a few miles (probably less than a hundred, depending on contributing environmental factors).
At times, there may be patches of earth in which you may not be able to communicate with at all - on any frequency. These areas are called skip zones, and are defined as an area where ground-wave propagation is out of the question (too far away), but at the same time is too close for sky-wave propagation to be effective.
CIRCUMVENTING VHF-UHF LIMITATIONS
Because long-range line-of-sight propagation is so limited for VHF and UHF bands, repeaters were developed to increase the effective range of transmission and reception for mobile and low-power stations. A repeater is a device that receives an input signal, and then simultaneously transmits an amplified version on the output frequency. Repeaters operate in duplex mode, meaning the input is received on one frequency and then broadcasted on another. It sounds complicated, but repeater must operate like this because the input and output can't occupy the same frequency. On two meters, the input/output frequencies are separated by 600 kHz, either above or below ("+" or "-") in reference to the input frequency. The separation is called an offset. Anything above 147.000 MHz will carry a positive frequency offset, and anything below will be negative. You'll become well acquainted with this concept when programming the frequencies on your first 2-meter rig. For the 1.25 meter band, the separation is 1.6 MHz and for the 70-centimeter band, 5.0 MHz.
Other repeater features and characteristics: Most repeaters have a "courtesy tone" that will indicate to listeners that a transmission has been completed. Many repeaters have a nifty little feature called an autopatch which allows users to access the telephone system that is local to the area of the repeater. If repeaters have an autopatch, they most likely are programmed to quickly put you in touch with the local public safety authorities - check with a club contact to find out how to access these functions. If you happen to be ragchewing and get cut off in the middle of a call, don't' worry - some repeaters have a time-out timer that cuts you off if you transmit too long. Repeaters periodically identify themselves also via programmed CW at a maximum rate of 20 wpm.
Simplex mode can and should be used when operators are within range of communicating without going through the repeater. This should be done whenever possible to keep from unnecessarily tying up the repeater. Simplex simply means that both the input and output frequencies are the same. You can tell if you can communicate on simplex without going through the repeater if you can receive the other station on the repeater's input frequency.
OTHER MEANS OF HIGH FREQUENCY PROPAGATION
There are other means to propagate on UHF and VHF so that you can practically communicate world-wide, with some limitations. For starters, you can work a "moon-bounce", also known as EME: get a beam or array of beams, point them at the moon, and transmit a signal on high power. This bounces the signal over onto the other side of the planet. And its not too hard to do, as long as you keep re-positioning the antenna(s) to follow the moon. High gain is required during moon-bouncing to compensate for path loss and poor reflectivity of the moon's surface.
Third, in the event of a meteor shower, you can utilize an effect called meteor scatter. As meteors break up in our atmosphere, they leave behind billions of particles that yield the charged effect. It's not a very dependable means, but it is somewhat predictable, and you will more than likely notice a vast increase in your transmitting / receiving capabilities during a meteor shower.
VHF and UHF signals are also prone to reflection by metal structures; this property makes it possible for cellular phones and VHF-UHF HT's to communicate with the outside world, even though you may be in a building.
The term "tropospheric ducting" is a temperature inversion effect created by a stable-high pressure system over oceans where a warm, moist mass of air becomes sandwiched between two cool air masses. VHF signals can ricochet through this "tunnel" by bouncing off the dense boundaries of air within the system to propagate waves for hundreds of miles. This effect can typically be seen during the warm seasons. Path loss for signals increases as frequency increases, so the UHF propagation via tropospheric ducting is virtually nil.