One of the more interesting questions a new or prospective ham will have is, “how far can I communicate?” The frustrating answer is, “it depends…” (don’t you hate hearing that?)
There are many factors involved in the limits to distance of radio communication. Carrier frequency is the huge one, followed by operating mode, antenna characteristics and transmit power. Time of day, solar activity and the season (spring, summer, autumn, winter) also have a big impact on range. Natural (thunderstorms, aurora, geologic, cosmic) and man-made (crowded band, power lines, noisy electronics) interference can also limit or disrupt a radio contact. Also factor in the use of repeaters (terrestrial and space satellites) or reflective objects (structures, moon, meteor showers), plus unusual weather conditions and you have a lot to consider.
Since many hams get started using VHF/UHF radios for local communication, let’s talk about this first. VHF/UHF radio wave propagation is normally limited to line-of-sight, meaning the antennas at each end must have a clear path between them (no obstructions such as buildings, trees, and particularly, the earth).
3-20 miles is a realistic range for VHF/UHF hand-held radios on the ground, depending mainly on a clear path and relative height of the two parties. Throw in a repeater with a high antenna and that range extends considerably. Raising your own antenna up higher buys you much more range, as does more transmit power. A directional antenna pointed at the desired station will also help a great deal but cannot overcome the line-of-sight limitation. If vegetation or buildings block your line of sight, sometimes your signal can reflect off structures along a clear path, which is what hams living in big city canyons sometimes have to do.
So VHF/UHF radio work is generally local, typically 50mi range at best. Occasionally hams will enjoy an unusual condition know as tropospheric ducting (tropo) where atmospheric conditions favor VHF sky-wave propagation far beyond line of sight. Making contacts a few hundred miles away is possible under these conditions. Because tropo is infrequent and unpredictable it is not reliable enough for normal long-distance VHF/UHF radio communication. It is more of a surprise and a fun occasion, although some VHF enthusiasts live for this and try to make as many long-distance contacts as they can in the narrow window of opportunity. Because it is a weather-related situation it can and will change quickly. Similarly, some hams enjoy the occasional ionospheric condition known as Sporadic E (Es) where this lower ionospheric layer provides some VHF reflection over long distance.
Hams who operate VHF/UHF bands can work great distances by using space satellites to repeat their signals. You can also communicate with the International Space Station (ISS) when it is in view since many astronauts are licensed hams themselves and it provides a fun way to make contact with ordinary humans on the ground. If you are truly adventurous and can afford high-power amplifiers and tracking antennas, you can reflect VHF signals off the moon, which allows you to work stations thousands of miles away. This popular activity is known as Earth-Moon-Earth (EME) or moon bounce communication. Some hams also have fun using meteor showers and auroras to reflect their VHF signals over great distance as well, although this is not predictable or reliable.
OK, so hopefully we gave enough basic info on VHF and UHF radio communication distance to satisfy the casual reader. If any of these details interest you, plenty of information on them and more can be found on the internet.
Now let’s talk about propagation on the high frequency (HF) bands where long-distance radio communication is routine. The HF bands commonly used by hams are (from lower to higher frequencies): 160m, 80m, 60m, 40m, 30m, 20m, 17m, 15m, 12m, 10m and 6m. On all of these it is common to make radio contacts hundreds and thousands of miles distant.
The nature of this is that under certain conditions HF radio signals bend through the ionospheric layers back down to earth, sometimes back up and down again. By this your transmitted signal may reach anywhere around the world. It’s quite possible to contact a station literally on the other side of the planet, and anywhere in between. Ironically, due to the nature of ionospheric propagation, it’s difficult to work stations 30–400 miles (50–650 km) distant because they are too close for normal “skip” and too far for ground-wave propagation. NVIS antennas optimized for this short distance are used to reflect the signal back down instead of letting it refract away.
Long-distance ionospheric propagation via ordinary sky waves is very complicated and not fully understood; it behaves magically sometimes. However, there are some well-known generalities we can talk about.
Ionospheric refraction and reflection of radio waves is what sky wave (as opposed to short-range ground wave) HF propagation is all about. This is largely influenced by the sun and happens in two ways.
First, the sun’s UV radiation activates and energizes the ionosphere in the day but not at night. We get more ionosphere-influencing daylight in the summer and less so in winter so that’s the seasonal factor. The net result is certain bands perform better in the day and others at night, and better in summer vs winter.
Second, the sun’s natural 11-year (approx.) activity cycle of sunspots and solar flares also greatly influence our ionosphere. The net result here is that the higher-frequency bands (15m, 10m, 6m) provide excellent long-distance propagation in times of high solar activity but these bands perform poorly when the sun is quiet.
Beyond this simplified and generalized explanation we don’t want to spend much time on the details. Research it yourself if interested; lots of info on the web. An excellent summary is The HF Bands-For HF Newcomers by Gary Wescom N0GW.
Moving beyond the important ionospheric factors, HF communication range is strongly influenced by transmit power. This is a general truism for all bands and all modes. That doesn’t mean you need cloud-burning kilowatt amplifiers to reach long distance, but more power puts more signal energy into the atmosphere. Now the guy on the other end may be running low power, so having lots of power doesn’t always ensure good two-way communication.
Operating mode also makes a big difference in how far your signal can travel. Wide-bandwidth voice and SSTV modes simply don’t reach as far as a narrow-band CW (Morse code) or digital modes; their intelligence dissipates more easily. The CW and digital emissions focus more of their energy into a narrow chunk of the electromagnetic spectrum and will propagate farther with less interference. Bottom line here is that you may not be able to reliably converse with a ham in Peru but you are more likely to establish and complete a contact using Morse code or a PSK digital signal.
With HF bands here again the use of directional antennas will significantly enhance distant communication but these are not essential. Many hams work all US states and many other countries and all continents using simple wire antennas.
Getting technical here, 160m is officially in the Medium Frequency (MF) spectrum but is generally treated like one of the HF bands and is supported by most modern HF transceivers. Antenna length is one of the biggest considerations in using this “Top Band”. Similarly, 6m is really part of the VHF spectrum and shares some of those characteristics. However, 6m is not always limited to line-of-sight propagation and can reach great distances as a sky wave. The 6m band is a feature of many modern transceivers but its usefulness is highly dependent on solar activity.
Let’s introduce the term DX here. DX is ham-speak for distance and generally means communicating with stations outside your own country. However, in a large entity working across the country could also be considered DX since it may be well over 1000 miles. For example, Maine to southern California and Florida to Washington state would be good DX. So is working Alaska or Hawaii from any of the 48 contiguous states. Florida to Cuba could be out-of-country DX but not from a distance perspective. DX is usually a big goal for HF operators who try to make contacts with many different countries, chasing awards such as ARRL’s DX Century Club and the QRZ DX100 (both require 100 confirmed country contacts).
Note that in the USA the entry-level Technician license has limited HF operating privileges. Tech licensees are only allowed to do Morse code (CW) emissions on the HF bands below 28MHz. They can operate voice and digital modes on 10m and higher frequencies but in a solar minimum (current as of this publication) those bands are mostly dead for many years. This limitation on HF operating modes is a big incentive for Techs to upgrade to a General class license which would allow them to use all modes on most of the HF bands.