This is the first in a series of beginner-friendly posts on radio concepts. In amateur radio, you’ll hear a lot of terms like “VHF”, “2 meters”, “FM”, “single sideband” and so forth. These give essential information about a radio signal. So, what do all the terms mean? I’ll try to keep it simple, with some more technical info in links and footnotes.

Frequency and wavelength

The most basic property of a radio wave is its frequency. As a wave moves past an antenna, it creates a positive voltage, then a negative voltage, then back to positive and so on. The number of complete cycles per second is the frequency, measured in hertz (Hz). So a radio wave with frequency 500 kHz (500,000 Hz) goes from positive to negative and back 500,000 times per second. A FM radio station on 96.5 MHz completes a cycle 96,500,000 times per second.

Instead of frequency we can also talk about wavelength, which is the distance between consecutive wave peaks (or troughs) in space, at a single instant in time.

(image source)

Radio waves travel at the speed of light, so there is a strict relationship between frequency and wavelength:

Because the speed of light is very close to 300,000,000 meters per second, we can use this approximation:

For example, that FM radio station at 96.5 MHz has a wavelength of approximately 300 / 96.5 = 3.1 meters. WiFi at 2.4 GHz (2400 MHz) has a wavelength around 300 / 2400 = 0.125 meters = 12.5 centimeters.

Wavelength is particularly important because the size of an antenna, generally speaking, is proportional to the wavelength of the signals it needs to transmit or receive. Longer wavelengths (lower frequencies) require larger antennas.

Bands

It is useful to group ranges of frequencies into bands, because different frequencies have different physical properties and applications. People have found uses for radio waves all the way from a few Hz to hundreds of GHz and beyond. However, the most widely-used frequencies in ham radio fall into three general ranges:

As you can see, the terminology is archaic; “high frequency” is actually towards the low end of frequencies in use today!

Amateur bands

When hams speak of “bands” we usually mean a specific range of frequencies reserved for amateur use. These are identified by approximate wavelength, e.g. “80 meter band” or “70 centimeter band”. You’ll need to consult a chart to find the exact frequency limits and privileges, which vary by country and by license type.

(excerpt from ARRL band chart)

The most important bands for local communication are 2 meters and 70 centimeters. Most handheld radios, including the ubiquitous Baofengs, will support both. Because of line-of-sight limitations, repeaters are very common on these bands. A repeater is a fixed station on a hilltop or tall building, which receives a signal on one frequency and simultaneously retransmits it on another. Because the repeater has a better location, better antenna and more power than your handheld radio, it can vastly extend your effective range — sometimes to 100 miles or more. A full write-up about repeaters is out of scope here, but you can find a wealth of information online.

(own work)

To complicate things, hams will sometimes say “VHF” to mean 2 meters and “UHF” to mean 70 centimeters, even though there are other amateur bands within VHF and UHF as defined above.

On HF a whole bunch of bands are available. The best choice at any point in time depends on many factors such as time of day, ionospheric conditions, the space available for antennas and the license privileges of the operator. At the current sunspot minimum, lower frequencies like 40 and 20 meters are open on a daily basis, while the higher frequencies like 10 meters are only sporadically open for long-distance communication.

Voice modes

A constant radio wave on a single frequency doesn’t convey any information beyond “I’m here!”. In order to send a message — be it voice, text, or something else — we need to vary the radio wave over time. This is where modulation modes come in.

Amplitude modulation (AM) is one simple way to transmit audio. The amplitude (strength) of the radio wave varies according to the amplitude of the sound wave. Because AM transmitters are easy to build and AM receivers are even simpler, AM was the first voice mode. It is still used for broadcasting and some other things, but has largely been replaced by FM and SSB on ham radio (see below).

Frequency modulation (FM) is another audio mode. Instead of varying amplitude over time, we vary frequency. FM produces clear audio and is the dominant mode for local communication on VHF and UHF. Most handheld radios do FM and only FM. The main downside is that a FM signal has a large bandwidth: it occupies a large range of frequencies. For this reason and others, FM is not used on HF bands.

(image source)

Single sideband (SSB) is a more efficient form of AM. Although AM is different from FM, the varying amplitude nonetheless spreads the signal over a range of frequencies.

(image source)

In this waterfall plot of an AM signal, the horizontal axis is frequency, the vertical axis is time, and the color indicates signal strength. The strong red signal in the center is the carrier and contains no information by itself. Also, there are two copies of the audio signal: one above the carrier, the other below. These are the upper and lower sidebands. In other words, the AM signal has a lot of redundancy. By transmitting just one sideband and no carrier, we can save on both power and bandwidth.

(same image source)

This waterfall shows an upper sideband (USB) signal. The lower sideband (LSB) would be the same, except flipped left-to-right. For historical reasons, LSB is used on the lower HF bands (up to 40 meters) while USB is used at higher frequencies.

The main disadvantage of SSB is that it requires more complex electronics in both the transmitter and receiver, which is why SSB did not catch on until the 1960s. With modern electronics it is easy to send and receive SSB. Aside from a few diehard AM users, SSB has taken over as the voice mode for HF ham radio.

Digital voice modes encode audio as a stream of bits before sending it. There are a number of digital modes in use, mainly on VHF and up. I don’t do digital voice so I don’t have much to say about it!

Other modes

Continuous wave (CW) is the oldest radio mode. It works by switching a transmitter on and off to represent the dots and dashes of Morse code. Although Morse proficiency is no longer required to get a ham license in most countries, it is still a very popular mode. CW occupies a very small (though nonzero) bandwidth, and works well even when signals are weak, because the human ear is very good at picking out the rhythmic beeps of Morse code. CW-only radio kits are the most affordable way to get into HF.

The downside of CW, or upside depending on how you look at it, is that you have to learn Morse code! Skilled CW operators don’t hear individual dots and dashes; they recognize the rhythms of letters or entire words and phrases. It’s like learning a spoken language or a percussion instrument. Many tools and resources are available for learning Morse code, too many to list here.

Digital data modes allow sending digital data over ham radio. Typically this involves an audio connection between a radio and a computer’s sound card. The computer translates data to and from audio tones, much like an old-school phone-line modem. Sometimes a dedicated device takes the place of the computer, or the functionality might be built into the radio itself.

A large number of digital modes are used for different purposes. Some of the more popular ones are FT8, JS8, RTTY, PSK31, and APRS.

Slow-scan television (SSTV) is a way of sending images over ham radio. Despite the term “television”, we are talking about grainy still images sent over the course of several seconds or minutes. If you miss the excitement of waiting for JPEGs to load over dial-up internet then this is the mode for you. Because images are sent so slowly, they fit into the same bandwidth as a single SSB voice signal.

Amateur television (ATV), by contrast, is full-motion video and audio sent using the same technology as commercial broadcast TV. This requires a lot of bandwidth (6 MHz) so it’s only done on the higher-frequency bands (70 centimeters and up) where there is more room.

That’s it for now! Hope you found this informative and stay tuned for more content. There is lots more to say about modes so check out Wikipedia or the ARRL website if you’d like to know more.