Bandwidth Explained

Lately I have doing a lot of studying about ham radio in my spare time. I get a monthly magazine from the American Radio Relay League or ARRL called QST and there are a lot of concepts I don't understand as I read articles. Rather than continuing to read and glossing over the complex subjects, I am trying to understand them more. This has me thinking about bandwidth and what it actually means.

We use bandwidth in our casual conversations with phrases like, "I don't have the bandwidth to work on that right now." That generally means that you are doing so much you don't have room in your schedule to do anything else or that the task consumes more time than you have free. We also talk about bandwidth for WiFi and how much data we can stream to our devices like smartphones, TV's, and computers. Here is a quick history lesson of what that all means and why it relates to ham radio.

When we first discovered that we could send messages over the air without having to use wires or connections, we found that we needed a carrier frequency for the message. At first, we sent messages using Morse code which uses short (dot) and long (dash) pulses of energy to convey a message. To do so, you might use 14.025 MHz as your carrier frequency with the bandwidth of your message only being a hundred Hz wide. What that means is that if someone else tries to send a message at a frequency too close to 14.025 MHz, say at 14.0251, the messages might interfere with each other. To fix this you would spread the carrier frequencies further apart. Perhaps you might use 14.026 MHz instead.

Now not everyone knows Morse code and so we learned how to encode voice messages in those same radio frequencies. We started with amplitude modulation or AM and later developed frequency modulation or FM. Anyone who has spent much time with a radio knows FM has higher quality sound than AM and so most of our music listening stations are on FM. The FCC has limited AM radio stations to a total bandwidth of 10 kHz. That means if you still use a carrier frequency of 14.025 MHz, your signal will fall in the 14.020 and 14.030 MHz range as AM splits the signal equally on either side of the carrier frequency (5 kHz below and 5 kHz above). In contrast FM signals are 200 kHz or almost 20 times wider than AM signals. It is no wonder that FM signals are higher quality than AM as you can fit more information in that space. It also means that FM radio stations have to space out their carrier signals much further than AM stations. By contrast, FM ham radio signals are only 16 kHz wide as they only need to transmit voice and not music.

Now lets tie this all together with how we think of bandwidth of our Internet links at home. When signing up for home Internet service, vendors like to use terms like 10 mega-bits per second (Mbps) or 1 gigabit per second (Gbps). With 10 Mbps it means you will be able to download 10 million bits every second. 1 Gbps is much better as it is 100 times more data in the same second at 1 billion bits every second. Now remember that there are 8 bits for every byte or character in a file so if you have a 10 mega-byte file, it won't take a single second to download it at 10 Mbps, the fastest you can download it would be in 8 seconds but even that is not true. Networks use parity and error correction bits to ensure you get the data correctly. While you are paying for 10 Mbps speeds, you are really only seeing about half that because of how the network works.

So how do kHz and Mbps relate? There is a bit of math that would make this post really long and confusing, not to mention boring. Suffice it to say that when you are talking about radio signals, you generally use kHz and when you talk about data, you use Mbps or Gbps. It all translates down to how wide the radio (in the case of WiFi) or network (in the case of wired connections) signal is needed to provide the information you are trying to receive.