Radio waves are a huge part of our lives every day. If you are using satellite radio or AM/FM radio, Over the Air TV, or even a cellphone or wireless network, then you are dealing with radio signals. Understanding how these waves work helps us to be better IT professionals, as well as amateur radio operators (if that is your hobby of choice). As a side note, if you ever wondered why microwave ovens are built the way that they are, it’s because they use radio waves to heat the items inside of them. And you don’t want them heating things outside.
What are radio waves?
A general answer for this is that radio waves are a form of radiation. Now, before you start worrying that your cell phone is going to mutate you, there are a few things we need to clear up. There are two types of radiation–ionizing and non-ionizing. Ionizing radiation is radiation that changes your cells and/or genetic code (think X-Rays or nuclear bombs). Non-ionizing radiation is radiation that primarily creates light or heat. Radio waves are non-ionizing radiation. On the spectrum, they are even more non-ionizing than sunlight.
There are two components to radio waves. The first is an electrical component, and the second is a magnetic component. If you imagine the radio signal as an electric current flowing along the length of an antenna, that is the electrical component. As if flows back and forth, it also radiates out in every direction (see radiation?). That is the magnetic component. Fun fact: if you hold a fluorescent light bulb near an antenna while it’s transmitting, it will light up (if the watts coming out of the antenna are higher than the rating on the bulb). You can keep it lit a few inches away from the antenna. This is the electromagnetic property of a radio wave at work.
Signals, deciBels, and Signal to Noise Ratio (SNR)
We use terms like signals to describe how well the radio waves are received. These signals are measured in deciBels dB. The strength of the signal determines how far away from the transmitter it will be picked up before it is overwhelmed by the surrounding noise levels (Signal to Noise Ratio).
Imagine you are hanging above a perfectly smooth lake. The water on the surface of the lake is the “noise floor”. If you drop a tiny pebble straight down into the lake, the ripples will flow out a short distance before they disappear. The ripples are the signal in your radio wave, and when they disappear, the amount of distance they traveled is their range. How high the ripple is compared to the surrounding water at any point is the signal-to-noise ratio.
Circles on the water. Wet texture.
Now, imagine you drop a bigger rock into the lake. You’ll notice that the ripples travel a bit further before they are absorbed back into the water. As you drop bigger rocks, you will notice the ripples go out even further, until you get to a point where they hit the shoreline and start to come back to you. And you will also see that the size of the ripples is actually bigger with each size of the rock you drop.
Putting this together
Radio waves are measured in terms of frequency. The wave is a sine wave, with a specific length between any two points on the wave. The frequency is often a given point on the wave is visible in a specific timeframe. In terms of radio waves, the timeframe is one second, and the frequency is denoted in terms of Hertz (Hz, kHz, mHz, gHz, etc). The higher the frequency, the shorter the wavelength (hence, the more points are visible in a given timeframe–one second). Looking at our lake example above, the tiny pebble would be an extremely high frequency, while the rock that created ripples that returned would be an extremely low frequency.
Ranges of wavelength and examples of each
- LF (Low Frequency) ranges from 30 kHz to 300 kHz. The wavelength ranges from around 10km down to 1km. This would be something like navigation beacons for ships and aircraft.
- MF (Medium Frequency) ranges from 300kHz to 3MHz. The wavelength ranges from around 1km to 100m. This would cover the AM Broadcast band.
- HF (High Frequency) ranges from 3MHz to 30MHz. The wavelength ranges from around 100m down to 10m. This would cover shortwave radio stations and even CB radios.
- VHF (Very High Frequency) ranges from 30MHz to 300MHz. And the wavelength ranges from 10m down to 1m. If you have a TV that gets the signal from an antenna, this would be channels 3 to 13. It also includes the FM Broadcast radio stations and aircraft bands.
- UHF (Ultra High Frequency) ranges from 300MHz to 3GHz. The wavelength is from 1m to 10cm. Channels 14 to 36 (83 on older TVs) on TVs, cell phones (to an extent), and the 2.4GHz wireless routers would fall into this band. Finally, your microwave oven operates in this range.
- SHF (Super High Frequency) 3GHz to 30GHz. The wavelength ranges from 10cm to 1cm. 5.8GHz routers would be the most well-known item in this range.
There are other ranges (ELF, SLF, ULF, EHF, etc), but they are beyond the scope of this page. In terms of amateur radio, you will rarely use anything in the lower ranges, and there isn’t a lot of commercially available equipment in the higher ranges. In IT, your routers and wireless devices mainly operate in the UHF/SHF ranges.
A note about antennas
Antennas are usually described in terms of wavelength. Typically, you will see an antenna that is 1/2, 1/4, or even 1/8 of a wavelength long. If you think about it, an AM Radio broadcast antenna would have to be about 600m tall if you were to use a full wavelength. Because you can get away with using a shorter length, they are typically 100m tall. Likewise, the length of an antenna for a wireless router is only a few cm long, because it doesn’t have to be the full wavelength. There is another length that is considered, which is “electrical length”. If you coil a wire around a metal rod, you would use a longer piece of wire than the rod’s length. The radio wave will resonate from the length of the wire–not the rod that it surrounds. So, you are able to cheat the antenna length a little more. There are drawbacks to this, though. If you get the coils too close together (especially with bare wire), it will short out. And the magnetic field collides with itself, which changes how the signal radiates. In a perfect world, you want to use a rod that is as close to 1, 5/8, 1/2, 1/4, or 1/8 of a wavelength as possible. In the real world, you compromise to get the best antenna that works for your situation, which is why you’ll hear people say that any antenna is a compromise.
POWER, Gain, and Signal Strength
For wireless equipment, power is measured in milliwatts. Radios are typically measured in watts and kilowatts. And broadcast radio stations are measured in megawatts. Watts are the effective transfer of the electromagnetic portion of the radio signal into space. As a general rule, if you place an antenna at a specific distance above the ground, the more power (watts) you transmit out, the further the signal will travel. This doesn’t take into consideration things like signal bounce or the curvature of the Earth (but both of those are important factors to consider). Height above Ground Level is a huge factor as well. Here’s a real-world example. I have a wireless router that puts out 27mW. If it’s 10 feet above the ground, I can pick it up a few blocks away. Now, I have the same router with the same power output, sitting 263 feet above the ground on an antenna tower. That signal can be picked up by another router with the same power output, which is located on an antenna tower 15 miles away. This is how the AREDN system operates.
Decibels to Power to Signal Strength
So, if the signal strength is measured in deciBels, the power is measured in watts, and the signal strength (which is how readable the transmission is at a given location) is denoted in SI units, how do they relate? In a perfect scenario (typically referred to as an isotropic antenna), for every 3 dB increase in signal strength, the power output doubles. So, if you put out 1 watt, and it’s measured at 3dB, then 2 watts would be 6dB, four watts would be 9dB, and so on. This corresponds to how well your antenna radiates the power that is being sent to it. Likewise, if you have a power output of 100watts and you lose 3dB, then your effective power out is 50watts. 6dB would be 25 watts, and so on.
Are you wondering about the SI units yet? Here’s the rub. If I told you that I was receiving your signal at 36dB, you would have to calculate how that relates to your effective power. And I would be throwing a lot of numbers at you, which might get lost if my signal is weaker than yours. So, a system was developed to give us a quick reference to the power and readability of radio signals. This system ranges from S0 (no signal at all) to S9+60dB (“Sixty over S9”, or “sixty over”), which is loud and perfectly understandable. This is also a reference to the SNR above. It’s how much louder your signal is compared to the surrounding static and noise. Above, I mentioned that every 3dB was a doubling of power. Every 6dB is an increase of one SI unit. So, given our 1-watt scenario above, you would have to put out almost 7 watts to get a change of 1 S-Unit on the scale.
A Word about Gain
Gain is a term that describes how well an antenna performs compared to another antenna. It’s measured in either dBi (deciBels compared to an isotropic antenna), dbd (antenna gain compared to a dipole antenna), or dBm (antenna gain measured in milliwatts). A quick note on dipole antennas. If you picture an isotropic antenna as a dot, then a dipole would be a line of dots where the feedpoint is in the center. There are alternate versions of this, where the feedpoint is off-center, but for this, we’ll talk about a perfectly centered feedpoint. The gain of a dipole is 2.15dB compared to an isotropic antenna. The gain is how much further a signal will travel in a specific direction. For example, if your antenna has a gain of +3dB, then it would be like transmitting 2 watts instead of 1 watt. Likewise, if your antenna has a loss of 3dB (-3dB gain), it would be like transmitting 1/2 watt instead of 1 watt. The signal won’t go as far, and won’t be as readable at the receiver. Signal strength, deciBels, and gain are all reflective of how well the signal is received–not transmitted.
Final Thoughts
This is an extremely simplified description of how radio waves work. There are a lot of factors involved, some of which might make my description inaccurate. There are entire books and sites dedicated to radio theory, antenna theory, and design, and even the math behind them both.
Here are some sources of information that will take your understanding to a whole new level.
Radio Waves (Wikipedia) Antenna (Wikipedia)
The Antenna Handbook (ARRL)
The ARRL Handbook (ARRL article which points to the various formats of both the Antenna Book and the ARRL Handbook)
Electrical Engineering (Khan Academy). This would be a good study aid for the Amateur Extra exam, specifically Signals and Systems. Both of these are more the electrical/electronic theory rather than the radio/antenna theory. But they will explain the math behind the magic.