Electromagnetic Radiation · 8:09pm Mar 9th, 2014
Propagation is their vocation.
Let's define a few terms first. "Radiation" is something that radiates, not necessarily bad nuclear stuff. Light is a kind of radiation–it radiates from the sun. Radio waves are another kind of radiation. Put it all together and you get the electromagnetic spectrum.
Let's start with basics. You've probably heard about these things called waves. Maybe you've heard of frequency. The image above represents a portion of the electromagnetic spectrum. Radio waves are down low, what we see as light and color are midrange, and other waves are higher up.
This is the basic structure of waves. No, they don't actually look like this. We can't really see them. The line gives you something to look at. Oscillation is how one wave goes through its cycle. The picture above shows two waves– the next cycle begins right where the last one ended. You can think of the amplitude, or height of the wave, as the amount of power it carries.
Wavelength is important to frequency. The number of cycles per second is called Hertz (Hz). Shorter wavelengths cycle faster and have higher frequency. Shorter wavelengths can carry more information (bandwidth). One thousand Hertz is Kilohertz (KHz). One million is Megahertz (MHz). One billion is Gigahertz (GHz).
The processor in your computer probably has a rating in GHz. This is its clock rate, a signal that signifies how fast it runs.
Let’s start at low on the spectrum. Radio itself is broken up into a lot of subdivisions.
Extremely Low Frequency (ELF) is from 3-300Hz. Why do you care so much? Math! For example, we’ll use 30Hz – thirty cycles per second. The speed of light is 299,792,458 meters per second. Therefore, the wavelength of 30Hz is almost 10,000,000 meters! (shown as 10^7 m on the picture)
ELF is used for things like submarine communications because low frequencies can do things like penetrate water.
The Very High Frequency (VHF) band is where a lot of commercial communication takes place. It contains FM radio, which in the United States resides between 88 and 108 MHz. (Notice that your radio dial goes from 88 to 108.) FM stands for Frequency Modulation– the transmitter modulates frequency to produce different sounds in your receiver.
AM radio is Amplitude Modulation. The signal tends to heard at a longer range than FM, but FM is a better quality signal. AM operates in a portion of a lower band, Medium Frequency (MF) from 500-1710 KHz.
Whether you’re using FM or AM, it’s pretty clear that the signal needs to change in some way to make the different sounds. The baseline frequency is called the carrier. The amount the signal has to change going up or down is called a sideband.
Because waves are symmetrical, the two sidebands are exactly the same, just mirrors of each other. This means if you have the right equipment, you can transmit just one half the signal to conserve bandwidth and still receive something meaningful on the other end. This is called Single Sideband (SSB) and is popular in ham radio.
Most commercial radio stations in the US legally operate at 50,000 watts of power. Back in the day, some stations across the border in Mexico used ten times as much power and their signals could be heard across much of the US. Wolfman Jack was a famous DJ on one of these “border blasters.”
You probably remember when television made the switch to digital and everybody had to buy the converter boxes. TV used to take up a larger swath of the VHF and Ultra High Frequency (UHF) band. In fact, the whole of FM radio was between channels 6 and 7.
Of course I’m going to use a picture of this movie poster. It’s got Weird Al and is relevant to TV and radio at the same time.
The switch to digital TV was made because digital signals take up less space on the spectrum than analog. The spectrum is actually pretty crowded. Ever gotten interference from something? That’s another signal crowding your turf. In the US, the Federal Communications Commission (FCC) sets rules on who can use what frequency.
Moving up the frequency band, we have microwaves. They range from 300 MHz to 300 GHz (wavelengths from 1 meter to 1 millimeter). Microwaves have significantly shorter wavelengths than lower frequency signals, and so they oscillate faster and can carry more data. Satellites often use microwaves. However, microwaves tend to be highly focused because they don’t travel as well as the lower frequencies. This is why it’s important to get your satellite TV antenna aligned, while your regular antenna doesn’t care as much. Almost all radar operates in the microwave band.
Your microwave oven probably operates at about 2.45 GHz (Wavelength= 12 centimeters. That’s why there are small holes in the metal door of the microwave; they’re too small for the waves to escape.) There’s no real trick to it, it just shoots energy at your food. This is why you should be careful around electronic equipment – it can burn you, too. It can cause cataracts in your eyes the same way that heat turns cooking eggs white and opaque.
Another important part of the spectrum is visible light. You’re using it right now. Remember that high-powered light can burn you too – lasers!
On either side of visible light are infrared and ultraviolet. If your eyes were bigger to handle the longer wavelengths of infrared, you might be able to see it. Some insects with their small eyes can see in ultraviolet.
Above that, you have x-rays and γ(gamma)-rays. This high up in the spectrum, their wavelengths are measured in nanometers and piquometers, and the frequencies are petahertz and exahertz (10^16+ cycles per second)
This is where you start getting into ionizing radiation. This is the bad stuff that goes along with nuclear. In large enough doses it can damage your molecules and DNA directly, not just by burning them or whatever.
Okay, so now that we’ve covered the spectrum itself, why don’t we swing towards applications? Most of us think of radio or communication when we talk about the electromagnetic spectrum.
Most of us.
To do your radio thing, you need an antenna. It helps if said antenna is the right shape and size. A basic antenna is just a piece of wire and works in all directions. Dish antennas are for sending or receiving in only one direction.
You can do fancy things, like partial-wave antennas. As long as the antenna length is pretty close to being exactly ½ or ¼ or some fraction of wavelength, it should work.
For example, your car probably has a quarter-wave antenna. The FM spectrum ranges from 88 to 108 MHz. That’s wavelength from 3.4-2.8 meters, 3.1 on average. Now, divide by 4 for quarter-wave = about 77.5 cm, plus or minus a few cm. Go measure, I’ll wait.
Pictured: Things I look at while waiting
So now that you’ve got the basic equipment, it’s time to use it. Assuming you’ve followed proper FCC band plans, nobody should interfere with your signal. But what if it happens? Let’s take another look at waves.
When two waves are of the same type, they are “in phase.” If you put them together, their amplitudes add together into a bigger one. This is called constructive interference. However, if two waves are exactly opposite, “out of phase,” then they cancel each other out– destructive interference.
Or maybe someone is just broadcasting a signal that is more powerful than yours. This is why radio stations are legally limited in power – so whoever has the biggest transmitter can’t rise above all the other signals. This is how the military does jamming. They broadcast noise at a higher power so whoever is trying to use that frequency isn’t able to.
Still, there are things you can do. Some high-tech equipment has encryption, which modulates the signal in such a way that it can’t be interpreted by someone listening in. Notice I didn’t say that it couldn’t be intercepted. If you broadcast, they can hear that something is going on, even if the encryption keeps the message secret. Using directional antennas, you can perhaps limit the likelihood that your signal will be intercepted.
In God we trust; all others we monitor.
Okay, so now you’re up and operating. What else should you keep in mind? The weather is a big factor. Generally, clear days are better for radio. Plus, there’s this nifty thing called the ionosphere. It’s a region of the upper atmosphere, from about 85 km (53 mi) to 600 km (370 mi) altitude. Some radio waves (called skywaves) can bounce off it, thereby going around the world. It takes a lot of math and smart people to calculate this stuff. Everyone wants to extend their signal over the horizon.
High Frequency (HF) is generally the highest signals that can take advantage of bouncing. This is why AM radio, at MF, has historically been long-ranged, especially at night when the lack of sun causes changes in the ionosphere. Higher frequencies just punch through the ionosphere into space instead of bouncing. Lower signals, like ELF, can curve with the ground – “ground waves.”
Of course, this means that meteorology is really important to radio. It’s why the military puts so much effort into studying the weather – even space weather.
Not a joke. Space weather, including the ionosphere and sunspots, is a big deal.
Going back to applications, I’ve mentioned ham radio a couple of times. Here’s the Wikipedia definition: Amateur radio (also called ham radio) is the use of designated radio frequency spectra for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication. Ham is a somewhat flippant term related to ham-fisted or ham actor. The community embraced it, though.
Your average ham operator has a radio and maybe a couple of antennas for different frequencies. It’s just like anything – you can spend as much or as little as you want to get varying degrees of performance. You can even do Morse code, and some frequencies require it. An operator has to have a license and callsign from the FCC so they don’t screw up the spectrum.
If you ever see a car with a big antenna and a funny-looking license plate (their callsign), chances are they’re a mobile ham.
A high end ham radio shack—yes, that’s where the company got the name. The maps on the walls are for locating stations the user has contacted. It’s like a game. Everyone is always trying to one-up everyone else and make longer distance contacts in different styles. It’s not unheard of to use the moon and other stuff to bounce radio waves. There’s ham equipment on the International Space Station.
CB (citizens’ band) radio doesn’t require a license. The classic CB operated by truckers is about 27 MHz (HF band). That’s a lower frequency than music radio, and you’ll notice CB antennas are longer. You’ll notice that a lot of your radio-controlled toys also operate at 27 or 49 MHz. Other public bands are used by walkie-talkies in UHF. CB has been losing popularity since cell phones were invented. Police now typically operate dedicated radios in the VHF/UHF bands.
Jeeze, these blogs just keep getting longer and longer. This is what happens when you guys let me talk about things I like. Nevertheless, this is just a basic lesson. I didn’t even go into the particle/wave debate. I’m sure there are a few of you out there who have more to add.
i alredy kno dis k
1911602 Then I invite you to proofread it.
1911605 3bizzy making a shitfic named "Edge In Equestria". The first chapter is "Ow, the edge!"
1911610 Edge came to Equestria without Bono?
1911613
fimfiction-static.net/images/story_images/173102.jpg
Now do a lesson on antenna theory :)
1911621 Ha ha, that black magic? No thanks.
So, if VHF is Very High Frequency, and VLF is Very Low Frequency, then VF is Very Frequency and used exclusively for Doge broadcasts, right?
1911648 ...I just noticed that. There's supposed to be a SLF and ULF in there. Now I feel bad for including a misleading picture.
But +1 to you for the joke.
My grandparents live near the deactivated Navy's Clam Lake ELF antenna in Northern Wisconsin. It is funny how people complain about the supposed low frequency sound/noise that wind turbines put out. (I've stood directly beneath a wind turbine. There was no sound whatsoever!) If they knew how much low frequency sound they've been exposed to, they would shut up.
1911623 Well, in a month or so when I know Antenna theory, I'll teach you, and you can reach everyone else :)
Taste the (extended) rainbow!
1911672 Sound is vibrations through the air. The electromagnetic spectrum is different, even if they both deal with frequency.
1911702 Exactly! I have heard people complain that they (the turbines) disturb cows because of the VLF sound they put out! I tried to explain to them that they were talking about 2 different things, but it was like trying to destroy Tom with a fire cracker.
Ok now I wonder what brought this on
1911731 It's possible that creatures can detect low frequency sounds. Human hearing only goes down to 20 Hz. Sounds lower than that might cause feelings, but if you can't hear them your body might interpret them differently.
1911764 A comment on my last blog about radios.
1911773
Ah
Interesting fact: Some people can see the very edge of the lower UV section, due to their blue cones being slightly more sensitive. Because its those cones being triggered, they interpret it as a very bright blue or purple.
http://m.youtube.com/watch?v=bjOGNVH3D4Y
It's fitting for this blog, I believe.
1911771 Off the top of my head, Elephants can hear sounds as low as 5 Hertz.
Unrelated, I wonder what the widest range of spectrum a animal can see is?
i stopped reding as soon as you said a thousand hertz is kilohertz.... becuase your wrong, 1,024 hertz is kilohertz, 1048576 hertz is megahertz, and 1073741824 hertz is gigahertz
1913867 I'm afraid TNAB is right here. That different scale is used in computer storage, not telecommunications.
Source- I am a licensed ham radio operator in the United States.
This brings back memories from high school science class...
Looking at your page, TNaB, I'm surprised to only see 37 blog posts. Whenever I glance at my feed it's always, always, these cool technical things you write that catch my eye.
1913960 I write a lot of blogs, but only save the ones that have lasting informational value.
There's a debate on that? Last I checked, it was proven that EMR acts as both a wave and a particle, and that is taught as high school physics.
1915660 Yeah, and I had an internal debate about whether I should include that in this blog
1911648 VF = Voice Frequency