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Other Radio Topics

Sure, maybe the frequencies on this site were helpful, but what good are they if you do not understand the basics. For a beginner, the word megahertz or wavelength may make no sense at all. This page was designed specially for those who like scanning, but do not know what the scanner is really doing.

Hertz, kilohertz, megahertz, gigahertz


Radio Range

Radio Bands

Frequency Mixing


Sideband Reception


Why Scanning?


To start off, we must first define what 'frequency' is. A frequency is the number of times an object performs a cycle in a specific time frame. Sound is made up of cycled air vibrations. The more vibrations, or cycles, there are, the higher the tone. 1 cycle per second is defined as 1 HERTZ, named after Heinrich Hertz, a man who studied radio waves in the ninteenth century.

If are familiar with the metric system, you know that for each additional zero you add to a number, you can shorten it by using a different term (1000 meters, 1 kilometer). The metric system is used in hertz also. Instead of saying 1000 hertz, you would say 1 KILOHERTZ. The same holds true for 1 MEGAHERTZ, 1000 kilohertz, and 1 GIGAHERTZ, 1000 megahertz. Most scanners receive only frequencies in megahertz, but some can climb in the lower gigahertz range.



Radio waves travel from the tower in waves. These waves move at the speed of light (186,282.3976 miles per second). The difference between the front of the wave and the back of the wave is defined as a WAVELENGTH.

As the frequency increases, the number of waves sent from the tower each second increases with it. The number of waves, or cycles, is defined in
hertz (1 wave, or cycle, equals 1 hertz. 2 waves, or cycles, equal 2 hertz. Etc.) When the number of waves increases, the size of each wave decreases. (Remember, 3 hertz means 3 waves per second. In order to fit each wave into that second, the size of each has to decrease.)

This is somewhat confusing for a beginner. To make it easier, imagine a rope 186,282.3976 miles long. This represents 1 wave in a second. Its wavelength equals 186,282.3976 miles. Now, imagine, cutting the rope in two equal segments. This represents 2 waves in a second. Putting the two ropes together still creates the accurate distance light travels in a second. But, the two waves needed to become less so they could both fit in that frame. Each of their wavelengths are half of 186,282.3976 miles, or 93141.1988 miles.

To go even further, imagine the rope as a whole again. Now, cut it into thirds. This represents 3 waves in a second. The lengths of all three still make the speed of light in a second, but all three needed to become a third of its size in order to equally fit in it. Each of their wavelengths are about 62,094.1325333 miles long.

You could go on forever with this, but there is not enough room on this page for that. A simple way to calculate the wavelength is take 186,282.3976 miles and divide it by the number of hertz (Remember, 1 wave equals 1 hertz).



Determining the distance your scanner can hear to is a difficult task to answer. The best way is by line of sight, or where your scanner can be in perfect alignment with a tower or transmission station without any large obsticles in the way. However, in open plains or other clear areas, your scanner could pick up transmissions from distances beyond the horizon (It is known that radio waves tend to curve their course as they travel freely across open areas instead of shooting into space). Basically saying, if you live in Nebraska or Kansas, you might hear transmissions from as far as sixty miles away.

Receiving transmissions from stations in your line-of-sight is the way to receive the strongest and clearest signal. If you live at the top of a mountain, you might be in the line-of-sight with a tower fifty miles away. If you live in a valley surrounded by mountains with radio towers, you will be able to hear those towers with "almost crystal clear" reception. However, you probably will not hear a tower ten miles down the opposite side of that mountain.

For those living on the open plains, being in the line-of-sight with a tower might not be so easy. If you want to be in the line-of-sight of distant transmissions, you will probably need a large atenna mounted on your roof.

This is a formula to determine how high above ground your atenna needs to be in order to see a point in the distance. This formula is quite tricky, so I'll try to make it easier to understand.

Attena Height=((Square Root ((Tangent (miles/cir * 360) * rad/pi)squared + (rad/pi)squared)) - rad/pi) * 5280

miles = number of miles to transmitter     cir = circumfrence of the earth

rad = radius of the earth     pi = 3.14159265359



Radio frequencies have been divided into many different categories, or bands. A normal AM/FM radio receives two of those located bands. Scanners can receive a lot others.

000.540 MHz - 001.710 MHz : AM Band

030.000 MHz - 050.000 MHz : Very High Frequency - Lower Band

072.000 MHz - 076.000 MHz : Mid-Band

088.000 MHz - 108.000 MHz : FM Band

108.000 MHz - 137.000 MHz : Civilian Aircraft Aeronautical Band

138.000 MHz - 174.000 MHz : Very High Frequency - Higher Band

225.000 MHz - 400.000 MHz : Military Aircraft Aeronautical Band

406.000 MHz - 470.000 MHz : Ultra High Frequency Band

470.000 MHz - 512.000 MHz : Ultra High Frequency (only in some cities)

806.000 MHz - 940.000 MHz : "800" band



In nature, a phenomenon occurs when two waves of different frequencies come together. This is best called Frequency Mixing. To figure out the following, you will need to understand basic math.

Imagine two frequencies coming together on an isolated section of earth where no other radio waves meet. Let's just assume that the two frequencies are 155.595 MHz and 161.565 MHz. One second before the frequency waves strike each other, there are only two (155.595 MHz and 161.565 MHz). But, two seconds later, there are four frequency waves departing from the point of collision (155.595 MHz, 161.565 MHz, 5.970 MHz, and 317.160 MHz)! What happened? Why are there four?

This is the process known as frequency mixing. When two radio waves strike each other, they produce two other waves, one equaling the sum of the two frequencies and the other equaling the positive difference of the two frequencies. Here is an example:

161.565 MHz + 155.595 MHz = 317.160 MHz

161.565 MHz - 155.595 MHz = 5.970 MHz

Simple, isn't it. These new frequencies can distort any transmissions covered on those frequencies. However, these new frequencies do not have a lot of power, nor a lot of energy to travel.



Like mixed frequencies, there is another kind of false radio frequencies. These are called HARMONICS. When a transmitter creates an audio or radio frequency, it also creates multiples of that frequency. For example, if a person transmits on the frequency 155.010 MHz, that transmission can be heard on 310.020 MHz, 465.030 MHz, 620.040 MHz, 775.050 MHz, and so on. If you do a little multiplication, you can see that the frequencies are indeed multiples of the first frequency (155.010 * 2 = 310.020, 155.010 * 3 = 465.030, etc.).

Most scanners have the ability to ignore harmonics, but some can be easily radiated through the filtering process. Harmonics are strongest on the odd multiples (3, 5, 7, 9, etc.).



This is a common error for first-time scanner listeners. You're searching through the VHF band when you hear your local fire department transmit on 154.335 MHz. You know for a fact that the fire department only transmits on 154.340 MHz. Then, why are they on this frequency?

The frequency 154.340 MHz, if the story was true, would be considered the CENTER FREQUENCY. Basically, it means the exact frequency which the transmitter transmits on. However, the transmission can occasionally drift into its SIDEBANDS, the frequencies between 0 and .005 above and below the center frequency. In this example, 154.335 would be a sideband of 154.340.

Sidebanded frequencies can be a burden and a blessing at the same time. Imagine your local police transmitted on the frequency 156.030 MHz. Also, your local highway department transmitted on the frequency 156.045 MHz. If the highway department had a very strong transmitter, their transmissions might register on your police channel, even if they are .015 MHz away. This could easily make it impossible to hear your police. But, thanks to the sidebands, you can possibly tune down to 156.025 MHz or 156.020 MHz, where the interference does not exist. It may be harder to hear, but at least you'll be able to hear it at all.

Some new scanners have the ability to zero in directly on the center frequency when searching. These scanners cost a little more than other scanners. The best thing to do if you do not own that kind is to collect information on where frequencies are located to determine whether you have found a sideband or not.



Radio signals created within your scanner are called BIRDIES. Every scanner has them. Birdies are signals that have no sound, yet stop a scanner on its search mode anyway.

Birdies can easily be found within a scanner. First, disconnect all antennas and move the unit to a place far from other elctronic equipment. Then, run a search, scanning every frequency your scanner can tune to. When it stops and indicates it has found a transmission, you have located a birdie. Most scanners come with four or five of them.



Try to imagine the first time you became interested in radio scanning. What sparked the interest? Did you see something on TV? Did you know someone who owned a scanner? Well, you came to this site for some reason, and chances are it has something to do with scanning.

Most people do not see any purpose in listening to a scanner. They do not understand what is really going on. They may ask you, "Why do you listen to that radio-thing?" An easy response would be, "Why do you watch TV?" Listening to a scanner and watching TV can be considered almost the same. Local TV news channels deliver news that has alreay happened. Scanners, on the other hand, can tune you in to the action as it unfolds. If news isn't your thing to hear, you can listen in to aircraft or trains as they travel through your area, almost like watching your favorite TV show.

Scanners provide you with the best first-hand link at what is happening throughout your city or village. If you ever plan to work with radios as a career, such as an air traffic controller or a police dispatcher, you can hear what is involved almost any minute of the day.




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