LITMIR.BIZ

Скачать книгу

surplus equipment. Commercial and public-safety radios require a license to operate.

       Mobile or cell phones: Obviously, you don’t need a license to use a mobile phone, but you can communicate only through a licensed service provider on one of the mobile phone allocations from 700 MHz through 2 GHz. (The new 5G services go much higher in frequency.) Although the phones are actually small UHF and microwave radios, they generally don’t communicate with other phones directly and are completely dependent on the mobile phone network to operate.

       WiFi: Your wireless network router, gateway, or access point is really a radio transceiver operating on the 2.4- or 5.6-GHz bands. That’s what those little moveable antennas are for! Your phone or tablet has small antennas and a WiFi transceiver inside, too.

Hams have always been interested in pushing the envelope when it comes to applying and developing radio technology — one of the fundamental reasons why ham radio exists as a licensed service. Today, ham inventions include such things as creating novel hybrids of radio and other technologies, such as the Internet or GPS radio location. Ham Mesh networks, for example, consists of wireless local area network technology adapted to ham radio. Ham radio is also a hotbed of innovation in antenna design and construction — in short, techie heaven!

Understanding the Fundamentals of Radio Waves

      Getting the most out of ham radio (or any type of radio) is greatly improved by having a general understanding of the purpose of radio: to send and receive information by using radio waves.

      Radio waves are another form of light that travels at the same speed: 186,000 miles per second. Radio waves can get to the Moon and back in 2½ seconds or circle the Earth in second.

      

For the exam, you’ll need to know that the energy in a radio wave is electromagnetic. That is, the waves are made up of both electric and magnetic fields. (A field is just a way of storing energy in space, like a gravitational field that makes you experience weight.) The radio wave’s field makes charged particles — such as the electrons in a wire — move in sync with the radio wave. These moving electrons are a current, just like in an AC power cord except that they form a radio frequency current that your receiver turns into, say, audible speech.

      This process works in reverse to create radio waves. Transmitters cause electrons to move so that they, in turn, create the radio waves. Antennas are just structures in which the electrons move to create and launch radio waves into space. The electrons in an antenna also move in response to radio waves from other antennas. In this way, energy is transferred from moving electrons at one station to radio waves and back to moving electrons at the other station.

      

You will see radio waves referred to as electromagnetic radiation. Don’t let the word radiation alarm you (or your neighbors). This is just a general term for any kind of electromagnetic energy flying around. There isn’t nearly enough energy in a radio wave to cause the same kind of concern as nuclear or ionizing radiation. It’s not even close! Radio waves are non-ionizing radiation and can’t cause the genetic effects or other damage associated with radioactivity.

      Frequency and wavelength

      

The fields of a radio wave aren’t just one strength all the time; they oscillate (vary in direction back and forth) the way a vibrating guitar string moves above and below its stationary position. The exam asks about the time a field’s strength takes to go through one complete set of values — it’s called a cycle. The number of cycles in one second is the frequency of the wave, measured in hertz (abbreviated Hz).

The wave is also moving at the speed of light, which is constant. If you could watch the wave oscillate as it moved, you’d see that the wave always moves the same distance — one wavelength — during one cycle (see Figure 2-3). The higher the wave’s frequency, the faster a cycle completes and the less time it has to move during one cycle. High-frequency waves have short wavelengths, and low-frequency waves have long wavelengths.

      Courtesy American Radio Relay League

      FIGURE 2-3: As a radio wave travels, its fields oscillate at the frequency of the signal. The distance covered by the wave during one complete cycle is its wavelength.

      If you know a radio wave’s frequency, you can figure out the wavelength because the speed of light is always the same. Here’s how:

       Wavelength = Speed of light / Frequency of the wave

       Wavelength in meters = 300,000,000 / Frequency in hertz

      Similarly, if you know how far the wave moves in one cycle (the wavelength), you also know how fast it oscillates because the speed of light is fixed:

      Frequency in hertz = 300,000,000 / Wavelength in meters

Frequency is abbreviated as f, the speed of light as c, and wavelength as the Greek letter lambda (λ), leading to the following simple equations:

      f = c / λ and λ = c / f

      The higher the frequency, the shorter the wavelength, and vice versa.

      

If you need some help with the math in this book (although I’ve used very little) there is a handy Radio Math supplement on this book’s web page (see the Introduction). The supplement also lists a number of online references for even more help!

      

Radio waves oscillate at frequencies between the upper end of human hearing at about 20 kilohertz, or kHz (kilo is the metric abbreviation meaning 1,000), on up to 1,000 gigahertz, or GHz (giga is the metric abbreviation meaning 1 billion). They have corresponding wavelengths from hundreds of meters at the low frequencies to a fraction of a millimeter (mm) at the high frequencies. As an example, AM broadcast waves have frequencies of about 1 MHz and wavelengths of 300 meters or so. FM broadcast radio has a much higher frequency, around 100 MHz, so the wavelength is shorter, about 3 meters. WiFi waves (WiFi is a radio system, too!) are about ⅛ meter long. The exam includes several questions about frequency and wavelength.

      The most convenient two units to use in thinking of radio wave frequency (RF) and wavelength are megahertz (MHz; mega means 1 million) and meters (m). The equation describing the relationship is much simpler when you use MHz and m:

      f = 300 / λ in m and λ = 300 / f in MHz

      For example, a wave with a frequency of 3.75 MHz has a wavelength of 300 / 3.75 = 80 meters. Similarly, a wavelength of 2 meters corresponds to a frequency of 300 / 2 = 150 MHz.

      

If you aren’t comfortable with memorizing equations, an easy way to convert frequency and wavelength is to memorize just one combination, such as 300 MHz and 1 meter or 10 meters and 30 MHz. Then use factors of ten to move in either direction, making frequency larger and wavelength smaller as you go.

Скачать книгу