Radio Frequency Communications

Dramatic changes in the way RF communications is used have taken place over the last several years. However, RF communications is equivalent to any other communications channel or pipe, like fiber optic cable or copper wire. The physics of RF communications remains the same as always, but the electronics implementing it have been combined with data communications technologies to radically enhance and alter RF applications. Today RF communications provides anywhere cellular phones, paging, Internet access, television distribution, data communications, and more.

Everyone is familiar with radio because of the radios in our cars. They work by receiving transmitted electronic waves and converting them into sound. RF communications is the same, but the radios use both transmit and receive radio signals. These signals are low-power signals so that receiving and transmitting hubs need to be placed close together. All radio communications share one immutable resource, the electromagnetic spectrum. Two basic pieces of knowledge are needed to put RF communications in perspective. One has to do with the physics of radio communications and the other with the regulation of radio communications. RF telecommunications are governed by:

• The properties of RF transmission.

• Government and international regulations.

First, let's examine the properties of RF communications. When I was in college and first married, every appliance was a big expenditure. My first wife, Mary, and I purchased a TV, but we lacked the money to buy a TV antenna. Some TVs came with rabbit ears at the time, but ours did not. So I proceeded to make an antenna. I went to my general physics textbook and looked up the RF spectrum. Then I selected the frequency right in the middle of the VHF TV band and divided it into one to arrive at the wavelength. It calculated out to be 6 feet. To make a half-wave antenna, I cut 3 feet of 300-ohm (the wide TV cable that is difficult to find anymore) cable, twisted the ends together to get a reasonably precise 3-foot length, and soldered them. Next I nailed the cable to a board, and in the exact center, I tapped another 300-ohm cable into the side to form a "T." This was the antenna. It worked, but not very well.

Good TV antennas, you see, have many elements (the spikes) that run perpendicular to their length. Each spike or element is a very precise length. The length matches the wavelength of a single broadcast TV channel. In that manner, the antenna filters the signal received and maximizes it for the broadcast TV channel frequencies. The antenna elements range from about 4 inches to about 3 feet in length.

This illustrates that each RF frequency has a wavelength that can be physically measured. There are very long waves (miles in length) that can penetrate the earth and ocean. They are used to communicate with submarines. As the broadcast RF gets higher and higher, the physical wavelengths get shorter and shorter until they reach the size of a raindrop. At this point, when they travel through the air and it is raining, three things can happen:

• The radio wave hits the raindrop head-on and gets soaked up—it heats the raindrop slightly—being soaked up is bad for us.

• The radio wave hits the raindrop obliquely, bounces off, and goes someplace that we do not want it to go—this is also bad for us.

• The radio wave misses all raindrops and is received correctly by the radio receiver. This is good for us because our message gets through.

You can see that rain and water are the enemies of radio communications. Of course, radio waves that are 3 feet long do not need to worry much about raindrops. As broadcast radio frequency increases more, getting higher and higher, the physical wavelengths get shorter and shorter still until they reach the size of a molecule of water in the air. Water molecules are always in the air (it is called humidity), they are much more prevalent than rain drops, but they are mostly invisible to us. At this point, when radio waves travel through any air, three things can happen:

• The radio wave hits a water molecule head-on and gets soaked up—it heats the water molecule slightly—being soaked up is bad for us.

• The radio wave hits a water molecule obliquely, bounces off, and goes someplace that we do not want it to go—this is again bad for us.

• The radio wave misses all water molecules and is received correctly by the radio receiver. This is the good thing for which we hope, because our communications get through.

What is illustrated here is that the higher the broadcast frequencies, the more susceptible to attenuation (absorption) are the radio waves. To overcome this absorption, transmitters and receivers must be closer to one another at the higher frequencies, or much more power must be used to burn through the atmosphere. The strength of a radio signal drops quickly as the distance from the broadcast antenna increases. So, high-frequency radio waves need more power or short distances between transmitting and receiving devices to get through (see Figure 8-1).

Sonic and Ultra Sonic Frequencies—

Frequency 10 KHz I

Wavelength 30 km

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