A radio communication system transmits a radio frequency signal and depends on a substantial amount of that signal being received at the receiver. Now we are going to take a closer look at that problems interference causes in TV and FM radio systems since we all can identify with these. We all also discuss some methods used to resolve interference problems.
After completing this section, you will be able to:
- Identify different types of interference
- Describe three methods to reduce interference
- Troubleshoot various antenna installation problems
This discussion will be limited to TV and FM reception since we are most familiar with these. Source of unwanted signals (noise and interference) happen naturally or are man-made. Good circuit and antenna design will reduce noise to negligible levels. Man-made sources generate most of the interference that usually disturb the quality of the signal at the receiver site. There are various solutions to remedy interference problems at the receiver. The first step in eliminating an unwanted signal is to pinpoint the source of it. After finding the source of the interference that is causing the disturbance, remove it if possible. When it is not possible to remove the interference source, try increasing the distance of the undesirable source from the receiver. This will usually reduce the effects of interference on the receiver and may clear the problem. Using filters is another practical approach for removing unwanted signals. One more method is to protect the receiver by shielding the antenna, the ac input power line, or the whole receiver from the interference signal. The following paragraph talk about the kinds of interference the communication technician may encounter and practical methods to resolve the problems.
Capture and Co-Channel Interference Effects. In area of congested radio, TV, and communication channels, such as metropolitan areas, receivers and subject to the capture effect and co-channel interference. The capture effect causes a stronger station to overpower and replace a weaker station at the receiver. The weak station is usually lost completely. Co-channel interference takes the form of two or more broadcasting stations bleeding into each other at the receiver. To the listener, this bleeding-over effect turns into bothersome noise. The best solution for these kinds of interference is to rotate the antenna or obtain a more directional antenna.
EMI and RFI. Electromagnetic interference (EMI) shows up on TV as vertical bands of dots moving on the screen. ON FM, EMI causes distortion to the audio. Radio frequency interference (RFI) displays itself as several bars or wavy lines on the TV screen. Strong RFI will cause complete loss of the TV’s picture. FM audio is affected by RFI to produce garbled sound, often causing it to be pure gibberish.
The automobile ignition or spark plugs and kitchen appliances like the blender or microwave oven are sources of EMI. In addition, computers and electric motors produce EMI. EMI can enter the receiver through the antenna, lead-in wire, or power line. To decide which is bringing in the EMI, disconnect the lead-in wire from the receiver and short the receiver antenna terminal together. If the interference disappears, then the source was the antenna or lead-in wire. If the interference continues, then the unwanted EMI is coming through the power line. When it is not possible to remove the EMI source or relocate it, use shielding or filtering to remove the interference. When the interference is entering the receiver by way of the antenna, it may be necessary to relocate the antenna.
Ham radio and CB radio transmitters are a common cause of RFI. If the source can be found (towering antennas in the neighborhood are usually a dead giveaway), attempt to contract the owner and let them know the problem exists. Install a high-pass filter between the antenna and receiver input on the lead-in wire to eliminate RFI. However, the best way to eliminate the interference is to remove the source of it.
Fading: fading is one of the most troubleshooting hindrances in communications. Fading is the result of the signal arriving at the receiver from two different paths-a direct path and the skyway path. A typical example of this type of problem occurs when an airplane flies over an area where outside TV antennas are used. The airplane causes reflected signals to mix with the direct signal, and a fluttering result in the picture on the TV receiver. Using a high-gain directional antenna will often resolve this kind of interference.
Reflections: Figure 1 shows a problem that exists between any type of broadcast station and its receiver. In practice, the reflected wave is quite strong, almost as strong as the direct wave, but the path taken by the reflected wave is longer than that of the direct wave. The important thing to remember is that even through the wavelength may be only 1 m and the path is several miles, every time the path difference is equivalent to ½ wavelength of 1800, there will be a null in the signal. Conversely, when the path difference is a multiple of wavelengths the signals add, potentially doubling the signal strength. Equation 1 will enable you to determine where a peak in signal strength might be found.
Remember, every times is an odd multiple of 1800 you are in a null. Ht is the transmitter height in feet, Hr is the receiver height in feet, D is the distance in miles, and f is the frequency in MHz.
The point is, when you need more signal, move the antenna. Intuition would tell you to increase the height, but you may actually be able to lower the antenna and find more signal.
Diffraction: Diffraction is much more complicated than reflection but the solution is the same. As you move away from the mountain shown in figure 1 you will find hot spots. The technician who finds the hot spot can save a great deal of money on an antenna installation.
Ghosting in TV Reception: It may be possible to fight the ghost with knowledge of your antenna patterns. Most TV antennas will have a fairly broad main bean and several null and sidelobes (see figure 1).
The technician should try orienting the antenna to place the ghost signal in a null and the desired signal somewhere on the main beam. If this isn’t possible, try a single frequency antenna such as a corner reflector. Single-frequency antennas have a high rejection of side lobes.
Sky-Wave Propagation: Commercial use of the shortwave frequencies is steadily declining, but it is still the cheapest way to communicate with remote area of the world. The technician or engineer will need some knowledge of system planning. People interested in shortwave propagation or forecasting should obtain a copy of a computer program called “Ioncap”. It was developed by the National Bureau of Standards, now called National Institute of Standards and Technology, and can be purchased from the U.S. government printing office. Several commercial programs are available that use Ioncap as a core and are easier to use.
Satellite Communications: when servicing or installing a satellite system, your chief problem will the aligning the antenna with the satellite. Beams are no more than 20 wide and polarity might be unknown. All are usually adjustable and will often need adjustment when performance is not satisfactory.