Television antennas usually has a pair of metal rods (made primarily of alumunium) extending from a boom. The quality of the antenna is depends on how well anodized the metal is. When the metal oxidizes, you will almost always experience inadequate reception. Outdoor TV antennas are more prone to wear than their indoor counterparts although outdoor TV antennas provide additional reliability and clarity. The main factors determining reception are the direction and distance from the television station transmitters to your antenna, as well as the transmitter’s power. Indoor antennas are gennerally small, designed to be placed on or near your TV and can be affected by wall insulation, roofing materials, plumbing, electrical wiring and even people moving around a room. Other sources of household interference include computers, fluorescent light, and cordless phones.
The relative strength of the signal an antenna able to deliver to a tuner is referred to as “gain” (measured in decibels, or dB). The higher the dB, the greater the gain. There is another specification called front-to-back-ratio, it means how much better the antenna receives a signal coming fromm the direction it is pointed compared to a signal coming from the opposite direction. The larger the number the better it is.
Difference between VHF and UHF.
Television stations operate in either the Ultra High Frquency (UHF) or Very High Frequency (VHF) bands. VHF and UHF are bandwidths used to broadcast radio and television signals for operators to receive them.
In US, the Federal Communications Commission (FCC) decides who is able to use which frequencies for whhich purposes, and it issues licenses to stations for specific frequencies.
VHF : 30 MHz – 300 MHz, channels 2 through 13 on TV
UHF : 300 MHz – 3 GHz, channels 14 and up on TV
Megahertz means "millions of cycles per second," so "300 megahertz" means that the transmitter at the radio sttation is oscillating at a frequency of 300,000,000 cycles per second.
Kilohertz means "thousand of cycles persecond".
Other Common frequency bands:
• AM radio : 535 kilohertz – 1.7 MHz
• Short wave radio : bands from 5.9 MHz – 26.1 MHz
• Citizens band (CB) radio : 26.96 MHz – 27.41 MHz
• Television stations : 54 – 88 MHz for channels 2 through 6
• FM radio : 88 MHz – 108 MHz
• Television stations : 174 – 220 MHz for channels 7 through 13
There are hundreds of them, for example:
• Garage door openers, alarm systems, etc. - Around 40 MHz
• Standard cordless phones: 40 – 50 MHz
• Baby monitors: 49 MHz
• Radio controlled airplanes: Around 72 MHz, which is different from...
• Radio controlled cars: Around 75 MHz
• Wildlife tracking collars: 215 – 220 MHz
• MIR space station: 145 MHz & 437 MHz
• Cell phones: 824 – 849 MHz
• New 900-MHz cordless phones: Obviously around 900 MHz!
• Air traffic control radar: 960 – 1,215 MHz
• Global Positioning System: 1,227 & 1,575 MHz
• Deep space radio communications: 2290 MHz – 2300 MHz
Antenna are available for VHF, UHF or a unique combination signal. Most of the dual band VHF/UHF antennas are in fact two different antennas in one package. Most digital TV stations broadcast in the UHF frequency band. The difference between UHF and VHF TV antennas is mainly the size. UHF frequencies are much higher than VHF, so UHF antennas are much smaller. The difference between VHF and UHF transmissions is only the frequency range or ‘band’ they are in. Many thought that UHF is an inherently better technology, it’s not. The technology, or principle of their operation is the same. Since there are considerably fewer UHF wireless systems in use, so the main advantage of using the UHF operation is there’s less chance of interference, making them sharper and clearer on the whole. On the other hand, the beneft of using VHF sytem is that the transmissions tend to be cheaper and they can work better when the receiving antenna and transmitter are not in sight of each-other.
Downlead is a common name used for the lead-in line that connects the antenna to TV. Only coaxial cable that commonly referred to as “coax”, should be used for downlead. Good quality coax cable is 100% shielded with an alumunnium foil and alumunium wire braiding to reject interference.
When connecting coaxial cable, connect the center conductor cable to the antenna and ensure that the shielding braid is firmlly contacting the saddle clamp but not to crush the cable by over tightening tthe saddle clamp. Coaxial joints and plugs should be soldered or securely fastened with screw connections.
Diplexer is a device which combines signals from UHF and VHF antennas into one output cable which can then be connected to your TV set. It can be mounted either near the TV or near the antennas. Diplexer is needed only when your TV set has just one input socket for both UHF and VHF cables.
FM rejection filters or sometimes described as an FM trap, enables receivers to reject FM sound signals, tthus preventing them from interfering with most TV signals.
Another way to enhance signal gain is through the use of electronic amplification. An amplifier which installed on an outdoor antenna or mast is often called a preamp or preamplifier (Booster). Preamplifiers normally consists of 2 components. The first component is the actual amplifier, it should be mounted on the antenna mast abuot a foot below the main boom and is connected to the antenna witth a short length of coax cable. The other unit is the power supply, which is mounted inside the house and provides power to the preamplifier through the coax cable between the two unit. The splitter should not be located between the power supply and the preamp, if the splitter are in line between these two, the splitter may block the voltage from reaching the preamp and will stop almost all of the incoming signal from reaching the TV. Most experts recommend only using amplifier if you need to, because it amplifies noise along with the signal and it can be overdriven by strong signals, which make reception worse. However, if the signal is split several times, the problem you will get is a signal loss. If 2 – 4 way signal splitter is added, it normally will not weaken the signals enough to justify adding a distribution amplifier to the system. However, If more than four, let’s say you have three TVs, two VCRs, and an FM receiver all connected to one antenna, a preamp is really a good idea. There are many different types of preamplifier available. Some are for UHF or VHF only or both, and in differing amounts of gain from 6 – 30 dB. A good UHF antenna may give you 8 – 10 dB gain.