FM Reception Guide: Glossary
Attenuator/Pad
An attenuator or pad is a simple resistor network, which is placed directly before the antenna input of a tuner or receiver. Pads are available in 75-ohm versions at most electronics supply stores with a number of attenuation values. A pad is generally required when using high-gain antennae in strong-signal locations.
Baluns
Baluns are matching transformers designed to change one impedance to another: 300-ohms to 75-ohms and vice-versa. They are small, inexpensive devices available in many different formats, one of which is sure to fit your specific application. There is no difference between TV and FM baluns and they may be obtained at any electronics store.
Booster Amplifiers
Booster amps are useful only in fringe areas, when the antenna lead-in is excessively long causing substantial signal loss, or when multiple or a large number of sets are to be fed from a single antenna. Booster amps are always best placed at the antenna itself. The rule is to put the amplifier as early in the chain as feasible, thereby, compensating for any loss in the signal before it occurs.
Coupler
If you have a good strong signal from your FM outdoor antenna, you may feed two separate audio systems from the one antenna via a two-set coupler. Couplers are sometimes incorrectly referred to as splitters. Remember, however, that couplers weaken the signal slightly, and then divide it between the two sets. You can obtain all versions of couplers at an electronics supply store. Ask for a VHF/VHF coupler. A VHF/UHF splitter will not work properly.
Lead-In
Lead-In is the wiring that connects the antenna to the antenna input of the tuner. There are two basic kinds of lead-in: flat, twin-lead, ribbon wire and round, coaxial cable. The common, flat, ribbon wire is sometimes referred to as a 300-ohm twin lead because of the impedance of the wire.
Impedance is one of the electrical characteristics of circuits. For the most efficient transfer of a signal from one circuit to another, the impedance of all the circuits, including the connecting wire, must match; hence 300-ohm and 75-ohm cable are used in different impedance circuits.
Although twin-lead is the most inexpensive of antenna connecting wire, it has certain shortcomings that may make it unsatisfactory in specific cases. Twin-lead tends to weather and age rather quickly and as it ages losses of the antenna’s signal become greater and greater until replacement must be undertaken if the antenna ’s signal is to reach the receiver in any useful amount. But the most negative characteristic of twin-lead is that it permits injection into its wires, and therefore into the antenna's signal, of all sorts of electrical interference: static from passing automobiles or two-way radios, or other man made interference, even undesired FM signals in the bands that may have been rejected by the antenna. This last occurrence in strong-signal areas, such as urban locations, can almost negate the improvements an outdoor antenna can effect. There are “shielded” versions of 300-ohm twin-leads available. However, their initial cost is rather high in comparison to and its longevity not much better than standard twin-lead.
75-ohm coaxial cable is readily recognized. It is tubular, about 1/4 inch in diameter, and the outdoor type is almost always black. Coax cable is constructed with a wire conductor as its core surrounded by a sheath of plastic insulation which in turn is covered completely with a copper braid grounded to shield the inner, central conductor. An outer plastic sleeve protects the entire assembly. This type of construction minimizes both injection of interference and unwanted signals into the main, center conductor and weathering of the wire itself. Coax cable is strongly recommended as lead-in for any antenna installation. Although initially more expensive and exhibiting slightly greater signal loss than twin-lead, it will far outlast any twin-lead, it will insure unsullied delivery of the antenna’s signal to the tuner and its signal losses will remain fairly constant throughout its life. Regardless of which lead-in you select, though, remember to keep it as short as possible. The longer the lead-in the greater the signal loss will be.
It should be pointed out that FM antennae are available in both 75-ohm and 300-ohm versions for use with both types of leads. Tuners and receivers are in many cases supplied with both impedance inputs and in cases where only a 300-ohm input is supplied, an inexpensive, readily available balun or matching transformer can be used. If one needs to be installed at the antenna itself, because it is a 300-ohm antenna and you are using 75-ohm lead-in, make certain that you use an outdoor-type, waterproof balun.
Multipath Distortion
Signal reflections that arrive at a receiver out-of-phase, slightly delayed in time, with the primary signal. This tends to blur or distort the principal signal. The effect on FM can range from a barely audible fuzziness to severely distorted sound quality, particularly at the high frequencies or treble. The degree of distortion will depend on the number and relative strength of the signal reflections.
RF Overload
Common signs of radio frequency (RF) overload include any or all of the following: stations which show up at many points on the dial in addition to their assigned frequency, stations that “blanket” other adjacent stations, audio distortion on voice and music peaks and excessive noise despite a strong signal level. The specific effects of RF overload will vary depending on the tuner, its characteristics, and the strength of the signal supplied to the tuner.
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An attenuator or pad is a simple resistor network, which is placed directly before the antenna input of a tuner or receiver. Pads are available in 75-ohm versions at most electronics supply stores with a number of attenuation values. A pad is generally required when using high-gain antennae in strong-signal locations.
Baluns
Baluns are matching transformers designed to change one impedance to another: 300-ohms to 75-ohms and vice-versa. They are small, inexpensive devices available in many different formats, one of which is sure to fit your specific application. There is no difference between TV and FM baluns and they may be obtained at any electronics store.
Booster Amplifiers
Booster amps are useful only in fringe areas, when the antenna lead-in is excessively long causing substantial signal loss, or when multiple or a large number of sets are to be fed from a single antenna. Booster amps are always best placed at the antenna itself. The rule is to put the amplifier as early in the chain as feasible, thereby, compensating for any loss in the signal before it occurs.
Coupler
If you have a good strong signal from your FM outdoor antenna, you may feed two separate audio systems from the one antenna via a two-set coupler. Couplers are sometimes incorrectly referred to as splitters. Remember, however, that couplers weaken the signal slightly, and then divide it between the two sets. You can obtain all versions of couplers at an electronics supply store. Ask for a VHF/VHF coupler. A VHF/UHF splitter will not work properly.
Lead-In
Lead-In is the wiring that connects the antenna to the antenna input of the tuner. There are two basic kinds of lead-in: flat, twin-lead, ribbon wire and round, coaxial cable. The common, flat, ribbon wire is sometimes referred to as a 300-ohm twin lead because of the impedance of the wire.
Impedance is one of the electrical characteristics of circuits. For the most efficient transfer of a signal from one circuit to another, the impedance of all the circuits, including the connecting wire, must match; hence 300-ohm and 75-ohm cable are used in different impedance circuits.
Although twin-lead is the most inexpensive of antenna connecting wire, it has certain shortcomings that may make it unsatisfactory in specific cases. Twin-lead tends to weather and age rather quickly and as it ages losses of the antenna’s signal become greater and greater until replacement must be undertaken if the antenna ’s signal is to reach the receiver in any useful amount. But the most negative characteristic of twin-lead is that it permits injection into its wires, and therefore into the antenna's signal, of all sorts of electrical interference: static from passing automobiles or two-way radios, or other man made interference, even undesired FM signals in the bands that may have been rejected by the antenna. This last occurrence in strong-signal areas, such as urban locations, can almost negate the improvements an outdoor antenna can effect. There are “shielded” versions of 300-ohm twin-leads available. However, their initial cost is rather high in comparison to and its longevity not much better than standard twin-lead.
75-ohm coaxial cable is readily recognized. It is tubular, about 1/4 inch in diameter, and the outdoor type is almost always black. Coax cable is constructed with a wire conductor as its core surrounded by a sheath of plastic insulation which in turn is covered completely with a copper braid grounded to shield the inner, central conductor. An outer plastic sleeve protects the entire assembly. This type of construction minimizes both injection of interference and unwanted signals into the main, center conductor and weathering of the wire itself. Coax cable is strongly recommended as lead-in for any antenna installation. Although initially more expensive and exhibiting slightly greater signal loss than twin-lead, it will far outlast any twin-lead, it will insure unsullied delivery of the antenna’s signal to the tuner and its signal losses will remain fairly constant throughout its life. Regardless of which lead-in you select, though, remember to keep it as short as possible. The longer the lead-in the greater the signal loss will be.
It should be pointed out that FM antennae are available in both 75-ohm and 300-ohm versions for use with both types of leads. Tuners and receivers are in many cases supplied with both impedance inputs and in cases where only a 300-ohm input is supplied, an inexpensive, readily available balun or matching transformer can be used. If one needs to be installed at the antenna itself, because it is a 300-ohm antenna and you are using 75-ohm lead-in, make certain that you use an outdoor-type, waterproof balun.
Multipath Distortion
Signal reflections that arrive at a receiver out-of-phase, slightly delayed in time, with the primary signal. This tends to blur or distort the principal signal. The effect on FM can range from a barely audible fuzziness to severely distorted sound quality, particularly at the high frequencies or treble. The degree of distortion will depend on the number and relative strength of the signal reflections.
RF Overload
Common signs of radio frequency (RF) overload include any or all of the following: stations which show up at many points on the dial in addition to their assigned frequency, stations that “blanket” other adjacent stations, audio distortion on voice and music peaks and excessive noise despite a strong signal level. The specific effects of RF overload will vary depending on the tuner, its characteristics, and the strength of the signal supplied to the tuner.
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