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Signal Amplifiers, Preamplifiers. Preamplifiers (Mast-mounted amplifiers). These should be mounted as close to the antenna as possible. Usually the amplifier comes in two parts:
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Preamplifiers (Mast-mounted amplifiers) • These should be mounted as close to the antenna as possible. Usually the amplifier comes in two parts: • The amplifier. This is an outdoor unit that is normally bolted to the antenna mast. It must have a very low noise figure, and enough gain to overcome the cable loss and the receiver’s noise figure. • The power module (power injector). This is an indoor unit that commonly lies on the floor behind the TV. It is inserted into the antenna cable between the amplifier and the TV. This module injects some power, usually DC, into the coaxial cable where the amplifier can use it. The power injector is the amplifier’s power supply.
Distribution amplifiers • These are simple signal boosters. They are often necessary when an antenna drives multiple TVs or when the antenna cable is longer than 150 feet. • Distribution amplifiers don’t need to have a low noise figure, but they need to be able to handle large signals without overloading. • Commonly, distribution amplifiers have multiple outputs. (Unused outputs usually do not need to be terminated.)
Caution • Never feed an amplifier output directly into another amplifier. • There should always be a long cable between the preamplifier and the distribution amplifier. • Placing the two amplifiers close together can cause overload and/or oscillation.
NOISE FIGURE • A mast-mounted amplifier’s most important characteristic is its noise level, usually specified by the noise figure. But many manufacturers don’t take this number seriously. • If it is given at all, it is often wrong. If all makers don’t do them right then comparison-shopping is not possible.
Rate amplifiers for their noise figures as follows • 0.5 dB superb • (anything better runs into thermal atmospheric noise) • 2.0 dB excellent • 4.0 dB fair • 6.0 dB poor • 10 dB awful
Noise figure • The noise figure is a number you must subtract from the antenna’s gain. • The noise figure tells how much of the antenna’s gain you are throwing away by not buying a quieter amplifier. • This loss is irretrievable. • It is gone and cannot be made up later.
Comparing some common antenna amplifiers * measured at channel 30 ** +13V=FM trap in, -13V=FM trap out. *** This is the longest RG-6 cable that satisfies the rule “The gain should equal the cable loss plus an extra 10 dB” at channel 30, assuming the power injector is at the TV.
Common antenna amplifiers • Note 1: The other 777x amplifiers probably behave the same. • Note 2: Winegard’s best. It has the best FM trap, but few people who need this amplifier need an FM trap. • Note 3: The 10 dB variable attenuator is in the power module. Be delicate when adjusting the attenuator. It will break easily. • Note 4: The 15-1170 is modest but problem free. It is a good 2nd amp in a very long cable. • Note 5: The 15-1108 is terrible. It often oscillates unpredictably. Very noisy. I bought a second unit to prove to myself that the first wasn’t broken. If you need 300 inputs, you can use a 15-1109 with a 15-1140 balun, but then the noise figure becomes 4.6 dB.
Channel Master 7777 • The Channel Master 7777 preamplifier has separate inputs (and separate amplifier circuits) for VHF and UHF, which are then combined without loss. • There is a switch inside that will allow VHF and UHF input via the same connector. • A second switch disables the FM trap.
Receiver overload • Signal amplifiers are supposed to be linear. That is, the output is a magnified but otherwise unaltered version of the input. • But too much signal can make an amplifier non-linear, usually clipping off the tops and bottoms of the sine waves. • When this happens, all channels are affected, not just the one that is too strong. In fact, the too strong signal is usually not a TV station. • A close FM station or police station is more likely.
Receiver overload • If you add a good amplifier to your antenna system and your results get worse instead of better then you have overload, and you need to reconsider more carefully what you are doing. • Use a RF Field Strength meter to measure your results, 0 – 10dB is a good rule to follow in most applications. • Overload never causes any equipment damage.
Attenuator • An attenuator is a resistor network that can be used to reduce the gain of an amplifier. • 3 dB, 6 dB, 10dB, 20dB attenuators are commonly available. • If an antenna system needs two amplifiers, where the output of one amp feeds into the other amp, too much gain (overload) can result and an attenuator is usually the simplest solution. • If you don’t have two amplifiers, it is unlikely that you will ever need an attenuator.
FM trap • If you are close to an FM station, there might be a narrow range between too much and too little amplifier gain. (Too little gain = dropouts, too much gain = overload.) • You can make that range larger by using an amplifier with an FM trap or by using a more directional antenna. • VHF preamplifiers usually include FM traps that can optionally be disabled. Freestanding FM traps are also available. FM traps can either cover the entire FM band or can be single frequency traps that you tune to the offending station. The former are less effective and tend to attenuate channel 6. If the FM station is close enough you might need more than one FM trap.
Grounding outdoor antennas • For TVs, the main benefit of grounding is lightning protection. • Lightning is a powerful radio wave generator and any elevated wire is an antenna for it. • A lightning strike in your neighborhood can generate hundreds of volts, even thousands, on the coaxial line. • These voltages can damage your equipment. (This is also called electromagnetic pulse, EMP.)
Grounding blocks • To reduce these voltages the antenna cable should have a grounding block (Radio Shack 15-923) at the point where it enters the house, and that grounding block should be wired to a ground rod driven into the ground as close as possible to the grounding block. An effective ground rod is one driven deep enough to reach into moist soil.
Grounding Rods • Driving a rod deeper into the earth decreases the resistance to earth ground • A second rod will drop the resistance by 40% • Doubling the diameter of a rod only reduces resistance by 10% • Treated rods have an outer coating that dissolves into the soil to lower resistance • Chemicals can be added to the soil to lower resistance but my need to be replenished
Grounding wire • The ground rod should also connect to the mast via a heavy wire. • #8 aluminum wire is readily available for this. Ground wires should be as short and straight as possible. • Turns should be curves with a 6-inch radius. Ground wires do not need insulation.
Grounding coax • Some people will tell you “Don’t ground the coax. That just makes the antenna a lightning rod”. • But the coax is already grounded through your receiver’s power cord, so you can’t prevent it from being a lightning rod. • All you can control is how much of your house the high current will go through before it reaches the ground. • Another advantage: Appliance RF noise can travel up the outside of the coaxial cable to the antenna, and then back down on the inside to interfere with reception. The grounding method described above will often eliminate that.
NEC • This 6-gauge wire, shown in red, connects the new ground rod to the breaker box (typically). This wire will help absorb the lower frequency components of a direct strike. • But if your antenna is situated where a direct strike is likely then installing this wire is strongly advised. • The wire should run close to the ground so that side flashes will likely arc to the ground. It is OK to run this wire around the exterior of the building. In this case keeping the wire 6” to 12” above ground is best. The length of this wire is less important, but turns should still be curves of large radius.
Some additional NEC rules • Do not attach an antenna to the power line service entrance power mast. • Outside the building, the antenna coaxial and grounding wires shall not come closer than: • 2 feet from exterior power lines of 250 Volts or less. • 10 feet from exterior power lines of greater than 250 Volts. • 1 foot from underground power lines. • 6 feet from lightning rod wires. • (Although these are safety rules, they also reduce the pickup of appliance noise.)
Some additional NEC rules • If the antenna mast or wires come within 5 feet of a swimming pool, they must be bonded to the pool’s bonding grid. • Grounding wires and grounding blocks are permitted to be interior to the building. (An interior ground rod might be in soil too dry to conduct much.) • Grounding connections must be constructed so that they will not come loose or corrode away. (Any connection joining two different kinds of metal will corrode very rapidly if the connection can get wet.) • An interior cold water pipe is acceptable as a ground rod if the connection point is within 5 feet of where the pipe enters the ground. (You must verify that the underground water line is not plastic.) • Indoor antennas (including attic antennas) are not generally susceptible to direct strikes. In such cases a grounding block is not required by the rules, but is probably a good idea when the cable is longer than 30 feet.
