TV Antenna Amplifiers & How They Work
For this month’s post, we’re going to focus on TV antenna amplifiers. Everyone knows what an amplifier is, but there’s plenty of misunderstanding about how TV amplifiers work and when to use them. Amplifiers can do a lot of good, but they can also create problems for TV reception. There are good amplifier designs and cheap amplifier designs, and you get what you pay for.
Amplifiers ingest weak TV signals and make them stronger. But as a rule, amplifiers don’t discriminate: Without special filtering, any signals coming into an amplifier, including unwanted ones, are all boosted. And amplifiers also create noise of their own, like any other active electronic circuits. Depending on the location of the amplifier in the signal chain, noise can become a significant problem for reliable reception of weak signals.
The best antenna system requires no amplification at all! In such a system, signals from TV stations are all above the required signal-to-noise (S/N) threshold for reliable reception. The ATSC 1.0 digital TV standard defines that threshold is 15.3 decibels (dB) stronger than the noise floor - at least in an ideal environment. In reality; with signal reflections and echoes (the so-called Rayleigh reception environment), that number can be 20 dB S/N or higher.
If the connection from your TV antenna goes through a splitter, the signal level drops at each splitter output. That won’t be a problem with strong signals, but it might make receiving weaker signals a real challenge. Signal levels are also reduced by attenuation in the coaxial cable running from the antenna to your television - the longer the cable run, the greater the attenuation. The signal-to-noise ratio also decreases with longer lengths of cable.
And that’s where amplifiers come in. If you are trying to receive weaker signals and run them through a splitter, you’ll need an amplifier to overcome the signal attenuation. And you’ll want to use an amplifier that has a low noise figure, i.e., one that adds a minimal amount of noise while amplifying signals. The trick is to achieve enough signal amplification without overdriving the TV receiver.
ATSC receivers in televisions are designed with a relatively wide dynamic range. That is; they can receive both very weak signals and very strong signals reliably at the same time from channel 2 (VHF) to channel 36 (UHF). But too much gain from an amplifier can boost already strong signals to the point where they overload the TV receiver, creating distortions and unwanted interference. Choose carefully!
Amplifiers for TV reception fall into two categories. If you live in a suburban area with good signal levels and want to split the antenna connection to 3 or more televisions or set-top boxes, you’ll want a medium-gain distribution amplifier ahead of the splitter. This type of amplifier is typically mounted inside and won’t have the low noise performance of a mast-mounted amplifier, but a tiny bit of added noise isn’t going to cause you any reception problems when splitting strong local signals.
Channel Master’s Ultra Mini-series fit this requirement nicely as they combine splitting and signal amplification in one convenient package, providing one, two, four, or eight amplified outputs. For indoor antennas that need a little signal boost to overcome signal attenuation, the inline MicroAmp provides 10 dB of amplification in a super-compact housing not much bigger than a connector. Again; these amplifiers are intended specifically for indoor use to feed multiple TVs and set-top boxes.
Exurban and fringe areas require a different solution. Here, the goal is to boost signal levels from all stations to provide more reliable reception. And, you want to keep the system noise figure as low as possible. You’ll need a high-gain preamplifier that can be mounted outdoors as close to the antenna as possible. Gain figures for the Titan and Amplify series of preamplifiers start at 16 dB and go all the way to 30 dB. On two Amplify models, you can select between 17 dB and 30 dB gain simply by flipping a switch. And these are low-noise amplifiers, with noise figure specifications less than 2 dB.
At first glance, 30 dB appears to be a substantial signal boost! But if you are running a long cable from an outside antenna and splitting the TV signals two or three times, you’ll need that extra headroom to stay above the 20 dB S/N target we mentioned earlier. For an antenna installation with 100 feet of RG-6 coax and a three-way splitter feeding TVs, signal levels from the amplifier will be reduced by a total of 12.5 dB @ 500 MHz (channel 19) by the time they reach each output of the splitter.
And that’s not counting in the preamplifier’s noise figure. A low-noise mast-mounted amplifier should have a noise figure below 2 dB. Even so, the noise figure is also subtracted from the overall system gain. The antenna system we’ve just described has already offset half the 30 dB boost from the amplifier by the time any signals get to a television. Therefore, any improvements to a TV reception system should always start with the antenna first (their noise figure is zero, by the way), then add amplification as needed.
Can you use both a mast-mounted preamplifier and a distribution amplifier in the same system? Yes, but that would mostly likely happen in a large master TV antenna system, distributing signals to many televisions where there is significant signal attenuation at each splitter output. What you DON’T want to do is feed the output of one amplifier directly into the input of another! You’ll completely overload the second amplifier and render all TV signals unreceivable.
We mentioned earlier that antenna amplifiers don’t discriminate and boost all signals within their design range. Additional filtering may be required to block unwanted signals. For example, strong FM radio stations can overload preamplifiers and create unwanted interference, so Channel Master preamplifiers are designed with an FM “trap” circuit - a band-reject filter that attenuates signals from 88 to 108 MHz that can be switched in and out as needed. (If you are trying to receive a station on VHF channel 6, you may need to keep the trap switched “out.”)
Another example: The recent FCC frequency reallocation of TV channels above UHF 36 has resulted in mobile phone networks operating close to the upper edge of the UHF TV spectrum. Accordingly, Channel Master’s Amplify line of preamplifiers incorporate a low-pass filter that cuts out all signals above 608 MHz (the upper end of channel 36) to eliminate interference from LTE networks.
Follow these steps when choosing and installing a TV signal amplifier:
(1) Determine whether your installation calls for a distribution amplifier (indoors, feeding multiple TVs) or preamplifier (outdoors, mounted at the antenna)
(2) Calculate the total signal attenuation from the antenna terminals to your TV / set-top box
(3) Choose an amplifier with a gain specification that offsets the calculated signal loss + the amplifier’s noise figure and leaves some extra headroom for weaker signals
(4) For very weak signals, install larger antennas first to boost gain, then add a low-noise amplifier to create headroom for signals with multi-path and echoes - a switchable-gain amplifier is a good choice
(5) For preamplifiers with switchable gain settings, use the lowest gain setting that provides reliable reception across all channels