A mast-mounted preamplifier can turn a marginal UHF channel into a stable picture, or it can knock every channel off the air at a site that sits close to a broadcast tower. Same part, opposite results. This guide covers how to choose an antenna preamplifier: when one helps, when it makes things worse, and what the noise figure, gain and filter lines on a data sheet mean for the install in front of you.
How do you choose an antenna preamplifier?
Choose an antenna preamplifier by measuring first. A preamp helps when the signal at the antenna is adequate but long downlead runs, several splits or a fringe location push it toward the bottom of the tuner’s range. Mount it at the antenna, compare noise figure in the band you need, pick gain that covers the losses, and filter FM or 600 MHz signals where needed.
What a preamp fixes, and what it cannot
A preamplifier does not create signal. Whatever carrier-to-noise ratio exists at the antenna terminals is the best the system will ever see. What the preamp does is set the noise performance of the system at the antenna, before the downlead, splitters and the tuner add their own losses and noise.
The FCC’s own planning assumptions show why that matters. The planning factors the FCC uses to define digital TV coverage assume a downlead loss of 1 dB at low VHF, 2 dB at high VHF and 4 dB at UHF, a system noise figure of 10 dB at VHF and 7 dB at UHF, and a required carrier-to-noise ratio of 15 dB (FCC OET Bulletin 69, Table 3). Those numbers describe a single TV on a modest cable run. A long run, a four-way split or a whole-house distribution network adds loss that the FCC model never counted, and that loss eats directly into the margin above the 15 dB threshold.
Noise figure and gain: the cascade rule
Every stage in a receiving chain adds noise. Friis’ equation for cascaded stages shows that the noise contributed by each stage decreases as the total gain preceding that stage increases, so the first few stages in a cascade are the most critical (Linköping University, TSEK03 Lecture 2). In linear terms the total noise factor is F1 + (F2 − 1)/G1 + (F3 − 1)/(G1 × G2) and so on, where F is each stage’s noise factor and G its gain.
Three practical rules follow from that equation:
- Put the preamp at the antenna. The same lecture covers lossy circuits ahead of an amplifier: loss placed before the first amplifier adds to the system noise figure rather than being covered by the amplifier’s gain. A preamp in the basement after a long run of coax cannot recover what the cable already lost. That location is the job of a distribution amplifier, which solves a different problem.
- Noise figure is the spec that sets reach. When you compare two preamps for a weak-signal site, compare noise figure first, and compare it in the band you actually need. Data sheets usually list VHF and UHF separately because the numbers differ.
- Gain only has to cover what follows. Once the preamp’s gain comfortably exceeds the losses and noise of the stages after it, extra gain does not improve the picture. It only raises the level of every signal arriving at the tuner, which is how overload starts.
When a preamp helps
The ATSC A/74 receiver performance guideline asks a fixed DTV receiver to work with input levels from −83 dBm to −5 dBm at the tuner, for both VHF and UHF (A/74:2010, section 5.1). That is the window you are trying to keep every TV inside. A preamp earns its place when the signal at the antenna is adequate but the losses between the antenna and the tuners would push it toward the bottom of that window:
- Long downlead runs, especially on UHF channels, where cable loss is higher than on VHF (the FCC planning factors above assume 4 dB of downlead loss at UHF against 1 dB at low VHF).
- Several splits feeding multiple rooms, where each split divides the signal before it reaches a tuner.
- Fringe locations where the antenna is pulling in distant stations and the level at the antenna is already modest.
Measure before you buy. A signal level meter reading at the antenna and at the far end of the run tells you how much loss the system really has, which is the number the preamp’s gain has to answer.
When a preamp hurts: overload and intermodulation
The same ATSC guideline warns about the opposite case. It tells receiver designers to assume that multiple undesired signals, each approaching −8 dBm, will exist at the receiver input, and it names two sources: a high-gain antenna close to the transmitters, or a mast-mount amplifier and antenna combination. A footnote adds that typical mast-mount amplifiers and preamplifiers examined in 2003 could handle approximately −8 dBm output per channel before they generated intermodulation products internally (A/74:2010, section 5.2).
Intermodulation is the failure to watch for. A/74 explains that two strong signals at frequencies Fa and Fb produce third-order products at 2Fa − Fb and 2Fb − Fa, and that with UHF channels packed together these products generally fall in other UHF TV channels (A/74:2010, Annex F). The result is a channel that a meter shows as strong but that the TV cannot decode, or a set of channels that comes back as soon as the preamp is bypassed.
Signs that a site needs less gain, not more:
- One or more transmitters are close by and in clear view of the antenna.
- Channels drop out or pixelate even though their measured level looks healthy.
- Reception improves when you remove the preamp or switch it off.
At sites like these, look for a preamp with adjustable or switchable gain, add attenuation on the strong band, or leave the preamp off and use a distribution amplifier after the splits instead. Remember that overload depends on the total of every signal the preamp sees, not only the channel you are trying to fix.
Filtering: FM broadcast and 600 MHz wireless
A broadband preamp amplifies everything its passband admits, including signals that are not TV.
FM. The FM broadcast band runs from 88 MHz to 108 MHz (47 CFR 73.201). It sits between TV channel 6 (82-88 MHz) and channel 7 (174-180 MHz) (47 CFR 73.603), so a VHF antenna and a VHF preamp will pick it up. Near a strong FM station, a preamp with a built-in FM trap, or a switchable FM filter, keeps that energy from contributing to overload.
600 MHz wireless. The FCC rules now set aside the 600 MHz band, 617-652 MHz and 663-698 MHz, for licensed wireless service (47 CFR 27.5). UHF TV channel 36 occupies 602-608 MHz (47 CFR 73.603). A preamp designed for the older, wider UHF TV band still passes those wireless frequencies, and a nearby cell site can overload it just as a TV transmitter can. When a site is close to cellular equipment, choose a preamp whose UHF filter rolls off above the TV band, or put an appropriate filter ahead of the amplifier.
How to choose an antenna preamplifier: a checklist
- List the stations. Note each channel number, its band (low VHF, high VHF or UHF) and roughly how far away and in which direction the transmitter is.
- Measure at the antenna. Take levels before anything is amplified, and note any very strong signals, including FM and nearby cellular.
- Total the losses after the antenna. Cable run, splitters and the number of tuners served.
- Match band coverage to the channels. If the site uses separate VHF and UHF antennas, choose a preamp with separate inputs.
- Choose gain to cover the losses, not the maximum available. Prefer adjustable gain wherever a strong station is in view.
- Compare noise figure in the band that carries the weak stations.
- Check the filters. FM trap for sites near FM transmitters, a TV-band UHF filter for sites near cellular equipment.
- Plan the power path. Mast-mount preamps are normally powered up the coax from an indoor power inserter. Place the inserter so DC reaches the preamp without passing through a splitter port that blocks it.
- Finish the mast. Weatherproof the connections and bond the mast and coax to the grounding system as the electrical code and local inspector require.
Frequently asked questions
Do I need a preamplifier for my TV antenna?
Only if the losses after the antenna are the problem. A preamp earns its place on long downlead runs, especially on UHF, on systems with several splits feeding multiple rooms, and at fringe locations where the level at the antenna is already modest. Measure at the antenna and at the far end of the run before you buy, because that difference is the loss the preamp’s gain has to answer.
Can an antenna preamp make reception worse?
Yes. Near strong transmitters a preamp can overload and generate intermodulation products that land on other UHF channels. The signs are channels that pixelate even though their measured level looks healthy, and reception that improves when the preamp is removed or switched off.
At those sites, use a preamp with adjustable or switchable gain, add attenuation on the strong band, or leave the preamp off and use a distribution amplifier after the splits instead.
Where should an antenna preamplifier be installed?
At the antenna, on the mast. Loss placed ahead of the first amplifier adds to the system noise figure, so a preamp in the basement after a long cable run cannot recover what the coax already lost. That downstream location is the job of a distribution amplifier, which solves a different problem.
What matters more in a preamp, gain or noise figure?
For a weak-signal site, noise figure. The first stages in the chain set most of the system’s noise performance, so compare noise figure first, in the band that carries the weak stations. Gain only has to cover the losses after the preamp; extra gain does not improve the picture and only raises every signal toward overload.
How is a mast-mounted preamp powered?
Mast-mount preamps are normally powered up the coax from an indoor power inserter. Place the inserter so DC reaches the preamp without passing through a splitter port that blocks it.
Where to buy antenna preamplifiers
Bursma Distributing supplies antenna preamplifiers and the rest of the reception chain wholesale to dealers, installers and integrators. Browse the preamps category and the wider Antenna - Tower - Satellite division for antennas, mounts and accessories. Dealers see their own pricing and stock levels in Shop Online after logging in. If you are not a Bursma customer yet, open a dealer account, or call 616-831-0080 and ask your sales representative which preamp fits the site you are working on.
Sources
- FCC Office of Engineering and Technology, OET Bulletin No. 69, Longley-Rice Methodology for Evaluating TV Coverage and Interference (Table 3, Planning Factors for DTV Reception)
- Advanced Television Systems Committee, A/74:2010 Receiver Performance Guidelines (sections 5.1, 5.2 and Annex F)
- Linköping University, TSEK03 Radio Frequency Integrated Circuits, Lecture 2: Noise (Friis’ equation; noise figure of lossy circuits)
- 47 CFR 73.201, FM broadcast band (Cornell Legal Information Institute)
- 47 CFR 73.603, Numerical designation of television channels (Cornell Legal Information Institute)
- 47 CFR 27.5, Frequencies, 600 MHz band (Cornell Legal Information Institute)