Designing an RF distribution system
A practical introduction to cable television (CATV) and master antenna television (MATV) distribution. Learn how cable, passive components, headend equipment, and amplifiers work together to deliver consistent television signals.
1. Cable and passive components
Build each amplifier section for unity gain
When a distribution network uses several amplifiers in series, design it as a sequence of unity-gain building blocks. Each amplifier should compensate for the cable and passive losses in the section ahead of it.
This requires both gain and equalization. Gain restores signal strength, while equalization compensates for the greater cable loss at higher frequencies. Together, they maintain the intended signal levels through the amplifier cascade.
Protect the coaxial cable
Coaxial cable consists of a center conductor, surrounding insulation, and an outer conductive shield. The shield contains the RF signal and limits interference entering from outside. Depending on the cable, it may use foil, braid, aluminum tubing, or a combination of materials.
The center conductor must remain properly positioned within the cable. Kinks, crushing, and flattened sections disturb that geometry and can cause reflections. Shield damage can allow signals to leak out or unwanted signals to enter. Handle and install cable carefully.
Allow for frequency-dependent loss
Many channels can travel through one coaxial cable, but higher frequencies experience greater loss over the same distance. In the United States, cable attenuation is commonly expressed in decibels (dB) per 100 feet.
Use the cable manufacturer’s attenuation figures at the low and high ends of your operating band. That difference determines how much equalization the distribution section needs.
Account for temperature
The reference cable-loss data discussed in the source uses a cable temperature of 70°F, or approximately 21°C. Colder cable generally has less attenuation; hotter cable has more. Outdoor cable runs need to accommodate seasonal changes, while indoor and buried cable usually experience smaller temperature swings.
Use the correct channel-frequency plan
A channel number does not always identify the same frequency. Over-the-air television and cable systems use different channel assignments, and CATV systems can use standard, IRC, or HRC frequency plans.
This guide uses the standard CATV plan. Consult the channel-number and frequency reference when comparing assignments.
Include splitter and coupler losses
Splitters and directional couplers are often called flat-loss devices because their loss changes less with frequency than cable loss. The source treats their response below approximately 450 MHz as relatively flat. In the 750–860 MHz systems it discusses, high-frequency insertion loss needs closer attention.
Use each device’s published specifications across the full operating band when calculating the losses between amplifiers.
Splitters divide the signal among outputs
An equal-output splitter divides the input among two or more output ports. An ideal two-way split produces approximately 3 dB of loss at each output, but a real splitter adds insertion loss.
| Device | Approximate loss per output |
| 2-way splitter | 4 dB |
| 4-way splitter | 7.5–8 dB |
| 8-way splitter | 11–12 dB |
The source describes two-way losses of roughly 3.5–3.8 dB at lower frequencies, rising to 5–5.5 dB at the highest frequencies in its examples. Use these values for initial estimates and the actual product specifications for the final design.
Directional couplers divide the signal unequally
A directional coupler sends most of the signal through the main path and a smaller portion to a tap port. Typical tap-loss values in the source are 8, 12, and 16 dB. A higher tap-loss value generally means less signal is diverted and less insertion loss through the main path.
A directional coupler combined with a splitter in one housing is called a directional tap or multi-tap. The source describes tap-loss values from 8 to 32 dB, depending on the configuration and number of tap ports, with up to eight ports.
2. Headend equipment and distribution levels
Active equipment creates, processes, and amplifies the channels sent into the distribution network. The principal devices covered here are modulators, signal processors, strip amplifiers, and broadband amplifiers.
Modulators create television channels
A modulator takes video and audio from a source such as a satellite receiver, camera, or playback device and produces a modulated television channel. The analog equipment described in the source can provide RF output levels up to +60 dBmV.
dBmV expresses voltage relative to 1 millivolt. A level of +60 dBmV corresponds to 1 volt using the same voltage measurement convention, in the 75-ohm system described here.
Processors control individual channels
A strip amplifier amplifies a single received television channel without changing its frequency. Automatic gain control (AGC) keeps its output level steady, helping maintain consistent inputs to the broadband distribution network.
A signal processor converts the received channel to an intermediate frequency (IF). At this stage, it filters unwanted adjacent signals and controls the signal level. It then converts the channel to the required output frequency for distribution.
For the analog television system described in the source, processing also sets the sound carrier below the visual carrier. The incoming difference of approximately 6–10 dB is increased to about 13–17 dB to reduce interference between adjacent channels. Processor output levels may reach +60 dBmV, depending on the equipment.
Control broadband noise before combining
Some frequency-agile modulators and processors produce noise across a broad range of frequencies. Combining many such devices can raise the headend noise floor and produce snowy pictures before signals reach the distribution network.
Tracking bandpass filters help limit that noise in agile equipment. Fixed-channel devices may use an internal single-channel bandpass filter for the same purpose.
Balance the combined channel lineup
Combine the source channels using the appropriate couplers and splitters. Set the visual carriers to equal levels. In the analog examples covered here, each associated sound carrier is set approximately 15 dB below its visual carrier.
Combining introduces loss. Depending on the number of channels and the combining arrangement, the source describes combined levels of approximately +10 to +30 dBmV per channel. A broadband amplifier may be needed to restore enough level for distribution.
Balance output level against distortion
A level near +30 dBmV may be enough to feed some outgoing cable runs directly. Other systems may need amplifier output levels of +35 to +45 dBmV or more. These are examples rather than universal targets: the usable level depends on the equipment, channel loading, and number of amplifiers in cascade.
Excessive drive causes channels to mix in active components, producing intermodulation distortion. Composite triple beat (CTB) is one common result and can appear as lines or a busy background in analog pictures.
Use output tilt to compensate for cable loss
The source’s 750 MHz design example starts with +40 dBmV on cable channel 118 and +35 dBmV on channel 2. This 5 dB output tilt gives the higher-frequency signal more level to compensate for its greater loss in the cable that follows.
Operating the lower-frequency channels at reduced output also helps control the amplifier’s distortion contribution.
Choose cable for the distance and installation
Select cable according to the run length and whether it will be aerial, directly buried, installed in conduit, or routed inside a building. The source describes outdoor hardline sizes of 0.500 inch and 0.750 inch, measured by the metallic outer sheath. The larger cable has lower loss per 100 feet.
Indoor systems may use hardline or flexible coaxial cable, with the appropriate cable rating for the installation. The flexible types discussed are RG-11, RG-6, and RG-59. Of these, RG-11 is the largest and has the lowest loss; RG-59 has the highest loss and is less suitable for long runs or higher-bandwidth systems. Use plenum-rated cable where the installation requires it.