CATV installation basics

Residential broadband will become a whole new world for telephone company technicians. Over the next several years, these technicians may be called upon to support a wide variety of residential broadband installations, from wireless multichannel multipoint distribution service to direct broadcast satellite and hybrid fiber/coax.

Whether the installation involves twisted pair copper wire, optical fiber and coaxial cable, satellite dishes, microwave antennas or other network interfaces, there will be many familiar elements (Table 1).

HFC Systems Telco technicians will be most familiar with HFC installations. These installations will require a drop wire connection from an HFC tap to the side of a home or business, where a ground block or network interface unit is located. The drop media can be twisted pair, coaxial cable or both. Figure 1 shows a typical subscriber drop for an HFC installation.

It is important for installers to maintain proper clearances above ground level anywhere a drop cable must be run across a public right of way. That means 18 feet from an alley or street surface, 15 feet over a commercial driveway and 13 feet over a residential driveway. On a telephone pole, the drop cable must be no less than 40 inches from the power drop and 12 inches from the telephone drop wire if one is in place.

The mid-span clearance from any power drop is 30 inches, while the side-of-home clearance is 12 inches. At the side of the home, any telephone drop must be at least four inches from the coaxial cable drop.

Underground drops are buried 36 inches deep when crossing a street or alley, 24 inches deep when crossing sidewalks and driveways, and at least 12 inches deep when crossing under lawns on the property. HFC installations are different from wireless cable and DBS in one additional respect: It is necessary to control signal leakage into and out of the network.

Because HFC networks reuse the same frequencies used by over-the-air licensees, special attention must be paid to small details such as proper connectorization and splicing, maintenance of drop cable shielding, and inspection of tap housings and ports for cracks or other damage. All connectors must be matched to the size of the drop cable, and seals or other gaskets must be in good condition. HFC jobs also may involve passive leakage detection measures such as wearing a leakage monitor during a routine installation.

MMDS Systems DBS and MMDS are both line-of-sight technologies that require an antenna. Accordingly, they must have an unobstructed view of the system transmitter, whether that is a satellite in orbit or a transmitting tower on the ground. DBS and MMDS always require an on-site signal survey to determine signal strength and antenna positioning.

The principal equipment used by MMDS receivers includes the antenna and mast, downconverter, drop cable to decoder, decoder and internal wiring. Figure 2 shows a typical MMDS network.

The customer premises equipment includes a circular or rectangular receiving antenna between 12 and 18 inches wide. In some cases, an over-the-air VHF/UHF antenna is bundled with the microwave system, so a customer can switch from the MMDS feed to over-the-air broadcasting.

The MMDS antenna is generally mounted on a mast, which is affixed to a roof. MMDS masts normally are 3 to 5 feet long and are affixed to chimneys, walls or eaves. If a longer mast is required to clear trees or other obstructions, the mast must be guyed. The typical rooftop installation requires drilling holes in the roof, ideally into rafters.

Masts should never be affixed to plywood. On buildings with flat roofs, the mast must be affixed to the eaves or exterior walls because any holes drilled into a flat roof will provide a collection point for standing water. Sealant should always be applied to the holes, and the cables and connectors should be weatherproofed by applying grease and a protectant tape. All tapes, ties and other non-metallic materials must be UV-resistant.

The drop cable must always be grounded, using an in-place HFC-type ground block, cold water pipes or a ground rod. Cable entrance points into the home are also waterproofed using grease and a plug, while the cable itself is given a 6-inch "drip loop." Cable is affixed using plastic clips spaced about every 18 inches along the length of the cable.

MMDS transmitters send signals in either a horizontal or vertical polarization, and the antenna must be rotated to match the transmit polarization. Where a rectangular antenna is used, horizontal polarization requires antenna orientation parallel to the ground. Vertical polarization requires orientation perpendicular to the ground.

A downconverter mounted on the rear of the antenna takes the received energy and "block converts" the entire set of frequencies to a lower range. For example, frequencies in the 2.5 GHz range are downconverted to a range under 400 MHz. The technical reason for downconversion is the extremely high-loss GHz range signals experience when moving through coaxial cable. By changing the frequencies to a lower range, signal attenuation is dramatically reduced.

A coaxial cable drop runs signals from the downconverter into the house, where it connects with a set-top video decoder terminal, functioning just like a cable TV decoder. The decoder descrambles the TV signals, which generally are launched in encoded form. The terminal also handles frequency conversion operations, taking signals at various frequencies and changing them to a standard format, typically channel 3 frequency, for output to the TV.

As an alternative to a rooftop VHF/UHF antenna, new equipment available for both DBS and MMDS installations uses the in-home electrical wiring as the antenna for off-air signals. These power-line antennas use a module that is plugged into a wall socket and then connected to the decoder, which is typically manufactured with two input leads-one for the MMDS or DBS antenna and the other for an external off-air antenna. DBS Installations

All North American DBS satellites are positioned over the equator, so all "look angles" are to the south. A compass can be used to establish north/south orientation, and an inclinometer can be used to fix the antenna elevation in degrees above the horizon. DBS receivers often include circuits measuring signal strength, allowing the signal to be "peaked." The signal is peaked when looking directly at the transmitting satellite.

A DBS receive system includes the antenna and mount, feedhorn, low-noise block converter (LNB), drop cable, satellite receiver and inside home wiring. DBS antennas are small, between 18 and 30 inches in diameter. These "pizza dish" reflectors are used to gather received energy and focus it on the feedhorn, which is positioned in front of the reflector at precisely the point in space where the reflected signals converge.

The feedhorn collects the energy, while the LNB converts the 12 GHz signals to a lower range that can be passed through the coaxial cable drop without excessive attenuation. Some providers, such as DirecTV and Hughes Network Systems, are designing dual-feedhorn systems that can look at two closely-spaced satellites at once. This capability allows the reception of video entertainment and high-speed data services from a single reflector.

DBS reflectors, or antennas, are made of spun aluminum or steel, stamped and drawn steel, fiberglass or formed plastic. Plastic and fiberglass antennas have metallic reflectors embedded in them. Like the MMDS mount, the antenna mount holds the antenna in place. As with MMDS, all outdoor cables and fittings should be weatherproofed using a sealant. Also like MMDS, many customers will want to connect a separate off-air antenna to the receiver. DBS receivers accept both standard off-air antenna or newer power-line antennas.

The LNB detects the collected signals provided by the feedhorn and amplifies them about 40 to 50 dB, a gain of 10,000 to 100,000. The LNB also converts the high-frequency signals, in the 12 GHz range, down to an intermediate range, typically 950 to 1450 MHz.

The satellite receiver is located inside the house. It tunes to one specific channel and downconverts the frequency, typically to 70 MHz. The selected channel is then remodulated to a channel 3 or channel 4 output.

Although most DBS receivers now have only one tuner, allowing viewing of a single channel at a time, models featuring two tuners are also available, providing two separate and simultaneous video outputs. That configuration allows viewing of one channel while taping the other or feeding two outlets. The output from a single reflector can also be split and fed to two or more separate satellite receivers within the home.

Inside the Home DBS, MMDS and HFC connections are remarkably similar in the home and may involve a combination of twisted pair, coaxial cable and shielded 10BaseT twisted pair wiring for cable modem connections. MMDS and HFC systems are suitable for multiple dwelling units. In that case, there is normally a master building entry system, similar to a cross-connect, to which each customer drop is connected.

The antenna or wired network should always be grounded to the side of the building. Figure 3 shows the proper bending radius of a ground wire. When multiple outlets are fed, signal splitting is also done at the side of the building with a direct "home run" cable to each outlet inside. Each outlet uses a wall plate and F-connector interface. TVs, VCRs and video game players use an F-connector.

The in-building wiring normally is an RG-6 or RG-11 coaxial cable, selected for better signal loss performance than RG-59 cables because it retains cable flexibility and handling ease. Though larger-size cables feature lower loss, they cannot be easily bent around corners and fished through walls.

Most DBS installations will require connection of the decoder to the RJ-11 telephone network because that is the return path for pay-per-view ordering. HFC and MMDS installations, especially digital MMDS, also may use a connection to a telephone line for PPV ordering.

Matt Davis is Broadband Engineer/Instructor for the National Cable Television Institute, Littleton, Colo.