Automedia

by Gary Bourgois


A TVRO (TeleVision Receive Only) System consists of the following components:

THE DISH This is the most visible component, a parabolic reflector which may consist of Solid Aluminum, Perforated Aluminum or WIRE MESH. Dish sizes vary from small (3-4 feet KU BAND, EUROPE) all the way to 16-20 Ft (USA Commercial C-BAND) with the average falling between 7-12 ft, 10 ft being most common. This device focuses the microwave signals coming from the satellites much as the mirror in a reflecting telescope concentrates the light from distant galaxies.
THE MOUNT The most common type of dish mount is called a POLAR MOUNT, so named because it is oriented to coincide with the earth's axis, enabling it to "track" the satellites, which are spread out in geostationary orbit in a band of the sky called the "Clarke Belt", named after Arthur C. Clarke, who dreamed up the concept of geostationary communications satellites back in 1945. The mount is installed on a 3 inch pipe, which is sunk in concrete. Most (but not all) mounts require a 3.5" OD pole. In most (but not all) cases this is a 3" ID pipe. Schedule 40 works well, so does used "drill stem". In many areas, this is available from salvage dealers at very reasonable prices. (* OD refers to OUTER DIAMETER, ID refers to INNER DIAMETER)
THE POSITIONER ARM (Actuator) A device containing a motor that is used to swing the satellite dish to allow it to focus on individual satellites, which may be as close to one another as 2 degrees of arc. Actuator arms are most commonly found in 18- and 24-inch lengths (the longer the arm, the wider the arc that the dish can "see".) In addition, a more expensive aiming mechanism, called a "horizon-to-horizon" actuator, is capable of more precise aiming (important on Ku band and closely-spaced satellites), as well as being able to see the entire satellite arc from your location.
Many true dish heads have also added a second actuator to control the VERTICAL aspect (elevation) of the dish, to enable them to track inclined orbit satellites which "wobble" or vary in the vertical plane.
THE LNB This is the Little gizmo that amplifies the very weak signals from the dish, and also converts them to a more suitable band of frequencies. LNB means LOW NOISE BLOCKCONVERTOR. Older systems consisted of separate components: LNA (Low Noise Amplifier) and a downconvertor which changed the received signals (3-4 GHz, or GIGAherz ....A GIGAHerz is one thousand megaHz....) to 70 Mhz.
The "standard block" used today is 950 to 1450 Mhz. Both C and KU band (while they input on vastly different frequencies) output on the 950 to 1450 Mhz block. HOWEVER there are special considerations when dealing with KU (explained later)
There is also a device called an LNBF, which combines the LNB and feedhorn into a single unit. (See below for description of feed assembly.) The LNBF device uses a simpler method for adjusting the polarity (voltage right on the LNB cable). You can not adjust fine skew, just H or V. This device is practical for C band only systems. However if the LNB part goes bad, you need to replace ALL of it.
An LNBF is especially suitable for dedicated operations, such as a smaller dish used for only one satellite. I have a 6 foot dish which I use for Telstar 401 only, using an LNBF, and it gives excellent service.
THE FEED ASSEMBLY The feedhorn acts as a microwave "funnel" if you want to think of it that way. This device places the LNB at the focus of the dish, and is usually just called "the feed". There are several different types of feeds.
Single C band feed: Contains ONE C band LNB. A device called a POLAROTOR is located inside, which allows the reception of both vertically polarized and horizontally polarized channels. In general practice, the EVEN channels are on ONE polarity and the ODD channels are on the opposite polarity. The actual polarity scheme depends on the satellite in question.
Dual C band feed: Allows the use of Two C band LNB's one for horizontal polarization and one for vertical.
Dual Band Feed (C and KU): Has two separate LNB's, one for C band and one for KU band.
DUAL C band and single KU band feed: (There is at least one version of this type of feed.
The most popular Dual Band feedhorns are the Chaparral CO-ROTOR II Gardiner and ADL, though there are others on the market. While these are actually a compromise system, with some minor losses, in practice they work pretty well.
SATELLITE CABLES This is a bundle of various wires and cables that run from the dish to the receiver, and consist of cables for the C and/or KU LNB's, power to the LNB (Usually sent over the same COAXIAL CABLE) as well as power for the positioner arm, return signals for position readout, and control voltage for the polarotor. Power to actuator Arm (Two conductors) Return position readout from actuator (Two conductors).
The position readout cable from the actuator to the actuator controller should have THREE conductors for the position sensor. For pot sensors, one is for the tap, the other two are for the resistor. For pulse sensors, one is ground, one is +5V, and the other is pulse input. Not all arms require all three connections, but my scrounged Saginaw special has some nifty hall-effect gizmo that does require power to generate pulses.
Power to polarotor (Two conductors). Polarotors come in two different varieties. The "old style" just used a 12V motor to rotate the probe. Apply power, it moves. Reverse polarity, it changes direction. The new ones use a three wire connection. One for +5V, one for ground, and one for "pulse". RG-6U cable (rather than the often-seen RG-59U) is the cable of choice for the higher frequencies of satellite TV.
RECEIVER There are many receivers available by a number of manufacturers, both new and used. The receiver takes the signal from the LNB and produces a TV picture from the wideband FM video, and also allows you to tune SUBCARRIER audio, which can provide many different audio only services such as MUSIC and TALK shows and even DATA TEXT reception. Newer receivers work with the standard 950-1450 mhz block that comes out of a standard LNB. Older ones use a direct 70 MHZ feed and have an LNA (low noise amplifier) and downconvertor at the dish. If you are on a budget, one of these older type receivers might be a good place to start, though most folks today use the more modern technology.
Your receiver may also have a descrambler built in for decoding PAY services. This type of receiver is called an IRD or Integrated Receiver Descrambler. You need a descrambler for services like HBO and regular CABLE-TV type services (to be discussed later).
DESCRAMBLER The DE-FACTO consumer descrambler is the VIDEOCIPHER system which is manufactured by General Instruments (G.I) in the USA. There are stand-alone decoders still available. However a major consider- ation is that at the present time, Descrambler technology is changing and older descramblers will not work under the new standards. Beware when buying an IRD (INTEGRATED, RECEIVER DESCRAMBLER) that the unit can be upgraded to the new VC-II+ and forthcoming VC-II+ RS (Renewable Security) if you plan on watching scrambled (Mostly Cable) services or Pay Per View movies and sports events.
In Canada, the OAK-ORION system is another popular scrambling system, which is used by TELESAT CANADA on the ANIK (Canadian) satellites. These programs may NOT be subscribed to in the US Legally. There ARE some USA programs transmitted with the OAK-ORION system, primarily Horse Racing and Hospital programming. While it is LEGAL to own a OAK-ORION decoder, there are not enough services available in this format to warrant doing so. Unless a person is a REAL horse racing nut, and even then, getting the decoder AUTHORIZED to descramble the horse races is questionable.
There are other scrambling systems in use in Europe. In the USA, there had been a serious market for "pirate" or "chipped" decoders, which receive scrambled services without the payment of subscription fees. This situation has lead to the change to the revised VC-II+ and forthcoming VC-II+ RS systems, which hope to thwart this piracy through the use of a renewable "credit card" type of chip system. Of course, this technology costs the consumer additional money, on top of subscription fees that for the most part are higher than comparable cable prices. Since the majority of the scrambled channels are owned by the cable TV conglomerates, who would prefer people subscribe to cable rather than view by satellite, the reasoning here is obvious. For this reason, satellite TV popularity in the USA has declined.


illustration by Bart Nagel