Satellite Dish Technical Help

General "Tech Help" Information

A DBS satellite system basic installation is usually a fairly simple task, consisting of installing a single dish and a single receiver hooked up to a single TV set. Most homes have more than one TV set, however, and many customers want more than one satellite receiver, and there are other complicating factors which make many satellite installations much more difficult. All small dish satellite systems come with very good installation manuals, and we provide additional information in our "Installation" pages. This Tech Help page provides additionat information based on questions we have received from customers from time to time.

House Wiring Schematic with Diplexers

The LNB is the part that attaches to the end of the arm in front of the dish. The dish reflects the satellite signal into the LNB where it is captured, processed, amplified, and sent down the coaxial cable to the satellite receiver. The term LNB comes down from the "C" band (large dish) days and stands for Low Noise Blockdown converter. The LNB is powered by a DC voltage from the receiver which also controls polarity switching in the LNB (see "Loss of Channels" below).

There are several styles of LNB's used in the DBS (small dish) industry. The most popular are probably the RCA (rectangular mounting tube) and the Dish Network ("D" tube). Hughes uses a "flat pack" style, but also sometimes ships its systems with an RCA or a Dish Network style LNB. Sony has its own style which is not interchangeable with any other. Other less popular systems such as Hitachi and Uniden generally use one of the styles already mentioned, but occasionally an odd different style will show up. Although each LNB style must be used with a compatible dish style so it will attach properly to the dish arm, all DBS (Primestar not included) receivers will work with any of the DBS Dish/LNB combinations. In other words, an RCA receiver can be attached to a Sony dish or even a Dish Network dish.

Each LNB style is made in two versions - single and dual. A single LNB has only one output and can be connected to only one receiver. This signal cannot be split to two receivers because this would interfere with the polarity control and cause loss of channels. A dual LNB has two independent outputs which can be connected to two receivers each with full control of all channels. More than two receivers requires the use of a multiswitch.

If your system brings in only some of the channels you are subscribing to, you may have a polarity switching problem.

The satellite receiver sends out a DC voltage over the coaxial cable to the LNB at the dish to power the LNB circuitry and to switch between left and right hand circular transponder polarization. The polarity switching is accomplished by changing the DC voltage between 13 and 18 volts. If this voltage signal does not get to the LNB reliably, it will show up as a loss of signal on the odd or even transponders. DBS satellites generally are Ku band, with 32 transponders alternating between left and right hand circular polarization. Each transponder can have several TV channels or a mix of TV, music, or data channels.

When you use a multi-switch so you can feed more than two receivers, the two LNB outputs are connected to the multi-switch inputs, and one of the LNB's is fixed (by the multi-switch) at left hand polarity and one is fixed at right hand polarity. Each of the receivers is then connected to the proper polarity by the circuitry within the multi-switch

As you change channels on the satellite receiver, this polarity switching is accomplished automatically, and the viewer normally does not know or care which polarity a particular channel is on. There is no regular pattern as to which channels are on which transponders, and a problem with the polarity switching may appear as a random loss of channels. Go to the signal strength menu and change transponder numbers (1, 2, 3, etc.). If the even transponders all have a good signal strength and the odd transponders all are zero (or the other way around), there is a problem with polarity switching. A small difference is signal strength between transponders is normal, but they should all be in the 80’s or 90’s.

A polarity switching problem may be caused by a malfunction in the receiver or LNB (or multi-switch), or it may be caused by a bad connection in the cable between the receiver and the LNB. The cable must have a firm continuous connection on both the center wire and the shield to work properly.

There are several ways to track down a polarity switching problem. One is to switch components (receivers, cables, LNB's), if you have more than one, to isolate which component has the problem. If you can get to the dish location easily and have a small TV set, you can set everything (receiver and TV set) up right there using a short piece of cable between the LNB and receiver. This should tell you if the cable is the problem.

If you have a volt meter, you can test the system as follows: Remove the LNB input cable from the back of the receiver. Connect a short cable to the receiver and measure the voltage between the center wire and the outside of the connector. This should read either 13 or 18 volts DC. Change channels on the receiver until the voltage changes from one voltage to the other. This may take quite a few channel changes. You can also go into the signal strength menu and change transponders directly. Even transponders should be one voltage and odd transponders the other. Connect the LNB cable to the receiver again and try measuring the voltage at the other end of the cable (at the dish). If you find that the cable is the problem, it may only be in one of the connectors and not in the cable itself. Make sure the center wire is clean and not shorted to the connector, and that the connector is making good contact with the shield.