Russell Brown /
10.16.2009
Originally featured on BroadcastEngineering.com
Satellite TVRO Part 5

In this tutorial, we will cover the steps and equipment required to bring in a satellite feed, including the site survey, feed horn and LNB selection, cables and connectors, spectrum analyzers and satellite finders and other equipment used to tune in a satellite feed.

Site survey

To begin you need to know the satellite, transponder (with polarity), frequency with modulation type, symbol rate and forward error correction (FEC). This is all you need to be able to receive the feed. Using one of the satellite Web sites, select the required satellite and look up its map or footprint page. Here you will see the predicted contours of its signal strength; find where your location is, and mark the signal strength. Typically, the signal in the middle of the United States is higher than at the coasts, where the signal starts to fall off. Using the chart in Figure 1, you can see what size dish is required. Opt to go with at least the next size up to give yourself some headroom when the signal fades due to weather or misalignment.

When selecting the site for the dish, be on the lookout for obstacles such as buildings and trees. You can use a compass and a paper towel tube with an inclinometer attached to check the site lines for obstacles. Remember that trees grow, and the tree that’s not in the way today may be a problem in the future if it’s not trimmed on a regular basis.

Sun outages can occur when the sun, satellite and the dish form a straight line, and the sun’s energy overwhelms the low-noise block converter (LNB) blocking the satellite’s signal. This occurs twice a year, and they can be predicted ahead of time. With the dish’s location and the satellite name, the date and times of the outage can be found by using online tools such as http://www.satellite-calculations.com.

Feed horn

There are several different types of feed horns that can be used depending on what is required. There are both single- and dual-polarity feed horns, for when you need signals from all transponders in a single band. There are even dual-polarity C-/Ku-band feed horns that supply all of the signals a satellite has to offer. Fine-tuning the alignment of a fixed dish will be quicker and more accurate by using Ku-band reception, which is three times more sensitive to dish alignment compared to C-band. So first, be sure that the satellite in question is transmitting Ku-band signals before starting. For C-band, an adjustment screw can be rotated several times before the signal starts to change, but with Ku-band, that same screw may only take a one-fourth or one-half turn to see a change. If the dish will not be receiving Ku-bands feeds, it’s a good idea to temporally install a Ku-band feed horn and LNB just for alignment.

There are only two possible adjustments to any feed horn: polarity and focal point. The focal point is the distance from the bottom of the dish’s bowl to the lip of the feed horn. It may be supplied by the manufacturer of the dish, or it can be calculated. (See Figure 2.)

Polarity is the alignment of the antenna within the feed horn to the signal sent by the satellite. For steerable dishes, the feed horn’s polarity is rotated, or adjusted by a small servo motor. As the dish moves from satellite to satellite, the polarity must change with each one, because the dish actually rotates in relation to the satellites as it moves through its arc, and this must be compensated for. On fixed dishes, the feed horn is manually rotated to find the correct polarity and peak it. Once you have the focal point set, it’s a good idea to run a felt tip marker around the upper edge so the focal point can be maintained when polarity is adjusted.

Be sure to install the cap on the open end of the feed horn to keep moisture and bugs out. There have been instances of insects making a nest within the feed horn. (See Figure 3.)

When feeds from two adjacent satellites are required, multibeam feed horn assembly can be used. Two separate feed horns can be attached and focused on two satellites that are from 2 degrees to 8 degrees apart. Overall gain is reduced, but with proper planning, this system reduces the number of dishes required.

LNB

LNBs contain a local oscillator (LO) that is used to downconvert the incoming satellite signal to the intermediate frequency (IF). Ku-band uses 10750MHz for its LO, and C-band uses 5150MHz. The stability of the LO is stated on the label in kilohertz and can range from 2kHz to 500kHz. The smaller the number, the more stable the LO and the higher the cost. Today’s newest digital modulation methods now require more stable LNBs (about 100kHz), so the receiver does not have to track the IF from the LNB. It is best to purchase the most stable LNB, because it will be feeding any future digital receivers you install. (See Figure 4.)

If you need to feed more than one receiver, a power splitter will do the job. Because LNBs require about 18V to work, the power has to come from a source. That source is usually the receiver, but with two of them (and only one feeding power to the LNB), the power splitter will only pass DC from one of its two inputs.

But if you shut down that receiver, the signal is lost to the other one. Another way is to use a power injector that uses a separate power supply to feed the LNB. This way the loss of either receiver will not cause the loss of the other one. Also, adding a surge protector is a good idea to prevent lighting strikes from damaging your receivers.

Cables and connectors

Generally, if the distance from the dish to the receiver is about 100ft or less, then RG6 cable will work very well. For longer runs, it best to move to a lower-loss cable such as RG11. If the distance is very long, then you should consider fiber optics. Some converters will take the entire IF and transport it over a fiber-optic cable, and then feed it to your satellite receivers at the other end. This is rather specialized work, and a qualified professional should be consulted about the installation.

When running the cable from the LNB, put a connector on it within a few feet of the dish. You will need to use a barrel to complete the run, but this will allow you to easily test the system without accessing the LNB. With large dishes, it can be difficult to gain access to the LNBs, and this will save time and effort.

Except for the simplest/shortest runs, avoid using satellite ribbon cable. These come with two coax and two control cables configured side by side. They are nearly impossible to pull through conduit, and they will all have to come out if you need to change just one cable.

Water and moisture are some of the most common reason for TVRO systems to fail or loose signal level. This is why watertight F connectors must always be used for outdoor connections. The most common type is compression-fit F connectors. Before screwing them on, a small boot is placed over the threaded female and the connector is attached, ensuring a watertight fit.

Always use the correct tools to attach the connectors, especially the RG11 connectors. A poor fit will cause the connector to come off or add attenuation, at the least. When using RG11, always use a barrel at the end and run the last several feet using RG6, which is more flexible. This will make the installation and servicing much easier.

Lastly, make sure all connectors are tight. Loose-fitting connectors cause many avoidable problems.

Finding the satellite

Today, there are many devices out there that will help you find the satellite you want, and some that just help you peak the signal strength. There are dedicated satellite finders that have the built-in capability of a spectrum analyzer and digital receiver with PSIP decoder. These most often are used by professional installers, and some also receive and decode DTV as well. Then there are the signal strength meters used to just peak the signal at the dish once it’s acquired and verified by the satellite receiver downstairs.

An alternative that is available to most TV stations is the spectrum analyzer at the transmitter. It can easily be used to find satellites and fine-tune the dish. For much less than a traditional analyzer, there are also spectrum analyzers that only cover the IF spectrum of LNBs. Both this and the traditional spectrum analyzer work in the same way. With the analyzer from the transmitter, you will need to supply power to the LNB. A power splitter that supplies DC power from only one of the two ports will work; you can connect that one to the line from the receiver. An inline power inserter will also do the job. Just make sure you don’t feed DC to the analyzer.

Looking at the IF band from 950MHz to 1200 MHz, the satellite signal can be peaked while monitoring the screen. Polarity needs to be adjusted for the deepest valleys, not just the peaks. The stated frequency of a transponder is at the center of the channel, so to find which polarity you are on, look at the center frequency of one channel and compare it to the transponder frequency list for that satellite. (See Figure 5.)

Conclusion

With the right equipment and direction, you can set up a satellite dish yourself and align it to find the signal you are looking for. It may take a few tries, but the satisfaction of understanding how to do it and being able to do so make it well worth the effort.



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