Satellite TVRO Part 3

The past two tutorials have covered satellites and how signals are processed and transmitted. This tutorial will cover how to receive them. It used to be as simple as moving a satellite dish around and watching an analog receiver for a picture, but not anymore. With digital satellite transmissions, it takes a bit more to find and identify a satellite.

Back in the 1980s, the way to find a satellite with your 5m satellite dish was to move its azimuth over a degree or two and them move elevation up and down while looking for any sign of a sync bar from the analog receiver. This would go on until you found a signal, and then you fine-tuned the azimuth, elevation and polarity and hoped it was the satellite you wanted. These days, it’s a little different.

There are very few analog signals to look for, which means you need specialized equipment to monitor the satellite signals and to identify them. This tutorial will cover today’s satellite equipment and how to use them to locate the desired satellite.

Antenna basics

There are three types of satellite dishes in use today: Cassegrain, parabolic and offset. (See Figure 1.) Each has advantages for its particular application. It’s common to refer to the dish itself as the “antenna,” but it is only the reflector. The actual antenna is contained within the feed horn, where the RF is received and turned into electrical signals.

Satellite dishes range in size from 18in to 70m. The most common for broadcast use is a parabolic dish in the 2m to 4m range, which is measured across the dish’s width. Cassegrain dishes are more commonly found in very large dishes, 5m and larger, and the LNBs are located within the hub of the dish where it’s more easily accessible. Small offset dishes are used for direct-to-home satellite reception and can be seen in most neighborhoods. But larger offset dishes of up to 6ft work very well for Ku-band reception.

Figure 2. Satellite dish configurations

Satellite dishes come in one of three basic configurations: solid, mesh and petal. A solid dish is just that — solid, made of a single piece of metal — which makes for higher delivery costs but will assure a longer trouble-free lifetime. (See Figure 2.) Mesh dishes are very common in backyards for “free-to-air” satellite programming. The largest is usually 10ft to 12ft. Mesh dishes suffer from a shorter lifetime, but this can be offset by their lower cost and less wind resistance. The petal configuration is easier to ship because it comes in pieces, each shaped like a triangle or petal. These are almost always made out of fiberglass with a metal structure to support the pieces and attach them to the mounting post. While they do work very well, petal dishes do not age well, and if you need to move them, there is the definite risk of unintentional damage. The only variable for dishes is their gain, which is directly related to their size. The bigger the dish, the more gain it has, so it can pickup weaker signals with less noise.

There is one more type of dish: the very wide-angle dish that can see the entire arc of satellites, or over 100 degrees, all at once. These are massive structures that weigh several tons and require very precise installation. Their advantage is the ability to see all satellites at once; to receive a new satellite, only a low-noise block converter (LNB) must be installed at the correct location. The downside is their cost and amount of space they require, but it’s the only dish you ever need to install.

The components of a satellite dish include the mounting pole, the mounting frame, the dish itself, the feed horn and the LNBs. The mounting pole is sunk into the ground and cemented in place, attached to a stand that sits on the ground or, in the case of a roof, a “nonpenetrating” mount is used. These last two must use weights to hold the mount, and the pole, in place. How much weight depends on the size of the dish, the type of dish and where it is being mounted. Without enough weight, the dish can move in high winds, throwing off the satellite dish’s aim. The pole must be plum for a steerable dish to be able to track the satellite arc correctly; fixed dish imitations only require the pole to be close to plum because the adjustments will compensate for errors. The mounting frame is either Az-El or polar. Az-El stands for azimuth-elevation, where each parameter can be adjusted independently, most common on fixed dish mounts. A polar mount allows the dish to track the arc of the satellites in the Clarke Belt with a single movement from east to west. The polar mount causes the dish to change elevation during the east-west movement. This is almost exclusively used on motorized, steerable dishes. The feed horn is the antenna of the satellite dish. Its position above the center of the dish sets the focus point for the reflected satellite signals from the dish. This focal length adjustment is a critical part of setting up a dish. It is also where the polarity of the received signal is selected. Feed horns also come in a number of configurations. The LNBs attach to the feed horn to receive the satellite signal and downconvert it to a frequency band that the receiver will accept. For broadcasters, LNBs are either for C-band or Ku-band signals. Some feed horns will accept one C and one Ku on the same polarity or two C or two Ku each on a separate polarity, so all transponders on a satellite can be received at the same time. (See Figure 3.)

Aligning a fixed dish

The three parameters for satellite dish positioning are elevation, azimuth and polarity. Elevation is the angle of the dish above the horizon; Azimuth is the angle the dish is facing on the compass scale; and Polarity is the angle of the receive antenna within the feed horn in relation to the polarity of the signal sent by the desired satellite. Properly adjusting all three of these parameters will allow you to pick up the satellite you want. (See Figure 4.)

If your satellite dish was located on the equator, the dish would be pointed straight up at a 90-degree angle, and then swung from east to west and all the satellites would be picked up — this is not the case for most of us. That straight line (east to west) at the equator is the line of geostationary satellites; as our dish moves north of the equator, that line becomes elliptical. The further north we move (higher latitudes), the more pronounced the elliptical curve of the geostationary satellites becomes. So we not only move the satellite dish from east to west, but also raise and lower its elevation to be able to track all the satellites in the Clarke Belt.

For fixed or stationary satellite dishes, you simply raise it to a fixed elevation above the horizon and aim it at a particular point on the compass, and the satellite you are looking for should be there (or close to it). There are many Web sites that allow you to enter your satellite dish’s location and which satellite you are looking for, and it will provide you with the elevation and azimuth readings to align your dish.

To find your dish’s elevation, it’s easiest to use an inclinometer or digital level, which measures the tilt or incline of an object. There are mechanical inclinometers with a large dial and the electronic variety that look like a carpenter’s level and have a digital readout — either one should work fine. The hardest part of setting a dish’s elevation is to find a flat space on the back of the dish where you can place the inclinometer that is perpendicular to the dish’s line of sight to the satellite. The back of all dishes is rounded, so that makes it harder to use; a part of the mounting frame can be used if it is parallel to the dish. The most assured way to measure elevation on a parabolic or Cassegrain dish is to place a straight board across the face of the dish, make sure it’s resting on the edges and then measure the elevation on the board.

Offset dishes pose their own set of problems in setting elevation, because the line of sight to the satellite is not perpendicular to the face of the dish. To align an offset dish, you must contact the manufacturer for the correct placement of the inclinometer.

Next time

The next “Transition to Digital” tutorial will cover steerable dishes and their associated problems.

Continue reading part four of the Satellite TVRO series.