Use of Large Antennas for Direct Broadcast Satellite (DBS)

Periodically in the on-line news groups, someone asks about the benefits of installing a larger antenna, hoping to overcome some of the outages caused by rainfall attenuation during summer downpours.

The design of the DBS system is complex so there is not really a simple yes-no answer. First, the antenna that comes with your DBS system is well optimized for most locations and for use most of the time.

The first consideration is that with a basic system, the receive antenna is looking not at one, but at four satellites together in orbit. The standard receive antenna has a half-power (3 dB) beam width of about 3.5 degrees, permitting a fixed antenna to see all four satellites more or less equally.

As the diameter of an antenna is increased, its beam width becomes narrower, making pointing the antenna more critical. The standard antenna's half-power beamwidth angle is halved with a 3-foot (0.91-meter) diameter antenna while at the same time, the antenna gain is increased by about 6 dB.

With a 10-foot (3-meter) diameter antenna, the half-power beam width is only about 0.55 degrees with an increase in gain of about 16 dB over the standard antenna. With such a large antenna and forward error correction, there will still be outages during periods of heavy rain, but the outages will be a somewhat shorter. Also, accurate pointing or aiming of the dish becomes very difficult with a 10-foot (3-meter) diameter antenna, an important consideration.

A problem with large antennas is caused by our planet's daily wobble about its axis. A geostationary satellite viewed from the earth describes a daily small figure 8 in the sky. Aiming a large dish will need to be done when the satellite is in the middle of the 8. (Professional satellite systems with large-diameter antennas use a steerable sub-reflector to compensate for the earth's wobble).

Wind loading becomes a problem as the antenna size is increased as a satellite antenna acts like a sail. In a 50 mile-per-hour wind (80 Km/hour), the standard DBS antenna wind loading or maximum thrust is about 35 pounds (16 Kg). With a ten-foot, 3-meter diameter antenna, the thrust increases to 850 pounds (386 Kg)! Unless you have mounted your large antenna on a concrete-secured mast with lots of steel bracing, the twist and tilt of the antenna during high winds will cause additional outages because of loss of aiming accuracy.

In summary, for most of us, be happy with the great signals received most of the time from your DBS system. The receiver package that you purchased was well designed and does a good job, far more reliably than many alternative solutions.