Understanding The Offset Dish

The offset dish is one of the most common dish antennas in a fta system. The offset dish is slightly longer on the vertical axis (straight up and down) than it is on the horizontal axis (left and right). The lnb is placed off-center, towards the bottom of an offset dish which allows the antenna to be more efficient.

To find the correct antenna size, an offset dish should always be measured on the shorter horizontal axis instead of the longer vertical axis. As an example, the Geosat 1.2 meter dish measures 1.2 meters on the horizontal axis, but it measures 1.35 meters on the vertical axis. The dish is longer on the vertical axis because the extra length is needed to keep the lnb from seeing the ground instead of the sky. The extra vertical length is not active electrically and it does not receive any signals from the satellite.

A good lnb that is designed for the offset dish will illuminate the dish in a perfect circle. The lnb should be matched with the dish. Two ways to match the lnb are (1) Choose a lnb that is designed for an offset dish and (2) Choose a lnb that has the same f/d as your dish. The f/d is a ratio of focal length to dish diameter. The f/d ratio for the dish or a prospective lnb may be found in the specification sheet for the particular item. Match the f/d numbers of the dish and lnb as close as possible.

An offset dish will not appear to be aimed as high in the sky when compared to a prime focus dish because the offset dish has some degrees of elevation built in. Degrees of offset will vary but they are usually somewhere between 22 and 27 degrees. All prime focus dishes have an offset of 0 degrees.

Always look for a specification or spec sheet when buying an offset dish. Cheap and poorly constructed dishes will not have a spec sheet and this is done for a reason. Cheap dishes that do not have a spec sheet may have irregular surfaces on the dish and this can cause a tremendous loss in received signal strength. The dish may also have been measured to include the dish valley, or its size may be advertised using the longer vertical axis length instead of the shorter and correct horizontal length.

Understand what gain the offset antenna has before you buy it. Gain is a measure of how well the dish antenna performs at a given frequency. Gain is measured in decibels or db for short. The higher the frequency, the more gain a particular dish antenna will have. Most ku band dish antennas rate their gain at 12.5 GHz. We normally use 11.7 to 12.2 GHz so we will actually have less gain than what is available at 12.5 GHz.

Antenna gain is expressed in db, db(d), or db(i). If antenna gain is expressed solely as db then you have no idea what it is being referenced to and its number is useless. Gain expressed as db(d) means that it is being compared to a half wave dipole antenna. This is the expression I like the best because I know what a half wave dipole is. Gain may also be expressed as db(i) which means that the gain is being compared to a theoretical isotropic radiator. Dish antenna gain is commonly expressed as db(i) because it will have 2.15 db of gain over a dipole. As an example, a dish antenna may be said to have a gain of 43db(d) at 12.5GHz OR 45.15db(i) at 12.5GHz. If a dish antenna is rated only in db, try to find out which reference antenna (half wave dipole or isotropic radiator) was used to determine the gain in db.

El Bandido 3-28-2011

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