Shielding Satellite Antennas from Terrestrial Interference - 2003-07-19

Shielding Satellite Antennas from Terrestrial Interference (TI) Small, consumer grade satellite antennas are sometimes found to be non-operable due to the presence of local, terrestrial radio interference in the same frequency band as the desired satellite signal. The techniques discussed below are ones that can be put into place by non-technical users without the need for special equipment or materials and without incurring large costs. The particular scenario involves the use of a 1 meter diameter antenna with an elevation look angle of about 10 degrees above the horizon. TI SHIELDING TECHNIQUES 1. ANTENNA SITE SELECTION If the direction of the TI is known, reception of the desired signal can be improved by moving the antenna to a location that puts a building, a wall, a fence or some other obstruction between the TI source and the antenna. Since TI can enter the antenna from a reflection off of a surface near the antenna, just putting a barrier between the antenna and TI source may not always be effective. The best situation is one where the antenna is entirely enclosed except for an opening in the direction of the satellite with an arc at least the size of the antenna beamwidth +/- 5 degrees. For the antenna considered above, this would be about 12 degrees. For a small antenna such an enclosure could be a building, a large pit in the ground (such as an unused swimming pool) or some similar local facility. If there are several large structures providing blockage, it is good practice to put the antenna closest to the tallest structure. The diagram marked as A shows the case where the antenna is installed between two buildings. Note that the antenna to the right may be subject to TI reflected from the tall building on the left. The best way to situate the antenna is near the tall building so that it makes the best use of the shielding properties of this structure. 2. ANTENNA SCREENING If there are no natural barriers available for blocking TI, metallic screens can be employed around the antenna to block the TI signal and its possible reflections. The screens should be constructed from either steel or aluminum mesh. Although copper mesh would be the best material, it is expensive and not always available. The distance between wires should be less than about 0.5 cm and normal window screening with a mesh size of about 0.1 cm would be best. The screens should surround the antenna and should extend in height about 1 meter above the top of the antenna. The screens should not be vertical but should slope away from the antenna as they get higher with a slope of 30 to 45 degrees. The base of the screens should be close to the base of the antenna (about 1 meter away). If a screen is deployed in the direction of the satellite, the angle of elevation from the bottom of the antenna reflector to the top of the screen should be about 5 degrees less than the satellite elevation. For example, if the satellite elevation is 10 degrees, the angle from the bottom of the antenna to the top of the screen should be no more than 5 degrees. In this instance, the distance from the bottom of the screen to the antenna should be about 2 meters. The screens should be electrically connected to a local earth ground. An example of a screened antenna is shown in DIAGRAM B. The screens are simple wooden frames with the wire mesh nailed or stapled to the frames. 3. ANTENNA POSITIONING It can happen that the optimum antenna look angles when there is no TI present may not be optimal in the presence of TI. This is because the TI may be entering on a direction that has a high gain for the antenna. In this case, a slight adjustment of the antenna direction may put the TI at a null in the antenna pattern while only decreasing the desired signal strength by a small amount. Such adjustments may be made in either Azimuth or Elevation. 4. COMBINED TECHNIQUES In most instances the use of multiple techniques may be needed to effectively reduce TI. If, for instance a suitable building is available to site the antenna inside; the application of wire mesh to the inside walls may improve the TI rejection characteristics. Even if a suitable pit is available for the antenna, screens may still provide another measure of protection from TI. This situation is illustrated in DIAGRAM C, where wire mesh is applied to a building that houses the antenna, thereby increasing the shielding provided by the building's walls. It should be noted that the building must have an opening in the direction of the desired satellite that provides an unobstructed path for the wanted signal.