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How to Avoid and-or Suppress the Terrestrial Interference to TVRO.

Terrestrial interference, a menacing condition disrupting clear reception on satellite television system, occurs because of other signals relayed on microwave towers traveling in the same TVRO (television receive-only) C band (3.7 to 4.2 GHz). These signals are comprised of TV programming telephone conversations and business data.

Careful avoidance and suppression techniques are weapons used to attack this condition. Natural barriers, such as trees and buildings, are most effective in screening unwanted signals, so careful attention must be paid to correct positioning of the antenna. In the event natural barriers cannot be used, filters must be built and installed into the system.

In-band microwave interfering carriers are in the TVRO 3.7 to 4.2-GHz receive band, and are allocated at [plus-or-minus]10 MHz off the center transponder frequency. (See Figures 1 and 2.)

Therefore, it is possible to have 25 2nterfering frequencies and 24 potentially damaged transponders. In a usual case, six separate 4-GHz carriers from the nearest tower, spaced 80 MHz apart throughout the TVRO band, will received. The interfering signals will have the same polarity as every fourth channel, hence problems occur.

The picture degradation caused by these transmitters is governed by the signal strength of the terrestrial interference (TI) seen by the satellite antenna. Should the interference be below the transponder in signal level, it will produce light to heavy sparklies over a recognizable picture. (See Figures 3 and 4.) As TI strength equals and/or exceeds the desired signal, the receiver's AFC circuit "locks on" to the TI and detunes the receiver to this signal. Picture "wipe-out" or blank screen is then experienced. (See Figures 5 and 6.)

Out-of-band signals also cause microwave interference. Signals from licensed transmitters outside the 3.7 to 4.2-GHz band, if strong enough, could overload the downconverter and cause picture degradation. (See Figure 7.)

One symptom of overload caused by out-of-band signals is disturbance of every other channel--channels co-polarized with the offending signal. The closer a TVRO channel is to a strong out-of-band, the more likely it will be disturbed. Therefore, if consecutive co-polarized channels near the low end of the band are affected, it may be attributed to an out-of-band signal below 3.m GHz. By the same token, if the higher co-polarized transponders are affected, it may be attributed to an out-of-band signal above 4.2 GHz.

The incidence of 4-GHz interference will increase substantially in the next few years, as more and more 4-GHz facilities are added to the original 6-GHz communications relay network. Since its inception in the late 1950s, the 6-GHz relay system has grown to more than 10,000 routes. (See Figure 8.) Four-GHz facilities have been added to about half of these. (See Figure 9.) As existing 6-GHz capacity is exhausted by many new common carriers, the rate of 4-GHz add-ons will accelerate rapidly.

Professional frequency-coordination forms can provide the origin, strength, frequency and format of terrestrial interference based on a specific location for a given antenna type. (See Figure 10.) This information is required before blocking or filtering TI.

The distinction should be made as to what criteria must be met to provide quality reception. The strength of the TI must be known to decrease the signal. The professional frequency coordinator uses a "rebroadcast" criteria, which states that the interference must be reduced to 25 dB below the transponder for quality pictures. (See Figure 11.) Once the criteria is understood, measures can be taken to cut TI to that level.

Utilizing the data provided by the frequency coordinator, a property map can be examined to see what blockage might be available to absorb or reflect microwave interference before it enters the antenna. (See Figure 12.)

Lower antenna positioning is better. As the antenna is raised, natural shielding is rapidly lost. Trees and evergreen thickets are effective microwave absorbers. Buildings act as both reflectors and absorbers. (See Figure 13.)

Properly constructed artificial screen shields provide up to an 18-dB reduction in interference. (See Figure 14.) A well-designed pit can help reduce interference levels before they enter the antenna.

If natural and artificial blockage have not completely eliminated TI, filtering is the next step.

Out-of-band is the simplest interference to eliminate. As previously mentioned, frequencies outside the TVRO receive band of o.7 to 4.2 GHz compose out-of-band TI. A microwave bandpass filter placed in the system will pass 3.7 to 4.2 GHz and eliminate any interference above and below the band. (See Figure 15.)

In-band terrestial interference occupies the 3.7 to 4.2-GHz band, and is spaced [plus-or-minus]10 MHz off the center transponder frequency. Information such as frequency and strength relative to the transponder is necessary before filters can be constructed for in-band TI. The most-straightforward method is to tap some signal from the low-noise amplifier-downconverter line into a spectrum analyzer.

If such instruments are not available, it may be possible to identify the offending microwave tower from an AT&T operator who has a list of radiated frequencies. Should this approach not work, a professional frequency coordinator should be consulted.

In instances of mild to moderate TI, the picture is still recognizable, meaning the AFC circuit has not detuned the receiver. At this point, IF traps placed after the downconverter will usually cure the interference.

When the receiver is tuned to a trouble channel, the [plus-or-minus]10-MHz microwave offset TI signal will downcovert to [plus-or-minus]10 MHz off the IF center frequency. Therefore, in the case of a 70-MHz final IF frequency, one needs to install traps tuned to reject 60 MHz, 80 MHz or both. Note that one set will cure all troubled transponders, and knowing all specific frequencies is unneeded.

A completely blanked picture means the AFC locked on to the TI and detuned the receiver. Placement of microwave traps in the system becomes necessary. There are microwave "traps" available that suppress the microwave signals before downconversion, and allow the AFC to work normally. These are installed between the LNA and the downconverter. In this case, unlike IF traps, the exact frequencies must be known.

Combinations of defensive installation techniques and filtering will create an avoidance/suppression approach to solving the frequent attacks of terrestial interference and, in the final result provide quality reception to satellite TV viewers.
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Author:LaGrow, D.
Publication:Communications News
Date:Aug 1, 1984
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Next Article:Designing an Effective RFP for Microwave Bypass System.

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