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RF/Microwave??

Protect satellite antenna from lightning

Posted: 16 Jan 2007 ?? ?Print Version ?Bookmark and Share

Keywords:lightning damage on satellite antenna? antenna design and selection? communication satellite for service broadcast? Richard Renard? LNB architecture?

Communication satellites operate within two frequency bands for TV/broadband service broadcast signals, C and Ku bands. The C band overall frequency spectrum is 4-8GHz, while the Ku band overall frequency spectrum is 10.7-18.4GHz.

Within these bands, each satellite has a specific uplink and downlink frequency allocation. The C band downlink frequency is 3.7-4.2GHz, while uplink frequency is 5.925-6.425GHz. The Ku band downlink frequency is 10.7-12.75GHz, while uplink frequency is 17.3-17.8GHz.

To use frequencies that are available for satellite broadcast as efficiently as possible and to accommodate additional channels within a given frequency band, the transmission signal can be formatted to be either vertical and horizontal, or circular right-hand and circular left-hand simultaneously per frequency.

Designers of protection circuitry must ensure that this functionality is not compromised in any way.

A low-noise block (LNB) is a module placed on the focus of the satellite dish antenna (parabola). An LNB provides the following functions:

  • Downconversion of the incoming signal from gigahertz range to the 910-2,150MHz (for Europe) range called "first conversion signal"This conversion allows the signal to be carried by an inexpensive coaxial cable towards the receiver;

  • Signal amplification with good noise figureThe LNB improves the first conversion signal level by using a built-in low-noise amplifier;

  • Selection of vertical or horizontal polarization;

  • Selection of operating band by switching its internal oscillator from low band to high band when the LNB "receives" a 22kHz toneSpecifically, the local oscillator (LO) frequency changes from 9.75GHz to 10.6GHz (C Band-LO frequency 9.75GHz, Ku band-LO frequency 10.6GHz);

  • Miscellaneous functions based on 22kHz tone pulse-position modulation (PPM) encoding.

Polarization gives a specific direction to a transmission signal and increases the beam concentration. The signal transmitted by satellite can be polarized in different ways: linear (horizontal or vertical) or circular (right-hand or left-hand). Consequently, the satellite can broadcast both horizontal and vertical or left-hand and right-hand polarized signals on one frequency.

The "universal" LNB switches polarization by looking at the voltage that it receives from the receiver. Generally, only two signals, 13V and 18V, are used with one type of antenna. The 13V signal (11.5-14V range) uses vertical polarization or right-hand circular polarization. The 18V signal (15.5-21V range) uses horizontal polarization or left-hand circular polarization. Also, 1V can be added from a receiver to any of these voltages to compensate for the voltage drop in the coaxial cable.

DiSEqC encoding
Besides selecting the polarization, the LNB must select the operating band. Each reception band is divided into two bands: low (10.7-11.7GHz) and high (11.7-12.75GHz).

Selecting the operating band is done by using a 22kHz tone frequency. A 22kHz PPM signal, amplitude approximately 0.6V, is superimposed on the LNB DC power rail. Its coding scheme allows the remote electronics to perform more complex functions. Traditionally, when other encoding functions do not require the 22kHz tone, simple presence or absence of this tone selects the operating band by changing the local oscillator frequency of the LNB.

The signal transmitted by satellite can be polarized in one of four different ways: linear (horizontal or vertical) or circular (right-hand or left-hand).

The complex encoding of the 22kHz burst is accomplished with a more sophisticated communication bus protocol called Digital Satellite Equipment Control (DiSEqC) standard. The open DiSEqC standard developed by the European Telecommunication Satellite Organization is a well-accepted worldwide standard for communication between satellite receivers and satellite peripheral equipment.

The 22kHz oscillator must be a tone generator with specific rise and fall time. The wave shape will be a quasi-square wave (sine with flat-top). The required frequency tolerance is 2kHz over line and temperature variations (Table 1).

The LNB is remotely powered from the satellite receiver STB. The same coaxial cable that carries an IF signal from the LNB to the receiver carries power from the receiver to the LNB. A dedicated IC, an LNB voltage regulator, generates the 13-18V DC. This device can be damaged by any lightning strike on the coaxial cable or the antenna, which can generate high current, high-voltage surge at the voltage regulator.

This surge can be simulated according to the IEC 61000-4-5 standard: Consider tr/tf = 8/20?s; Vpp = 3-6kV; R = 12; and Ipp = 250-500A.

In case of lightning events, the current surge at the LNB voltage regulator inputs ranges from 250A (when 3kV is applied) to 500A (when 6kV is applied). This IC cannot withstand such high-value energy.

To comply with the IEC regulation and to protect the LNB voltage regulator IC against any damage from lightning events, a dedicated and optimized protection device is required in front of the voltage regulator.

Segmented approach
Look for a solution based on a segmented approach to provide the most suitable protection device relative to the various LNB voltage regulator absolute maximum ratings capabilities. Depending on the LNB voltage regulator used in the application and on the applied lightning surge test level, a different solution may have to be implemented to optimize the total solution's cost and robustness.

The important considerations in the selected solution are the following: 3-, 4- or 6kV protection (8/20?s), axial or SMD package, low Vf, low clamping factor, fast response time, and UL-recognized.

- Richard Renard
Product Marketing Engineer, ASD & IPAD Division
STMicroelectronics




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