TELSAT® COMMUNICATIONS LTD

It is most important to maintain a good match between the feed and the LNB in order to ensure that the maximum amount of signal is transferred to the LNB. The chart below shows approximate effects of VSWR on measured noise figures or temperatures of an LNB. An LNB with a measured C-band noise figure (NF) of 30 K is used as an example.
Two new LNB products are starting to appear on our markets, the first is a "Twin" LNB, this product can feed two separate receivers from two independent outputs. The twin LNB is ideal for households that own two receivers and wish to view two different program from the same satellite at the same time. The second product is a "dual" LNB, this product provides both horizontal and vertical outputs simultaneously through two separate F type connectors. Typically used in multi-receiver applications for IF distribution or channelised SMATV. There are times when LNB's work better - this is usually due to the temperature of the sky - "here seems to be a considerable improvement in signal reception during the colder weather", it is that because of the amount of water in the atmosphere during colder weather, the cool air holds less water than the warmer summer air - the warm water has a higher NOISE temperature than cool air.

FERRO MAGNETIC POLARISERS:

Magnetic Polarisers sometimes referred to as De-polarizers are popular in the European and British Ku DTH markets. These add-on units which are used to rotate " or skew" the incoming signal by using a magnetic polariser sometimes referred to as a "ferotor" - the Faraday rotation principal which twists the E field plan to that of the LNBF probe. By applying a small varying DC current of 50mA or less to a specially wound coil the signals orientation that passes through the device can be rotated in a controllable manner by changing the DC polarity and the current flow through the coil. Once set this current must be kept constant as the polarization sense is critically determined by the accuracy of the current flowing though the polariser winding. There are no moving parts and the device is inserted between the LNB and the feed horn at a 45 degrees to the incoming signal and the "skewing" is set between -45 and +45 degrees. This ensures that the required current is kept to a minimum and unwanted noise is also minimized. When using a polariser with a budget receiver be aware that generally it is not possible to memorize (store) any settings that were made to peak the signal - however the up market models do generally offer a memorized polariser feature on each stored channel. It is desirable that each selected channel can store the relevant trim because the amount of wave twisting that is needed for each polarization sense to maximise the signal is not only governed by the current through the polariser but to a lessor extent by the channel frequency being viewed A loss of .3db at Ku frequencies can be expected and a typical cross polar rejection of better than 20db is possible.. There are some combined offset feed/magnetic Polarisers coming onto some overseas markets where DTH services are available generally manufactured by "specialist" TVRO Company's. Ferro magnetic polarizers do not have any moving parts and can be expected to give a long and trouble free service.

FEED HORN:

The feed horn is a precision wideband antenna mounted at the dish focal point that collects the reflected signal from the dish surface - the truer the surface of the reflective surface the more focused the signal will be at the feedhorn. All C Band TV transmissions from Intelsat & Rimsat Satellites are circular polarised (a spiral pattern) either Right Hand (RHCP) or Left Hand (LHCP). To modify a standard Chaparral Super Feed II Linear feed horn for RHCP or LHCP a slab of Teflon is installed inside the feed horn throat at 45 degrees in-relation to the LNB pickup probe. TV transmissions from PAS-2, AsiaSat2 and JSAT3 are "linear" either Vertical or Horizontal - this enables the reuse of transponder frequencies. The most popular type feedhorn (ADL or Chaparral) available today is the scalar feedhorn - it is recognized by the large circular plate with a series of three or four concentric rings which assists the incoming microwave energy to be directed to the feedhorn waveguide and hence to the LNB pickup probe. The scalar rings conduct the incoming signal from the outer edges of the focal cloud to the large waveguide opening located at feed center. The scalar feedhorn primarily sees or illuminates the inner 70 percent of the antenna's surface area, while attenuating up to 15 dB the signal contribution from the outer lip of the dish. Molecular motion within the Earth itself generates random noise which covers the entire electromagnetic spectrum used for the transmission of satellite signals. This random noise is many times stronger than the satellite signals reaching any location. The 15 dB of attenuation or illumination taper provided by the feed sharply reduces the reception of the Earth noise which lies just beyond the antennas rim. The outer 30 percent of the antenna's surface area therefore acts more as an Earth shield for the feedhorn than as a contributor to the overall signal gain of the receiving antenna. The distance between the center of the antenna surface and the feedhorn's waveguide opening is called the focal length or (f). The focal length between antenna surface and waveguide should be initially set to the distance recommended by the antenna manufacturer. Adjustments of 1/8th inch or more in or out from the recommended distance should be made while using a signal meter or spectrum analyzer to determine the precise position required for maximum signal acquisition. This is particularly important for antennas composed of individual segments, especially those composed of mesh panels as antenna surface irregularities due to careless antenna assembly can actually shift the optimum position of the focal cloud from the value recommended by the antenna manufacturer. When adjusting the feedhorn in or out, be sure that the waveguide opening remains precisely centered over the dish at all times. You can check this by measuring from the antenna's rim to the outer ring of the waveguide opening from four equidistant positions around the rim. All of these measurements should be equal. In cases where the feedhorn is weighed down by two or more electronic amplifiers, guy wires may need to be used to ensure that the waveguide is precisely centered.

Seen in mostly commercial installation is the Orthomode or the Dual-polarity C Band feed horns. These feed horns feature two waveguide flanges set 90 degrees apart so that two LNB's can be mounted to pick-up both vertical and horizontal services simultaneously.

A Polarotor motor works in the same way as a model radio controlled vehicle - it works just like the servo in an RC model. The position is determined by the width of the pulse which is sent by the TVRO receiver, is repeated several times to allow time for the Polarotor to react. The pulse width is variable in width in the range of .8 - 2.2ms. The pulse is generated by a Pulse Width Modulator. 1.2 ms and 1.8 ms is the normal difference for a 90 deg turn

FEEDHORN TROUBLESHOOTING:

Proper installation and adjustment of the feedhorn is critical to system performance. It is particularly important if you are installing a feedhorn that receives Ku-band signals. In order to find the correct focal distance for tracking, the feedhorn must be at the correct distance from the center of the dish, properly oriented, centered and perpendicular to the plane of the antenna. Follow the alignment procedure outlined below: 1. Set the scalar ring adjustment for the f/D ratio that is called for in the antenna specifications. If you do not know the f/D ratio, you can calculate the focal distance and f/D ratio yourself using a formula. 2. Rotate the feed to it's proper orientation using the polar axis template. The polar axis is a line that runs through the center of the dish pivot points. It is the axis around which the dish will rotate. Another way to look at it is ......If your dish is positioned so that it is pointing at it's highest point of travel (the zenith of the arc).......when you stand directly in front of the dish, the polar axis runs from 12 o'clock to 6 o'clock. Proper orientation in these terms means that you point the arrow of the polar axis template (supplied with all Chaparral feed horns) at 12 o'clock (directly in line with the axis). If you do not have a template, you can get close by sitting down the long side of the servo motor pointing it at about 11 o'clock. 3. Centering the feed in the dish is also critical to proper reception. This can be done by measuring from the feedhorn to at least 3 different points around the rim of the dish (i.e. measure from the feed to the left side, right side and bottom). The 3 measurements should be equal. Use the adjustments in the feed support legs (or guy wires if you have a buttonhook support) to make any necessary adjustments. 4. The opening in the feedhorn (face) should be parallel to the face of the antenna (dish). The easiest way to check this is to use an inclinometer or universal protractor. Check the angle at the center of the dish and across the throat of the feed horn the measurements should be the same. The f/D ratio and scalar rings - why it is important to set properly? Proper setting of f/D on the feedhorn allows the feedhorn to take advantage of all of the signal being reflected off of the dish, without receiving interfering ground noise or terrestrial interference. The f/D ratio is the focal distance of the dish (f), divided by the diameter (D). When dealing with most prime focus antennas, the number should come out between .28 and .42. If you notice, most of those numbers are also on scale on the side of your CHAPARRAL feedhorn. You simply set the top edge of the scalar ring even with the line that corresponds to your correct f/D setting. What this adjustment actually does is determines how wide of an angle the feedhorn can illuminate. If the dish is very deep (example: 10ft diameter dish that is 24 in. deep), having an f/D of .28 for example, then the focal distance is relatively short. When that is the case, the focal distance is often only a few inches greater than the depth of the dish. Therefore, the feed needs to be able to see nearly straight to the side of the opening in the throat. Conversely, if the dish is very shallow (example: 10ft diameter dish that is 11 in. deep), the f/D ratio would be closer to .42 and the focal distance would be much longer. In that case, the feed would need to have an narrower field of view so it would "see" the whole dish, yet not see past the edge of the dish.

Formulas for calculating focal distance and f/D ratios To calculate the focal distance, you have to measure the diameter (D) and the depth (d) of the dish. Measurements should be in like units (you use feet for the diameter and inches for depth).

For the example, we will say we have a dish that is 120 inches in diameter and 18 inches deep. focal distance (f) equals the diameter squared divided by 16 times the depth or : focal distance (f) equals the diameter squared divided by 16 times the depth or : D x D After you have calculated the focal distance (f), you can use that figure to calculate the f/D ratio of your dish. In this case, using the same diameter (D) = 120; and the calculated focal distance (f) = 50 f /D = .416 which you would round up to give you a setting of .42 After you have calculated the focal distance (f), you can use that figure to calculate the f/D ratio of your dish. In this case, using the same diameter (D) = 120; and the calculated focal distance (f) = 50

TELSAT COMMUNICATIONS LTD can supply MS DOS software (at no charge) that makes these f/d calculations very quick and simple, just forward a formatted 3.5" floppy disk and a self addressed return envelope.

FEEDHORN TROUBLESHOOTING TIPS:

How to recognize a polarity problem. Polarity problems are usually very easy to recognize. They are usually indicated by the fact that every other channel is bad. You will notice that on some satellites, only the even numbered channels will come in, while on other satellites only the odd numbered channels will come in. This happens because the probe inside the feedhorn will not turn the 90 degrees that is required to change from a horizontally polarized channel to a vertically polarized channel. If your satellite system is several years old, the problem is most likely that the servo motor that drives the probe has failed. Here are some steps to take to find the problem. Use a volt meter to check the voltage at the back of your receiver to make sure that the voltage is coming out of your receiver. The connector to check is usually labeled Polarizer +5v or Polarity +5v. Disconnect the wires that go to the dish and measure the +5 connector to GND. You should have approximately +5 to +6.5 volts dc.

Chaparral receivers put out a constant +5 supply, so the voltage should be there as long as the receiver is turned ON. Other brands of receiver may only put out the +5 when the channel is being changed or when the polarity/skew is being adjusted. Check for dc voltage at the pulse connector. The pulse output is what tells the servo motor how far to turn the probe. You will read from .2 to .9 (+)volts dc here. In most receivers, this voltage will only be present when the channel is being changed or when the polarity/skew is being adjusted. If the receiver is putting out the proper voltages on the pulse and +5v connectors, re-connect the wires that go to the dish. Then, go out to the dish and remove the feedhorn cover. Disconnect the 3 wires that are connected to the servo motor. Measure to verify that you are getting the pulse and +5 voltage on each respective wire. If you are NOT getting the same voltage as you had at the receiver, then you have a wiring problem. If you are getting the same voltage, reconnect the 3 wires, proceed to step 4. Have someone inside change channels on the satellite receiver. If you hear the servo motor turning, but there is no apparent change in the position of the probe (remove the throat cover and look inside the throat to see the probe), remove the servo motor and pull up gently on the amber colored drive shaft that couples to the servo motor. If the shaft pulls out, you will need to purchase a replacement Chaparral Servo Motor. If the servo motor does not turn, and you have the correct voltages getting to the motor, that normally indicates that the motor is bad and needs to be replaced. You can purchase a replacement servo motor at TELSAT COMMUNICATIONS LTD, your satellite dealer. If you find that the servo motor seems to be buzzing all of the time or if you are watching a program that seems to fade out intermittently and will come back by itself or if you change the channel up or down and back, he problem is also likely to be a bad servo motor. But try these steps to determine if the problem is more serious: Take the servo motor off of the feedhorn and hook it up directly to the back of your receiver. You must disconnect the wires going to the dish for this test to be valid. Watch the servo while you change channels, then let it sit for a couple of minutes. If it turns when you change channels and does not drift or buzz when you are not changing channels, that tells you that the receiver and servo motor are working properly and the problem is likely to be noise being pick up by your unshielded pulse line. The only way to correct this problem is to make sure that the pulse line is shielded and the shield is grounded at one end. If the servo motor behaves the same way when it is hooked up directly behind the receiver as it did out at the dish, then it is most likely bad. You need to replace it.

We hope this information was helpful. If you can't solve your polarity problem after following the instructions and tips above, we recommend calling TELSAT COMMUNICATIONS LTD, your local satellite dealer to troubleshoot the system further.

Our thanks to Chaparral Communications for this Technical Document Feedhorn Troubleshooting Tips

ANTENNA:

The antenna, usually shaped in a parabolic curve, prime focus dish is a precisely curved metallic surface so that the incoming signal is reflected from the surface and focused at one point which is known as the focal point, at this point we position the feed horn who's function is to feed this very weak signal onto the Low Noise Amplifier (LNB) for further amplification. There are many factors that influence the f/D ratio of a TVRO dish, in practice it is usually .36 to .375. The minimum antenna gain required for good "C Band" TVRO pictures is at least 42dbi (db antenna gain relative to an isotropic source) which is obtained by using a 12ft mesh dish with good parabolic accuracy - within 2mm over the entire surface area. Commercial mesh dish has a efficiency of 65%. The ability of an antenna to form a tight focal point is related in part to its gain, this will influence the quality of picture you receive - higher the antenna gain the better the picture/sound. Satellite TV antennas are constructed from many materials, including fiberglass, metal (steel & aluminum) or wood or a combination of materials.

Which Dish type is best? Solid dishes are usually perform much better. The main reason is efficiency which is a measurement of the signal amplitude that actually reflects to the feedhorn. While the actual differences can be kept to a minimum, it is obvious that a solid surface should reflect better than a surface full of holes. If the hole size is kept to a small fraction of the wavelength that is being reflected the holes cause only a moderate reduction in efficiency, but it can be measured. An even greater factor in the performance of a dish is the accuracy of the parabolic geometry. If this is wrong the reflected signals will not focus properly. This poor focus can cause reduced signal recovery and poor sidelobe performance.

With today's powerful satellites some signal reduction can be tolerated. But when those satellites are in two degree orbital slots the control of the sidelobe characteristics is essential. It is probably also true that a stamped or spun single piece dish is inherently more accurate than one that is welded together from many smaller pieces. However because KU is nearly three times the frequency of C band it is much more critical of the overall dish integrity. BUT a solid dish requires a stronger more expensive mounting (concrete & steel work) and the purchase price is much more than a equivalent mesh/fiber dish. All have one thing in common: a curved metallic surface for reflecting and focusing the signal to the feed horn. Fiberglass antennas sandwich metal foil or screen inside the fiberglass or paint the reflecting surface with special reflecting paint. Steel and aluminum can be used as a reflective surface in either solid form or as open mesh screening. The framework can be made of steel, aluminum, wood or fiberglass just as long as a well defined curved metallic surface is maintained. Open-mesh antenna are generally 4 times cheaper than all other types of dish antennas and are generally the choice of most users. However, the screen surface is more easily damaged by hail, vandalism or even strong scrorly winds. Open-mesh antennas can be manufactured much lighter without affecting overall stability. The strength and rigidity of the outer circumference is very important because this is where much of the signal is reflected from. Fiberglass is affected over time by solar radiation, which is why fiberglass antennas are always white or other light colours, any dark colour would only hasten the deterioration.

OFFSET ANTENNA and FEEDS:

Many of the small aperture Ku band dishes 30 to 80cm s on the market use an offset antenna feedhorn design which places the focal point below the front and centre of the dish. This type of antenna, which is actually a small oval subsection from a much larger parabolic antenna design, is oval in shape. Because of its unique geometry, the offset fed antenna requires a specially designed feedhorn which matches the antenna geometry precisely. The efficiency of an offset antenna is high due in part to being a "solid" dish construction and that the LNBF is not shielding the incoming weak signal because of its mounting position.

TRACKING:

Azimuth and Elevation are the two basic coordinates required to aim the dish to satellites parked on the "Clarke Belt" once you have the unique pair of coordinates for a given satellite you can use a compass to accurately align your dish, a compass will give you magnetic bearings, to correct to a "true " heading you will need to subtract the magnetic deviation for your area, if this is not known call your airport control tower for the correct offset. Elevation setting is best done by placing a "inclinometer" on the back plate of your dish i.e. the same plane as the outer rim of your dish, set the elevation to the figure supplied by the dish supplier or a figure that is equal to the polar axis angle plus the declination offset for your area. If an inclinometer is not available you can use a protractor and plumb bulb. Declination is the offset angle between the polar axis and the dish which enables the dish to precisely track the Clarke Belt. The declination angle at any site is defined by the latitude at that location. The declination setting is usually given by the supplier - in NZ the declination is between 5 to 7 degrees depending on the latitude of the site. The elevation angle is the sum of the polar axis angle and the declination offset angle. A "Solar Outage" may give you the effect of a dish out of correct alignment, a Solar outage is the result of the loss of reception that occurs when the sun is positioned directly behind a target satellite which results in large amounts of solar noise overriding the satellite transponder - this effect occurs twice a year. There are computer programs available that give these dates and times.

DISH MOUNT & ALIGNMENT:

Your dish pole mount is usually a length of 3.5" or 4" OD of thick wall Galvanised steam pipe set in concrete, the size of pad depends on the dish diameter. Concrete is also poured inside the pole mount pipe for added strength and less whipping, this is especially important if Ku band reception is anticipated. Additional strength can be obtained by affixing two support members from just below the polar mount at an angle of 45 degrees behind the dish mount pole. Before the concrete is poured it is vitally important that the dish mount pole is plumb i.e. vertical in all planes. It is not recommended that large dish's not be mounted on roof's unless a registered engineer has certified such a installation.

TVRO RECEIVER:

The satellite receiver takes the amplified satellite IF signal with is between 60dbm & 30dbm from the feed horn/LNB, then delivers a Base band (base band signal is the un-clamped and un-filtered output of the TVRO RX demodulator containing video information as well as all the transmitted subcarriers) or composite Video signal and a audio signal suitable for viewing on your television set or multi-standard receiver or monitor. The Satellite TV receiver is basically the "second IF" at 479.5mHz or 70mHz as in older receivers and use a SAW filter/s to achieve a suitable selectivity usually 36,24,18 or 12mHz bandwidth depending on how the transponder is configured for full or half transponder operation.

Most TVRO receivers are microprocessor controlled and use digital PLL (phase lock loop) for both input and sound subcarrier tuning. The quality of a Satellite TV receiver lies in its ability to produce a good pictures from very weak signals, i.e. as little as -112 dbwm2 PFD (Power Flux Density) is available which translates to 6 picowatts of signal power falling on every square metre of surface area and not so much in all the extra gadgetry. A domestic TV receiver as used in most households is not suitable to directly receive satellite TV transmissions because satellite signals are very weak (as much as 165bdm2 is lost between the satellite and your TVRO site - Warkworth use a 30 meter (100ft+) dish to obtain TV pictures suitable for rebroadcast where the TV enthusiast is generally using a 12ft dish.

Present day TV receivers are suitable for receiving programs on VHF and UHF frequencies, the satellite transmits in the SHF band of frequencies (4gHz & 12gHz). The major difference is that our domestic TV receivers use AM picture modulation and FM sound modulation where-is Satellite TV uses wide band FM sound deviation of typically +/- 150kHz & a vision deviation of 40mHz. The frequency the audio subcarrier is transmitted varies between 6mHz & 8mHz and use different de-emphasis and pre-emphasis characteristics from the normal domestic 50us standard. A further difference is that the transmitted signal from the satellite is "dithered/wobbled" up and down in frequency over a range of 1mHz at a rate of 25Hz - this concept is referred to as "energy dispersal".

Because we are intercepting international TV signals we need to consider TV Standards". There are many different video standards in the world today including NTSC (National Television Standards Committee) which is used by USA, Canada, Japan el at, PAL (Phase Alternate Line) is used by European countries, Australia, England, NZ et at, SECAM (Sequence Couleur a Memoire) is used by France, USSR el at. This lack of international TV standardization can make life for the satellite TV viewer difficult - to overcome this dilemma by purchasing a multi-standard TV monitor/receiver for little more cost than a domestic TV receiver.

DIGITAL BROADCASTING:

As more and more information is being handled in digital format, the future for satellite is also digital. In the very near future, all TV, Video, Video on Demand and Audio transmissions will take place in digital format and this offers some advantages. The prime reason for digital broadcasting is that with analog broadcasting only one channel per transponder can be transmitted, whereas with digital broadcasting this can be 10 channels per transponder. This means a substantial cost reduction per channel. Due to compression techniques, more information can be put on the same channel bandwidth currently being used, which allows more flexibility. For instance, the sender can opt for higher resolution, or for a lower resolution but more channels. In general, digital broadcasting will bring an increase of choices to consumers. Besides a likely increase of the number of programs, the same programs will also be broadcast several times per hour or day, to give the consumer more flexibility in when to watch a program. Also, channels will become increasingly focused on specific subjects, such as documentaries, movies, sports, and perhaps even more specific than that for example only football or nature documentaries.

LINK BUDGETS - Analog:

Carrier to Noise (C/N), Signal to Noise (S/N) and Receiver Threshold The carrier to noise ratio, (C/N) is the waveform term relevant before demodulation in the receiver. Signal to noise ratio(S/N) is that relevant to the waveform after demodulation. The S/N ratio is thus dependent on both the carrier to noise ratio and the modulation characteristics. A below option c/n will show up as "sparklies" which are small black & white dashes in the TV picture which is and indication of an insufficient signal. To overcome this a larger dish is usually the only way to overcome this effect. Another important link parameter is the receiver's "demodulator threshold" figure. At present this is typically 7 dB but is expected to be a little lower as "threshold extension" techniques develop. Threshold is the point where the linear relationship between demodulator C/N input and S/N output begin to break down to major component areas.

To calculate a Carrier to Noise (c/n) ratio the following steps are required: Calculate the carrier power Calculate the noise power Subtract the noise power from the carrier power to arrive at the carrier to noise ratio (c/n).

To determine the carrier power: Satellite power transmitted i.e. EIRP LESS The Distance from the Rx to satellite i.e. the attenuation called the spreading. The Amount of rainfall PLUS the Size of the Rx dish dbi gain.

To determine the noise power: Waveguide losses + LNB noise temperature + Rx IF bandwidth + Earth noise.

It follows, that for continuous good reception, two criteria must be satisfied for the specified signal availability. Another important link parameter is the receiver's demodulator "threshold" figure. At present this is typically 7 dB but is expected to be a little lower as "threshold extension" techniques develop. Threshold is the point where the linear relationship between demodulator C/N input and S/N output begin to break down. The antenna system should provide a degraded sky C/N value exceeding the receiver demodulator threshold figure. The degraded sky S/N, for a given signal availability, should exceed 42dB for Grade 4 and 47 dB for Grade 5 reception.

The object of a Link Budget Calculation is to determine the suitability of satellite TV receiving equipment for a desired purpose and arrive at a suitably sized antenna and LNB (low noise block) combination. The chain calculation is tedious, rather than difficult, and often prone to human error when performed by hand. The calculation may be performed for each satellite transponder to ensure good reception for all required channels. It is particularly important where a system is put together from a variety of manufacturer's component parts since, at one extreme, poor results may be experienced and at the other, "over engineering" may unnecessarily add to equipment cost and may look less esthetically pleasing.

The final result of a link budget calculation is a signal to noise ratio (S/N) value which can be compared with the CCIR 5-point scale of impairment. These grades, obtained from a series of subjective tests, are conveniently used as the criteria for overall system performance.

Grade 4 or above is normally acceptable and translates to a weighted S/N value greater than 42.3dB. Short form link budgets tend not to quantify data such as atmospheric absorption, precipitation effects, and mechanical losses and simply allow a fixed C/N margin between a calculated clear sky value and demodulator threshold to allow for all these effects. If the margin for a particular earth-space path is not known there is a danger of lapsing into guesswork! Atmospheric absorption by water vapor and oxygen is basically a clear sky effect (happens whether raining or not) and depends mainly on the absolute humidity or vapor density measured in grams per cubic metre.

However, this is a relatively minor contributor below about 7.5GHz. Another clear sky effect is the loss due to tropospheric scintillations. Turbulence caused by wind in the atmosphere cause short duration fluctuations in the refractive index. These translate to small amplitude fluctuations in the received signal which can be significant particularly at low elevations.

Most LINK Budget programs calculates the statistical contribution of these scintillations and allows for the loss in the link budget The effects of precipitation become significant above about 8gHz. Rain, or to a lesser extent snow, fog, or cloud will attenuate and scatter microwave signals. The magnitude depending more on the size of the water droplets (in cubic wavelengths) rather than the precipitation rate itself. Heavier rain tends to comprise larger droplets so the two are normally related. In addition, rain has a noise temperature similar to that of the earth (290K average) which increases the sky noise temperature over the clear sky value. The DND figure is the total degradation of the signal due to precipitation expressed in dB and, for a given signal availability, consists of the sum of the attenuation due to precipitation and the system noise increase translated to an equivalent dB loss. There is also a small contribution due to the increase in atmospheric gaseous absorption during rain. DND is the major component of the link margin set aside for Ku band. Signal availability is normally taken as 99.5% of an average year & for domestic systems, and 99.9% or greater for SMATV or cable head systems.

We recommend the SWIFT supplied range of Satmaster Satellite Link Budget Software which is suitable for both digital and analog system design applications.

LINK BUDGETS - Digital:

Television picture signals are sampled at least twice the highest frequency present and converted to a digital steam of bits known as the information source. The output of the information source is input to the source encoder whose function is to reduce the average number of data bits per second which must be transmitted to the user over the channel. Source coding, basically another subject area, involves the study of data compression techniques such as that used in the MPEG-2 standard. An information bearing signal transmitted may not be correctly interpreted at the receiver due to distortion of the signal over the noisy channel so the output of the information source is fed to the channel encoder where redundancy (extra bits inserted) is introduced to reduce the bit error probability over the channel. This practice is known as Forward Error Correction (FEC) and is the only known method of providing error correction without calling for retransmission. In satellite communications in general, digital carriers in the great majority of cases use either QPSK or BPSK modulation with F.E.C. QPSK with coherent demodulation in conjunction with Rate ½ or Rate ¾ F.E.C is common, as is coherent demodulated BPSK with a Rate ½ code.

Bit Error Rate (BER) This is a measure of how well a receiver/decoder is working, the higher the BER the better the IRD will perform under marginal reception conditions - i.e. rain storms etc. The BER is measured in "exponential notion" i.e. a high BER of 5.0 x 11-5 is better than to 9.0 x 10-3.

Bit Rate (BR) This is a measure of how much data is transmitted within one second, to give some idea of what are typical number are, a 36mHz transponder could handle 50mBits, a video service could be for "broadcast" 8mBits while a VHS quality service typically 1.125mBits data stream.

The DVB Project Office has so far published the following standards as European Telecommunication Standards Institute ETSI standards:

DVB-S The 38 Mbit/s modem standard for satellite TV

DVB-C The 38 Mbit/s modem standard for cable TV

DVB-CS (S)MATV system for community antenna installations

DVB-SI Service Information -- Descriptive data that comes with every DVB channel, including Electronic Program Guide data

DVB-TXT Teletext for the new digital TV era

The DVB project selected the MPEG II Main Profile at Main Level (MP@ML) with a maximum data rate of 15 Mbit/s.

Main Level means that up to 720 x 567 pixels at 25 Hz (Australia, NZ & Europe) or up to 720 x 480 pixels at 30 Hz (USA). 4:3, 16:9, or 20:9 aspect ratios can be supported.

The bit rate utilized for a video program can be selected freely depending on the quality requirements.

2 Mbit/s: approx. VHS quality, suitable for programes with very little picture movement - talking heads etc

4-6 Mbit/s: approx. studio quality (e.g., for news broadcasts, community & talk shows)

8 Mbit/s: better than D2, SP and Digital Betacam, comparable to studio production quality - for sports,dramas, documentries etc

Have a look at our frequency listing where you will see the different digital services symbol and FEC parameters. Digital Modulation also known as Phase Modulation is very similar to FM in many respects. The most suitable digital modulation methods for digital TV via satellite is BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), 8-PSK, and possibly 16 QAM (Quadrature Amplitude Modulation). Of the four, QPSK is the most common. QPSK has the advantage that it can operate with transponder power close to saturation so is energy efficient.

INSTALLATION:

To install a domestic Satellite TV system some technical knowledge is required - assistance is usually available in most areas. In NZ the dish must have a direct unobstructed view to the North (to the South in the Northern hemisphere) and elevated to approx. 45 degrees - the correct aiming co-ordinates can be obtained from your TVRO supplier for your location. Due to the large surface area of the dish special consideration must be given to mounting the dish, whether elevated or at ground level - in some cases it will be desirable to seek professional advise. To install a domestic Satellite TV system some technical knowledge is required - assistance is usually available in most areas. The dish must have a direct unobstructed view to the North and elevated to max. 45 degrees - the correct aiming co-ordinates will be supplied for your location - this is for NZ only - for other regions please contact use for details. Due to the large surface area of the dish special consideration must be given to mounting the dish, whether elevated or at ground level - in some cases it will be desirable to seek professional advise. Note: The Intelsat (located at 180E) satellite which currently carries RFO and World Net is in an inclined orbit making tracking essential for 24hr viewing - some of the Rimsat leased satellites are also in an inclined orbit. As time goes by PanAmSat PAS-2 looks set to become the region's hottest bird.

CUSTOMER SERVICES:

For your protection, if purchasing Satellite equipment or services from another Company we suggest for your own "peace of mind" you confirm that they are members of a recognized TVRO professional organization or society - Telsat Communications are a members of the (SSPI) Society of Satellite Professionals International, SPACE-Pacific Trade Association and (ETSA) Electronic Technology Services Association . We have over 29+yrs experience in the Radio/communications & Satellite Television industry. With this experience and accountability you will be well serviced with your Satellite Television requirements. We purchase direct from the manufacturers to ensure you of the best factory based after sales service if required to protect your investment. Among our clients are local & overseas Embassies, large corporate and regional Companies and domestic users through out NZ & the South Pacific. We are proud to be Telecom NZ's preferred TVRO equipment supplier.

NOTE: In NZ generally any structure over 1.5m in diameter requires both a Building Permit AND a Resource Management Consent before erection commences.

FUTURE DEVELOPMENTS:

Digital techniques have opened the door of many new and exciting innovative customer driven services:

Pay-per-View (PPV) - this option allows the customer to receive and pays for television that he watches - PPV services will include special sports events and first run movies etc.

Video-on-Demand (VND) - making a visit to the local video shop will be the thing of the past, the customer electronically orders the movie of his choice, enjoying the flexibility of setting his own viewing time. He only has to switch on his TV and he's off - to the movies from the comfort of his lazy boy chair.

Near Video on demand (NVOD) - a simplified version of VOD, time-shifted programs, offset against each other by say 15mins are available on different channels. All the customer is required to do is make one telephone call to his cable operator.

Interactive Television - the consumer can actively influence the direction of the program. For example, in the case of a sports broadcast, he could select only those camera positions that are ideal from his point of view.

Special Interest Channels - the viewer will be able to subscribe to special programs for many special interest areas. Once a service is actuated, his fee will open the door to a host of information that was previously available only in special interest magazines.

Happy viewing - we have only touched the surface of this fascinating methodology.

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Telsat® Communications Ltd P O Box 1537, 17 Westhaven Grove, Palmerston North, (Knowledge City) NEW ZEALAND. Established in 1988. Tel: +64-6-356-2749, Fax: +64-6-355-2141, Toll Free 0800-TELSAT (NZ Only).

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