Important DocumentsIntroduction to C-Band Interference In October 2000, the Australian Communications Authority (ACA) sold off part of the 3.4GHz spectrum to three companies, raising over $112 million. In doing so, they set in stone a progressive rollout of terrestrial wireless broadband internet services in all populated areas of Australia that is set to disrupt the reception of over 100,000 C band satellite viewers across the country.
The ACA had, some time earlier, devised three bands within what they term the 3.4GHz spectrum. These three bands cover 3.425GHz to 3.575GHz, despite the fact that satellite systems such as Thaicom, Asiasat, Insat, and Telstar were delivering signals into Australia for commercial operators, on the same frequencies. In the roll out prior to the commencement of service in August 2004, hundreds of sites were equipped with terrestrial transmitters to ensure saturation coverage of Sydney.
Because these sites rely on microwave or optical fibre connectivity, they have been installed at many GSM towers and two way radio communication sites where these facilities already exist. In addition high-rise buildings have been used to ensure optimum coverage.
This is all very well for wireless Internet users, who can now experience the benefits of broadband internet speed without connection to a wired service provider, but for the thousands of C band satellite users in Sydney, a blank or pixellating TV screen is now the result.
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Analysis of the problem reveals that the most prolific operator is "Unwired Australia" whose sister company AKAL had paid $95 million for licenses in all areas where the spectrum had been made available.
The Importance of a Site Survey
An unfortunate by-product of the wireless age is microwave interference. This energy can interfere with satellite TV reception, as it operates on similar frequencies to those used by international satellites. Roof tops and other areas exposed to high amounts microwave energy (i.e. on excessively high mounting poles) are extremely susceptible to C-Band satellite interference, where there are no buildings or trees to shield microwave interference away from the dish and LNB.In the case of digital reception, a spectrum analyser should be used to measure any terrestrial interference. A small test dish should be set up and an identical LNB to the one supplied with the system should be used to test for any interference. If interference is present then it is suggested that the customer be advised of the possible increase in system costs necessary to achieve reliable performance. This may seem an unnecessary effort, but it is far easier to discuss the subject before the dish is installed on a customer's property, rather than to have to remove an unsuccessful installation and attempt to negotiate costs with the customer.
Not all receivers, LNBs, cable and dishes are born equal. All have different characteristics in relation to the amount of interference needed to fatally effect their performance. It will therefore take some time for installers to find the best combination of equipment to be used to overcome this problem. This is usually a costly and time-consuming exercise and is the main reason this Guide was put together. It is now very obvious that you will get what you pay for in relation to C-band equipment.
The use of components based solely on price will compromise system performance. Unfortunately many systems have been sold and designed with the end price in mind rather than quality, long-term reliability and performance. These systems usually have more than one shortcoming, allowing the interference to degrade performance. The cost of fixing these systems often exceeds the original purchase price.
The IF Solution
Whilst the best remedy is to remove the interference before it enters the LNB input, some relief from the effects of mild interference can be obtained by filtering the signal after the LNB and before it arrives at the receiver.
A satellite receiver is designed to accept an 800MHz wide band of input signals, and as such does not have any great selectivity. It is also designed to accept signals of a certain level, typically -25 to -65dBm. Feeding a high level signal into the receiver can cause the tuner to be overdriven.
Where the interfering signal is converted by the LNB (along with the satellite signal) it will appear as an IF signal, normally just outside the nominal 950-1450MHz IF band generated by 3.7-4.2GHz LNBs.
An IF filter designed to reject the interfering frequency and pass only satellite signals, can be used to enhance the satellite signal. Such filters can be inserted in the coaxial line to the receiver and can be configured to pass the DC voltage necessary to power the LNB. We recommend an IF filter be used in all installations to provide the customer with some degree of protection against the future effects of interference
We have designed an IF filter as part of our interference solution package which will eliminate interference that is passed by the LNB but does not drive the LNB into compression. These filters are the first recommended step in fixing interference problems in existing systems
The Input Frequency Solution
By far the best solution to eliminate the effects of interference is to prevent it from entering the LNB in the first place. This can be done in two ways, either by using a waveguide filter which is placed between the Feedhorn and the input of the LNB, thus filtering out any interference before it reaches the LNB, or by using an LNB with a built in notch or Bandpass filter. These solutions are costly but offer a high rate of success, offering up to 75dB of protection against interference. These items can also be used in conjunction with each other to offer the ultimate protection against any interference. However the waveguide filter solution does have insertions loss which may be unacceptable depending on the link budget information.
Apart from the expense of such filters, they are only made to operate in one polarity, so a dual polarity system will require 2 filters, 2 LNBs and an orthomode coupler or just one of our new LNB with built in Bandpass filter.
The LNBs used with a waveguide filter should have a range of 3.7-4.2GHz, rather than the more commonly encountered extended C band range of 3.4-4.2GHz. At least by restricting the operating range above the interfering signal, some protection is afforded. Finally for optimum reception, these single polarity LNBs should be Phase Locked (PLL) to ensure a high level of frequency stability.
Whilst this is the ultimate professional solution, it is expensive, and is reserved for those customers relying on the reception of satellite signals for the ongoing success of their business (eg hotels, radio stations, television stations, embassies, schools etc).
For commercial or extremely high interference sites we have designed a Single Polarity PLL LNB with a frequency range of 3.7-4.2GHz with a built in Bandpass filter. With this LNB we have our highest success in combating interference with 100% of sites fitted with this LNB have made a full recovery from any interference issues.
For sites where our PLL with bandpass filter has been to expensive we have come up with a combination of dual polarity lnb with frequency reception restricted to 3.7GHz- to 4.2GHz. This LNB when used in conjunction with a high effiency feed ring and IF filter give the best protection against interference. Also by using natural features at the installation site to shield the dish from interference sources as well has a high gain and quality dish and cable will give your system the best future proofing against any further interference sources. As with wireless broadband it is a two way service and individual modems also emit interference effecting C Band reception.
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PLL LNB |
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Waveguide filter |
We also use our orthomode feed for single polarity interference sites by using the side port with one of our Filtered LNBs and blocking the top port with any redundant single polarity LNBs. By doing this we have seen a vast improvement in the bit error rate.
For domestic customers, we have designed a more affordable solution. A dual polarity,3.7-4.2GHz LNBF with inbuilt 3.5GHz notch filter. This LNBF has the ability to look at both polarities without the use of an orthomode coupler. This LNB also offers effective protection against interference but does not have the performance of our PLL LNB. This means that the dual polarity LNBF is only effective in protecting against interference at lower levels, the LNBF offers 35dB of protection as compared to the 70dB offered by the waveguide filter and PLL LNB
With the solutions mentioned above it is highly recommended that a IF filter be place at the back of the satellite receiver to eliminate any possibility of receiver overload.
Summary
Where interference is suspected, try the easiest solutions first. Remember this is an imperfect science and there are no cheap cures. However, every individual problem that can be eliminated will contribute to the overall success in improving system performance.
For existing systems, if it is possible to still receive some satellite channels, it is likely that some of the economical fixes suggested here will cure the problem to the extent that normal viewing will be restored. It will NOT be possible to view any channel in the extended section of C band. Any channel falling between 3.4 and 3.7GHz will be lost. This will affect users of Telstar10, Thaicom 5, Insat 3a, Insat 2e, Asiasat 3, Apstar 6, Telstar 18 and Palapa C2.
For existing systems, the recommended procedure is to start by trying to identify the possible source of interference. It will most likely be a nearby mobile phone tower, or communications site. Use the ACMA website to search for your location, it will soon reveal if there is a transmitter nearby. If the dish is rooftop mounted, consider relocating it to ground level, where buildings can be used to shield the dish from the local source.
In some situations the amount of 3.5GHz radiated energy can be huge, and care must be taken to prevent cable ingress. Check that cabling is RG/6 quad shield (a reputable brand is essential... just because it is Foxtel approved doesn't mean it is suitable), check the fitting of connectors, and replace crimp types with compression connectors. Bury any cabling that is lying on the ground, and shield long vertical runs of cable by using steel or aluminum pipe.Replacing the LNB with one rated from 3.7-4.2GHz will certainly help. Replacing a dual polarity type with a single polarity model is a huge step in the right direction, as long as single polarity reception is acceptable.
If the interference is severe, locate a reputable installer who uses a spectrum analyser. The interfering signal may be 70dB stronger than the satellite signal in severe cases, so some kind of spectrum display is necessary to be able to observe improvements.
Our economical "Spectralook" spectrum adaptor is suitable for this application when used with a video monitor.
Use a single polarity PLL LNB, and if necessary purchase a waveguide filter for additional protection. Commercial installations requiring operation of both polarities will require two PLL LNBs, and two waveguide filters, fitted to an orthomode coupler.
Interference is likely to be more severe when watching satellites having a low look angle, although this does not preclude interference on dishes set to high elevation values
