Duplexer

The duplexer incorporates the receiver multicoupler system and the transmitter combiner system to a common antenna. Duplexers are especially useful in a microcell application where space is limited, costs are kept to a minimum, and the building owner or manager places limits on the number of antennas allowed in the building.

The types of duplexing that can be used in a wireless system are:

· Separate TX antenna, or no duplexing

· Single, or one antenna

· Double, or two antenna

When no duplexing is used, three separate antennas are connected, on for transmit and two for receiver. This configuration results in lower output power.

In single duplexing, one antenna is used for transmit and receive. A second receive antenna could be connected for receiver diversity. This configuration allows a single antenna to be used but also results in lower output power.

In double duplexing, two antennas are used for both TX and RX and results in the maximum possible output power.

Satellite Antenna

Overview, of the typical RF antenna design types used with satellites, both on the ground and on the satellite. This includes satellite television (tv) reception.

A variety of forms of antenna can be used for transmitting to and receiving from satellites. The most common type of satellite antenna is the parabolic reflector, however this is not the only type of antenna that can be used. The actual type of antenna will depend upon what the overall application and the requirements. 

Antenna gain

The distances over which signals travel to some satellites is very large. Geostationary ones are a particular case. This means that path losses are high and accordingly signal levels are low. In addition to this the power levels that can be transmitted by satellites are limited by the fact that all the power has be generated from solar panels. As a result the antennas that are used are often high gain directional varieties. The parabolic reflector is one of the most popular.

Antennas on satellites

Although there is fundamentally no difference between the antennas on satellites and those on the ground there are a number of different requirements that need to be taken into account. In the first instance the environmental conditions are very different. As conditions in space are particularly harsh the antennas need to be built to withstand this. Temperatures vary considerably between light and dark and this will cause expansion and contraction. The materials that are sued in the conduction need to be carefully chosen.

The gain and directivity of the antenna need to be chosen to meet the needs of the satellite. For most geostationary satellites the use of directional antennas with gain is mandatory in view of the path losses incurred. These satellites are more likely to cover a give area of the Earth, and as they remain in the same position this is normally not a problem. However the attitude of the satellite and its antenna must be carefully maintained to ensure the antenna is aligned in the correct direction. The antennas on board the satellite are typically limited in size to around 2 - 3 metres by the space that is available on the satellite structure.

For satellites in low earth orbits, considerably less directive antennas are normally used. Signals are likely to be received and transmitted over a much wider angle, and these will change as the satellites move. Accordingly these satellites seldom use parabolic reflector antennas.

Ground antennas

Ground antennas used for receing satellite signals and transmitting to the satellites vary considerably according to their application. Again parabolic reflectors are the most widely used, but Yagi antennas may be used on occasions.

The size of the antennas may vary considerably. The parabolic reflectors used for satellite television reception are very small. However those used for professional applications are much larger and may range up to several tens of metres in size.

The satellite antennas are carefully chosen by the system designer to match the particular requirements. It is possible to calculate the exact specification for the antenna, knowing the path loss, signal to noise ratio, transmitter power levels, receiver sensitivities, etc. A small 70 centimetre antenna may be sufficient for direct reception of satellite TV programmes but would not be suitable for transmitting programmes up to the satellite where a much higher signal level is required to ensure the best possible picture is radiated back to Earth.

Satellite television antennas

It has already been mentioned that satellite television antennas use parabolic reflector or "dish" antennas. They are also incorporate what is termed an LNB. This is a Low Noise Block converter. The satellite transmits signals at frequencies between 12.2 and 12.7 GHz. Signals at these frequencies would be very quickly attenuated by any coaxial feeder that was used. As feeder lengths may run into several metres or more in many installations, this would mean that the signals that reached the television would be very weak. To overcome this problem the LNB is installed at the feed point of the antenna. Its job is two fold. It amplifies the signal, but more importantly it converts it down to a frequency (usually 950 to 1450MHz) where the loss introduced by the coaxial feeder is considerably less. The amplification provided by the LNB also enables the loss introduced by the cable to be less critical. By performing these two functions it means that domestic coaxial cable can be used satisfactorily, while maintaining sufficiently high signal levels at the receiver.

Antenna RF Diplexer

The antenna diplexer or RF diplexer splitter / combiner used for combining and splitting RF fees so they can be used by multiple transmitters of receivers and possibly on different frequencies.

An Antenna diplexer is a unit that in one application can be used to enable more than one transmitter to operate on a single RF Antenna. Sometimes these units may be called Antenna duplexers. Typically an Antenna diplexer would enable transmitters operating of different frequencies to use the same Antenna. In another application, an Antenna diplexer may be used to allow a single Antenna to be used for transmissions on one band of frequencies and reception on another band.

Antenna diplexers find many uses. In one common example an Antenna diplexer or RF diplexer is used in a cellular base station to allow it to transmit and receive simultaneously. The Antenna diplexer enables the same Antenna system to be used while preventing the transmitted signal from reaching the receiver and blocking the input. In another application a diplexer may be used by a broadcast station transmitting on several different frequencies at the same time using the same Antenna. The use of the diplexer enables a single Antenna to be used, while preventing the output from one transmitter being fed back into the output of the other.

Small Antenna diplexers may be used in domestic environments to allow several signals to run along a single feeder. In one application this may allow a single feeder to be used for television and VHF FM radio reception, or to allow terrestrial television signals and this from a satellite low noise box (LNB) to pass down the same lead. These RF diplexers are normally relatively low cost as the specifications are not nearly as exacting as those used for professional RF diplexer installations.

Basic Antenna diplexer concepts

There are a number of ways of implementing RF diplexers. They all involve the use of filters. In this way the paths for the different transmitters and receivers can be separated according to the frequency they use. The simplest way to implement a diplexer is to use a low pass and a high pass filter although band-pass filters may be used. In this way the diplexer routes all signals at frequencies below the cut-off frequency of the low pass filter to one port, and all signals above the cut-off frequency of the high pass filter to the other port. Also here is no path from between the two remote connections of the filters. All signals that can pass through the low pass filter in the diplexer will not be able to pass through the high pass filter and vice versa.

A further feature of an RF diplexer is than it enables the impedance seen by the receiver or transmitter to remain constant despite the load connected to the other port. If the filters were not present and the three ports wired in parallel, neither the Antenna nor the two transmitter / receiver ports would see the correct impedance.

RF diplexer filter requirements

When designing an Antenna diplexer a number of parameters must be considered. One is the degree of isolation required between the ports labelled for the high and low frequency transmitter / receiver. If the diplexer is to be used purely for receiving, then the requirement for high levels of isolation is not so high. Even comparatively simple filters give enough isolation to ensure each receiver sees the right impedance and the signals are routed to the correct input without any noticeable loss. Even levels of isolation of 10 dB would be adequate for many installations. For diplexers that are used to split and combine television and VHF FM radio along a single line, te levels of isolation are likely to be very low.

The next case is when the diplexer is to be used for transmitting only. It will be necessary to ensure that the levels of power being transferred back into a second transmitter are small. Power being fed into the output of a transmitter in this way could give rise to intermodulation products that may be radiated and cause interference. It is also important to ensure that the transmitters see the correct impedance, and that the presence of the second transmitter does not affect the impedance seen by the first. Typically levels of isolation between the transmitter ports of 60 - 90 dB may be required.

The final case is where one of the ports is used for transmitting, and the other for receiving simultaneously. In this instance very high levels of isolation are required to ensure that the minimum level of the transmitter power reaches the receiver. If high levels of the transmitter signal reach the receiver, then it will be desensitised preventing proper reception of the required signals. Levels of isolation in excess of 100 dB are normally required for these applications.

Band pass filters

Under some circumstances band pass filters may be used. These may be used where comparatively narrow bandwidth is required for either or both of the transmitter / receiver ports. Sometimes a very high Q resonant circuit may be used. By using this approach high degrees of rejection can be achieved. Often repeater stations which receive on one channel and transmit on another simultaneously use diplexers that utilise this approach.