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.
.jpg)
