There is a significant difference between splitters and combiners, and one must also consider the point of view from the INPUT to the device.
When receiving, the antenna is the INPUT. Signals from the antenna are divided between two (or more) outputs. There may be frequency-sensitive FILTERS in a splitter that restrict the input signal to being available at a particular output terminal by its frequency range.
The typical "splitter" will impose a loss of signal to at least one-half power from input port to two output ports
Generally there is no problem in having two RECEIVERS connected to a common antenna. The input circuit of a receiver is an amplifier that will immediately increase the strength of the input signal. This amplification will usually overcome any loss in signal in the splitter itself, up to a point. The "point" will occur with input signals of extremely low level, which, if attenuated by the splitter, may become so weak that they arrive at the receiver input stage below the inherent noise
level of the receiver's amplifier, and thus will be lost
in the noise
. To overcome this, some "splitters" contain their own amplifier that may pre-amplify the signal before it is split. This method avoids losing really weak signals in the noise of the following amplifier due to loss in the splitter.
In a VHF Marine
which as an AIS receiver integrated in the same chassis, the antenna signal from the RECEIVE side of the RECEIVE-TRANSMIT relay is routed to multiple receivers. Typically there will be a VHF Marine
Band receiver, a dedicated DSC
receiver, and two dedicated AIS receivers.
From the point of view of TRANSMITTING into a COMBINER, there will be two INPUTs and one OUTPUT. An antenna will be connected to the OUTPUT, and two transmitters connected at the two INPUTs.
Note that there is NO simple means for a "combiner" to effectively allow two transmitters to send their signal to a single
antenna. While it is POSSIBLE to have two transmitters connected to a combiner and feeding one antenna, this sort of arrangement requires very precise conditions to be met. For example, if the transmitters are transmitting the same signal on the same frequency, the precise phase of the signals must be identical so they will combined in an additive manner. If the signals are on different frequencies, the combiner must be designed with extremely sharp, narrow-band filters so that the input signal from one transmitter does end up appearing at the input port of the other transmitter.
Devices that perform this sort of combining are called duplexers, and are usually limited to two transmitters. A duplexer will also allow a receiver to remain connected to an antenna while a transmitter is also connected to the same antenna, as long as the transmitter frequency is fixed and can be deeply attenuated with a notch filter.
For use with VHF Marine Band radios and with AIS transmitters, the typical device (called by various names) permits only one transmitter to be connected to the antenna at a time. This is accomplished by very rapidly detecting the presence of transmitter power at an input and switching the antenna to only that input for the duration of the transmission
. These devices should be called high-speed automatic antenna switches rather than "splitters" or "combiners."
When the antenna is switched to one input for transmission
, the antenna is no longer connected to the other input. A dummy load may be connected to the other input so that if that transmitter is keyed up it will see a proper load.
As a general consideration in designing a radio
system for highest reliability
, the number of connectors, cables
, and other devices inserted between the radio and its antenna should be kept as low as possible. For a device like a VHF Marine Band DSC
radio which is a primary safety
device aboard a ship, there is no good justification for inserting multiple splitters or combiners into the transmission line path between radio and antenna.