An power combiner as the name suggests, combines radio frequency (RF) signals from N ports into a single output port.
Enter
- Power levels in dBm at the input ports, each separated by a comma
- Additional Insertion Loss in dB
Background
An RF combiner shown in the picture below takes two input signals (applied to the ports on the same side) combines them and provides the output on the third connector.
A passive combiner is the same physical device as an RF splitter. The latter is used to split an input signal into two or more outputs. The combiner performs the reverse operation.
A passive device doesn’t need to be powered. It is constructed using either microwave transmission lines or resistors. Key specifications for a combiner include:
- Frequency Range
- Insertion Loss
- Isolation
- Phase and Amplitude Imbalance
- Power rating
Combiners may be used in an RF lab to perform a two-tone test for example.
The process for this calculation is explained in the dBm addition calculator. Note that dBm values cannot be added directly.
Example Calculation
If the inputs to a two port RF combiner are -10 dBm and 20 dBm, with zero additional insertion loss, the output is +20 dBm. In this case one of the inputs exceeds the other by 30 dB which is a large difference and hence the output will be closer to the larger value or +20 dBm.
If the inputs are -10 dBm and -10 dBm, the output is -7 dBm. In this case, since the input levels are the same, the output power is doubled or increased by 3 dB. The result is therefore: -10 dBm + 3 dB = -7 dBm.
In both these cases, we have assumed no additional insertion loss. That’s not practical. Typically all combiners have an insertion loss specified in the data sheet that increases with frequency.
The plot below shows the loss in a two port combiner measured as a splitter.
The frequency range is 200 MHz to 500 MHz. The loss should be 3 dB but as can be seen, it is not flat across the frequency range. Instead increases from 3.2 to over 3.4 dB. As well, the two ports are not identical with the loss on one port exceeding that on the other.
Can I combine two antennas with a combiner?
The quick answer is yes.
Suppose, for example, that two antennas were fed to a receiver input using a simple “T” as a combiner. If one antenna appears as a short at its resonant frequency, it would load down the other antenna and, in effect, wipe out the receiver input.
However, a properly designed power combiner would provide high isolation between inputs so that the antenna “short condition” at
one input would have little influence on the other input.
Notes
The calculator on this page assumes that the input signals are at different frequencies.
There is a special case for a two-port combiner: If the input signals are in-phase then the calculator provides the total combined power. However, in the event that the two signals are 180° out-of-phase, the insertion loss is infinite and there will be no signal at the output.
References
[1] Understanding Power Splitters – Mini-circuits Application Note
[2] Power dividers and directional couplers – Wikipedia
[3] dBm to Watt Calculator
[4] Watt to dBm Converter