Page for information on Transmitter Combiners
A combiner in this page is for combining different frequency Land Mobil Radio ("LMR") transmitters at higher power levels (100-250W) into a single transmitter antenna. A combiner is required to prevent the output of one transmitter from feeding into the output of another. At a minimum this would generate wide band inter-modulation ("IMD") (the output stage of most PA's looks like a bunch of mixer diodes in parallel). A good combiner provides isolation between the different transmitters and low insertion loss to the antenna, making is a non-reciprocal device.
In LMR most spurious signals must be attenuated down to at least -80 dBc (decibels below the carrier power). This can be more at co-location sites and certain frequency choices can exacerbate this, especially if the receiver frequency is close to a potential mix product frequency. Note that a combiner only prevents spurious mixing in the transmitter power amplifiers, it's still quite possible to have IMD in the antenna, cable, connectors or other parts of the system.
There are multiple ways to combine transmitters, but generally it falls into two major categories
- Frequency selective filters (cavities) and a ferrite isolator
- Hybrid Combiners (Wilkinson Combiners)
- a combination thereof
These have a frequency selective filter feeding into a common port on the output. The input from the transmitter typically uses an isolator with appropriate sized load, but this is optional if the isolation provided by the cavity is sufficient. Broadcast combiners may utilize such an isolator free design as high power isolators are expensive and hard to realize, while also adding more loss to the system.
In a cavity combiner system insertion loss is directly proportional to the separation of the transmitter frequencies. A typical 1/4 lambda cavity system will work best when the TX1 to TXn spacing is at least 500 KHz. Below this, 3/4 lambda cavities are used, along with adjusting the coupling loops for less coupling into the cavity at the expense of additional losses. This last point is the rule with combiners; there is no free lunch and you will loose dB's, 10th's of dB's add up quick.
Typically cavity combiners are only able to work to 80-100 KHz of separation, and at this close, the cavity may only be providing 10 dB of TX1 to TXn isolation with the rest ending up as power on the dummy load of the isolator. This loss will be reciprocal across the channels, and it's not uncommon to see 5-6 dB of loss on a closely spaced system.
A hybrid combiner is an excellent choice if one can accept the high loss. A hybrid provides a great deal of isolation between ports and is the same electrical design found in a receiver multi-coupler, but supporting higher power. A hybrid combiner has no minimum frequency spacing and great bandwidth. In most cases an isolator is still used, as a real world combiner provides a 30-35 dB port to port isolation.
Losses in these sort of combiners start at 3 dB for a two port system. In reality the loss is a bit more due to isolators, heating loss, etc. A typical 2 port system will be 3.3-3.6 dB, and every port adds another 2-3 dB of loss. An eight port system may realize 10-11 dB of insertion loss per channel. Contrast this to a cavity system where with proper TX spacing 2.5 dB per channel can be realized.
The major advantage to the hybrid combiner is it doesn't require tuning, and there is no minimum frequency separation.
Many times there may be a system with 10 TX channels all spaced 300 kHz with the exception of two 454 MHz channels which are 50 kHz. In such a system a cavity combiner would be ideal as the insertion loss will be under 3 dB per channel, excepting these close spaced channels. In this case a hybrid will be used to combine the two closely spaced channels feeding into a standard cavity combiner port tuned to the center of both channels. In this instance the 3db hybrid and 1.5 dB cavity loss will give 4.5 dB loss on both these close channels but retain the 3 dB loss on the remaining channels which are not close spaced. A 10 port hybrid combiner would see 13-14 dB of loss per channel, clearly showing the advantages to this "combination" approach.
This list is US centric.