When input power must be split among multiple transistors in an amplifier, the amplifiers themselves, antennas, or in an enormous number of other scenarios, power dividers are the answer. Not surprisingly, they are manufactured by dozens of companies throughout the world and represent a significant portion of the RF and microwave passive-component market. At first glance, RF and microwave power dividers appear simple. But each type has unique characteristics, so choosing the best one for a particular application can be confusing. This article will hopefully help to alleviate that problem.
For anyone not familiar with power dividing and combining, some of the confusion is caused by terms that tend to be used haphazardly over time. For example, power dividers are sometimes referred to as splitters—which is true. But the industry has deemed that power dividers should be called splitters when they are the simplest and least expensive devices, such as those used in home coaxial-cable systems. In addition, although the term coupler and power divider are often used interchangeably, key differences between the two exist.
Coupler or Divider?
Directional couplers are designed to “sample-off” a relatively small amount of power into one port for purposes like monitoring. Another factor to remember is that while the insertion loss of most passive components is simply the total attenuation from input to output, when applied to a power divider, it refers to the additional loss above that created by the splitting process itself.
Furthermore, power dividers can also be power combiners, but this does not mean that their specifications will be the same nor that every type of power divider is equally suited for use as a combiner. As noted, there’s more than meets the eye when it comes to power dividers.
A power divider splits an input signal into two or more outputs that are usually, but not always, equal in amplitude and phase. Regardless of its type, the goal of every power divider is to have the greatest port-to-port isolation, lowest insertion loss and voltage standing wave ratio (VSWR), and least amplitude and phase imbalance over the entire frequency range of the device.
The most basic type of power divider is the T-junction (Fig. 1), which if mechanically symmetrical will divide a signal applied to its input into two outputs that are equal in amplitude and phase. It’s basically just three transmission lines connected at a single point. The T-junction satisfies the basic purpose of dividing an input signal into two separate transmission paths, but it suffers from two very important deficiencies: There’s no way to match impedance at all ports or any means of providing isolation between them. Although these limitations have always been important, the increasing sophistication of multichannel receivers, along with other applications, has relegated them to obscurity.