Application Note: Parallel Capacitance in High Power RF Resistors

Most of the high power resistors used in RF and Microwave frequencies are found in Wilkinson power dividers/combiners in power distribution circuits.

For best performance, the 100 Ohm isolation resistor, when used in a Wilkinson divider, should have very low capacitance to minimize the effects on insertion loss. Additionally, the isolation resistor must be able to absorb half of the input power on each input port when used in a Wilkinson combiner.

Discrete resistors can also be used to design high power attenuators. For low frequency applications, this maybe practical; however, at high frequencies, the parasitics of each individual resistor will degrade the attenuator performance making this approach less desirable.

High power resistors come in different shapes and sizes. For example, the most popular are: SMT resistors, resistors with leads (with and without dielectric cover), and resistors with leads and dielectric cover mounted on a conductive flange as depicted on Figure 1.

Figure 1. Illustration of various high power resistors