RF Terminations

RF terminations are used in radio frequency circuits to soak-up incident power. They are matched to a specific impedance load and the line is terminated at the rated impedance. In measurement devices, all unused ports should be terminated because otherwise they will cause interference. They are also used in dummy loads for transformers, isolators and directional couplers. Matched loads provide a termination that is designed to absorb all of the incident power with very little reflection. This will effectively terminate the line or port in its characteristic impedance. High power versions of terminations are used in transmitter applications as dummy loads.

RF terminations are available in various categories such as N male, SMA male, BNC male, TNC male, 7.16 DIN, etc. Corresponding female versions are also available. They are classified as either high power RF terminations or low power RF terminations.

The physical differences in construction of these resistors can cause models that are DC-equivalent to have drastically different electrical characteristics at radio and microwave frequencies. Termination resistors for RF and microwave transmission lines don’t all work the same really. The same resistors or networks that are designed to perform termination functions adequately at lower frequencies may sometimes work poorly at higher frequencies. This is basically caused by unwanted capacitance and inductance that is present in the various package designs. Measuring how far from “ideal” the resistor performance these units exhibit will predict how they will perform in a high-frequency design. If there are sufficient changes in the impedance of line terminators at RF and microwave frequencies, then the resulting mismatch can often cause signal reflections on the transmission lines.

There are some important factors that should be considered when selecting RF terminations. These include the amount of power that needs to be absorbed and the tolerated amount of reflection over the given frequency range. While specifying the RF terminator you also need to mention: the peak power to be absorbed, the frequency range of the signals, and the size (length, diameter, weight). Contact pins are electroplated with gold or silver; connectors can be coated either silver-plated brass or nickel.

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