Types of Coaxial Terminations (Dry versus Oil-Filled)

A coaxial termination is a device that can be placed on one end of a coaxial transmission line to absorb the transmitted signals without impacting other system components. A termination presents a “matched load” to the system impedance (usually 50 or 75 ohms), so that a terminated transmission line will behave like an infinitely long line where the signal never has a chance to reflect back to the signal source. Without a termination, the signal will be strongly reflected back up the transmission line, generating false signals, “ghosting”, and possibly overheating or causing electrical breakdown (sparking) and equipment damage. Testing devices without a termination can leak signals in a way that could be detrimental to surrounding devices.

Standard coax terminations consist of a resistor with a heat sink. Any RF signal received by the termination can then be dissipated without impacting the rest of the system. Sometimes called a “dummy load”, terminations are often used in setup and testing scenarios, such as testing a transmitter before the antenna is connected. Key characteristics of terminations are frequency range, VSWR, connector type and power handling.

There are two basic types of coax terminations, based on the mechanism used to dissipate the excess RF energy: oil-filled and dry.

Oil-filled terminations use oil to cool the system and dissipate the excess energy. They are particularly useful for very high-power applications, with power ratings up in the thousands of Watts. However, because they are filled with a fluid, they must often be installed in a specific orientation, and work best within a stable environment. Depending on power needs, this type of termination can get very large.

Dry terminations use the conductive properties of the RF-absorbing material itself to direct the energy out and away from the line, through the body of the termination, either through metal fins into air (convection) or into an external heat sink (conduction). Dry terminations can be mounted and operated in any position, but have a lower power handling capacity (in the hundreds of Watts, not thousands) compared to oil-filled terminations. Dry terminations are usually easy to install (or uninstall), making them convenient for many moderate-power field applications. They also tend to be smaller and lighter than the oil-filled terminations, and can be manufactured in a variety of shapes and sizes to fit a wide range of needs. Cooling fans can be integrated for increased power dissipation capacity.