Thermal sensors can generally be divided into two main categories, thermocouple power sensors and thermistor-based power sensors. Thermal sensors depend on the process of absortbing the rF and microwave signal energy, and sense the resulting heat rise. Therefore they respond to true average power of the signal, whether it is pulsed, CW, AM/FM or any complex modulation. (Agilent 2008). Thermocouple power sensors make up the majority of the thermal power sensors sold at present. They are generally reasonably linear and have a reasonably fast response time and dynamic range. The microwave power is absorbed in a load whose temperature rise is measured by the thermocouple. Thermocouple sensors often require a reference DC or microwave power source for calibration before measuring; this can be built into the power meter. Thermistor-based power sensors such as the Agilent 8478B are generally only used in situations where their excellent linearity is important, as they are both much slower and have a smaller dynamic range than either thermocouple or diode-based sensors.Thermistor-based power sensors are still the sensor of choice for power transfer standards because of their DC power substitution capability (Agilent 2006). Other thermal sensing technologies include microwave calorimeters and bolometers,and quasi-optic pulsed microwave sensors.
Read more about this topic: Microwave Power Meter
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A thermal column (or thermal) is a column of rising air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation, and are an example of convection, specifically atmospheric convection. The Sun warms the ground, which in turn warms the air directly above it. Dark earth, urban areas and roadways are good sources of thermals.
The warmer air expands, becoming less dense than the surrounding air mass. The mass of lighter air rises, and as it does, it cools due to its expansion at lower high-altitude of 125415748 pressures. It stops rising when it has cooled to the same temperature as the surrounding air. Associated with a thermal is a downward flow surrounding the thermal column. The downward moving exterior is caused by colder air being displaced at the top of the thermal.
The size and strength of thermals are influenced by the properties of the lower atmosphere (the troposphere). Generally, when the air is cold, bubbles of warm air are formed by the ground heating the air above it and can rise like a hot air balloon. The air is then said to be unstable. If there is a warm layer of air higher up, an inversion can prevent thermals from rising high and the air is said to be stable.
Thermals are often indicated by the presence of visible cumulus clouds at the apex of the thermal. When a steady wind is present thermals and their respective cumulus clouds can align in rows oriented with wind direction, sometimes referred to as "cloud streets" by soaring and glider pilots. Cumulus clouds are formed by the rising air in a thermal as it ascends and cools, until the water vapor in the air begins to condense into visible droplets. The condensing water releases latent heat energy allowing the air to rise higher. Very unstable air can reach the level of free convection (LFC) and thus rise to great heights condensing large quantities of water and so forming showers or even thunderstorms.
Thermals are one of the many sources of lift used by soaring birds and gliders to soar.
Thermals on the sun typically form hexagonal prisms (Bénard cells).
... In wireless communication systems, the rise over thermal (ROT) indicates the ratio between the total interference received on a base station and the ...