RF Power Measurement using Thermocouple Instrument

An elementary method of measuring radio frequency power using the thermocouple instrument is shown below. In this method a thermocouple ammeter along with a non-inductive resistor are employed. The tank circuit of the device whose power output is to be measured will be linked through a suitable transformer, to yet another tuned circuit. This tuned circuit consists of the secondary of the transformer a capacitor, a known value of non-inductive resistance and a thermocouple ammeter. The value of the resistor must be known and the resistor must have a power rating which is atleast twice the power that is expected to be measured. It must also have the temperature coefficient that shall not bring any change in its value due to the dissipation of the power being measured.

(a) Description of the Circuit: 

An arrangement for measurement of R.F. power is shown in Figure. In this circuit the L1 is the tuning coil and C1 is the tuning capacitor of the tank circuit of the source whose power is to be measured.
Measurement of RF Power using Thermocouple Ammeter
The tank circuit is linked using the transformer T whose primary is connected to the source tank circuit. The secondary of the transformer is connected to the meter through a capacitor CS and the resistor R. The transformer T is termed the linking transformer and it is tuned in its secondary by the capacitor CS, The resistance R used in the circuit is the non-inductive resistance of the required power rating

(b) Measurement Method:  

The primary of the linking transformer will be connected to the source whose power is to be measured. The capacitor in the secondary of the linking transformer CS, will be adjusted in its value such that it tunes the secondary circuit to the same frequency of source. As the secondary is tuned to the frequency of source and is at resonance, the only resistance in the circuit is the resistance ‘R'. Hence the thermocouple instrument can be calibrated in terms of power. It indicates the power output of the source under that condition.

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