What is Magnetron? Working Principle of Balanced Bolometer Bridge method for Low Power Measurement. How can we extend the bolometer method for high microwave Power Measurement? Working principle of calorimeter wattmeter method for High power Measurement.

1. Al-amin
Department of Electrical & Electronic Engineering
Jatiya Kab Kazi Nazrul Islam University, Trishal, Mymensingh,
Banglade
1. What is Magnetron?
2. Working Principle of Balanced Bolometer Bridge method
for Low Power Measurement.
3. How can we extend the bolometer method for high
microwave Power Measurement?
4. Working principle of calorimeter wattmeter method for
High power Measurement.

2. •What is Magnetron?
The Magnetron is a vacuum tube which is
used to generate microwaves of high power.
Or, Magnetrons are the cross-field tubes in
which the electric and magnetic fields cross,
i.e. run perpendicular to each other.
• Its working principle is based on the
interaction between electron stream and
magnetic field.
• The Magnetron Tube works on Direct
Current power.
Working:
• Depending upon the relative strength of the
magnetic and electric field the electrons
released from the cavity move towards the
anode will navigate through the interacting
space.

3. •Working Principle of Balanced Bolometer Bridge method for Low Power
Measurement.
•Measurement of Low Power
The measurement of Microwave power around 0.01mW to 10mW, can be
understood as the measurement of low power.
Bolometer is a device which is used for low Microwave power measurements.
The element used in bolometer could be of positive or negative temperature
coefficient. For example, a barrater has a positive temperature coefficient
whose resistance increases with the increase in temperature. Thermistor has
negative temperature coefficient whose resistance decreases with the
increase in temperature.
Any of them can be used in the bolometer, but the change in resistance is
proportional to Microwave power applied for measurement. This bolometer is
used in a bridge of the arms as one so that any imbalance caused, affects the
output. A typical example of a bridge circuit using a bolometer is as shown in
the following figure.
The milliammeter here, gives the value of the current flowing. The
battery is variable, which is varied to obtain balance, when an
imbalance is caused by the behavior of the bolometer. This
adjustment which is made in DC battery voltage is proportional to the
Microwave power. The power handling capacity of this circuit is
limited.

4. •How can we extend the bolometer method for high
microwave Power Measurement?
•Measurement of High Power
The measurement of Microwave power around 10W to 50KW, can
be understood as the measurement of high power.
The High Microwave power is normally measured by Calorimetric
watt meters, which can be of dry and flow type. The dry type is
named so as it uses a coaxial cable which is filled with di-electric of
high hysteresis loss, whereas the flow type is named so as it uses
water or oil or some liquid which is a good absorber of microwaves.
The change in temperature of the liquid before and after entering
the load, is taken for the calibration of values. The limitations in this
method are like flow determination, calibration and thermal inertia,
etc.
•Measurement of Power by Means of a Bolometer
Bridge
•To measure unknown RF power, a small known AF power,
indicated by the voltmeter V1, is superimposed on the RF test
power. The direct current from voltage E is next adjusted by
varying R, which heats the bolometer element until its
resistance equals R1, which is the value required to balance
the bridge. The test RF power is then turned off, which
unbalances the bridge. Balance is now restored by increasing the
AF voltage to V2. Hence RF power equals.
Since power delivered to the bolometer element is 1/4th power given to the
bridge.
In a co-axial and wave guide transmission system, the bolometer mount
must provide the necessary impedance transformation (matching). In a co-
axial cable, a 50Ω transmission line impedance has to be transformed to a
125 —200Ω resistance value of a bolometer. This is usually done by
means of a tapered section, as shown in Fig. 20.2.

5. 1. Working principle of calorimeter wattmeter method for
High power Measurement.
 Calorimeter Wattmeter/Power meter
• The unknown RF power is checked against a 1200-cps (Hz/cycles
per second) comparison power in the bridge circuit.
• Two temperature-sensitive resistors serve as gauges.
• In operation, the unknown RF heats an input load resistor.
• This resistor and one gauge are in close thermal proximity so that
heat generated in the input load heats the gauge and unbalances
the bridge.
• The unbalanced signal is amplified and applied to the comparison
load resistor which is in close proximity to the second gauge, and
rebalances the bridge.
• The meter measures the power supplied to the comparison load to
rebalance the bridge.
• Efficient heat transfer from the loads to the temperature gauges is
accomplished by immersing the components in an oil stream.

6. This Photo by Unknown Author is licensed under CC BY-NC-ND