Dielectric constant measurement, equipments, test procedure
This page covers dielectric constant measurement of liquid and solid. It includes test equipments and test procedure for the measurement dielectric constant.
Consider a solid sample or length lε loaded in rectangular waveguide
against short circuit that touches it well.
D & DR are the positions or first voltage minima of the sanding wave pattern when waveguide is unloaded & loaded with the dielectric. The respective distance from the short circuit will be ( l + lε ) & ( lR + lε ).
The impedance are equal so
Zo & Zε are respectively the characteristic impedance of empty & dielectric filled waveguide β & βε are respective propagation constant. Expanding tangent sum angel we get following equations.
The dielectric constant calculation is done with this equation.
Test Tools or Test Equipments Needed
Following test equipments are needed for the dielectric constant measurement setup.
• Gunn power supply
• Gunn Oscillator
• Frequency meter
• Variable attenuator
• Detector mount
• PIN modulator
• Solid dielectric cell
Solid Dielectric constant measurement
The figure-1 depicts test setup used for solid dielectric constant measurement.
➨Do the setup and arrange test equipments as shown in the figure-1.
➨Energize the microwave power source & obtain suitable power level in the SWR meter.
➨Micrometer position of solid dielectric should be fully closed.
➨With no sample in the shorted waveguide, measure & record in table position of standing wave minima, starting from an arbitrary plane. Compute guide wavelength (distance between successive minima being λg/2).
➨Take position of first minima as reference minima i.e.DR.
➨Using frequency meter determine the frequency of the excited wave & compute free space wavelength (λ0 = c/F).
➨Remove the solid dielectric cell & insert gently the dielectric sample into the solid dielectric cell such that it should touch the plane of waveguide.
➨Now connect solid dielectric cell without disturbing any setting.
➨Measure and record shift in minima in the table format.
➨Refer the calculations Dielectric constant for finding the value of Dielectric Constant.
➨Measure and record waveguide dimension.
Record following in table format and use equation-3 to calculate dielectric constant for solid.
a.) Waveguide dimension a = ----- cm, b = ----- cm
(for x band a = , b = )
b.) Cut-off wavelength λc = 2*a/m in cm
c.) Frequency of operation = ------ in GHz
d.) Beta β = 2π/λg , Propagation constant
Liquid Dielectric constant measurement
The figure-2 depicts test setup used for liquid dielectric constant measurement.
➨Do the setup and arrange test equipments as shown in the figure-2.
➨Energize the microwave power source & obtain the suitable power level in the indicating meter.
➨With no liquid in the cell, read & record in table position or standing wave minima i.e. DR starting from any arbitrary plane. Compute guide wavelength the distance with alternate minima being λg/2.
➨Using frequency meter determine the frequency of the excited wave & compute free space wavelength.
➨Carefully fill the cell with known volume of the liquid sample. Calculate the height of the liquid in the cell (volume/area = V/ab). Read & record in table the position of voltage minima or the sanding wave pattern with respect to the same reference plane. Measure and record in table the position of standing wave voltage minima shifted due to dielectric constant. The position of first minima is taken as D.
➨Find out SWR as done in the above measurement.
➨Measure waveguide dimension.
➨Determine and enter values in tabular format with following parameters and do calculation as per equation-3 above.
a.) Waveguide dimension a = ---- cm, b = ---- cm
b.) Cut-off wavelength λε = 2a ----- cm
c.) Frequency of operation f = ---- GHz
d.) Free-space wavelength = c/f = ---- cm
e.) β = 2*π/λg, Propagation constant