ADJUSTABLE PLUNGER CAVITY TECHNIQUE
Abstract
A new technique, based on the cavity resonance, of measuring the complex dielectric permittivity of liquid samples is described in detail. The errors in the measurement of the real and imaginary parts of the complex dielectric permittivity are ~+ 1% and + 3% respectively. The complex dielectric permittivity values are measured using this method for some standard values are also presented. A comparison is made between the present values and the values obtained using standard techniques.
Review of Scientific Instruments, Vol. 64 (1), Jan. 1993, Pages 231-233.
Theory
An adjustable plunger waveguide is converted into a TE10p mode rectangular cavity, where p can be varied as p = 1,2,3 .. etc. The node number p indicates the number of half wavelengths inside the cavity when it resonates.
As one increases the length of the cavity, fixing the input microwave frequency constant, the node number ‘p’ also increases. The distance through which the plunger is moved between successive cavity resonances gives half of the guide wavelength of the microwave. At resonance, the standing wave pattern is formed inside the cavity.
Taking into consideration of the electric field variation inside the cavity, the average power inside the cavity for TE101 mode will be,
For
a ¹ 0And
= 1 For a = 0
Where b = 2p /l d is the phase constant.
The reflected power from the plunger cavity due to this average power is
P1 = 1 – Pnav1
The same argument can be extended for higher order nodes. By measuring the values of reflected power P1 and l d, attenuation constant a can be calculated.
The dielectric constant (
e’) and loss (e ") are calculated using the above formulae where l c is the cut off wavelength and l 0a is the free space wavelength.