The Ohio State University ElectroScience laboratory - NASA

[Pages:45]DIELECTRIC CONSTANT OF VEGETATION A T 8.5 GHz N. L. C a r l s o n

The Ohio State University

ElectroScience laborator-y (formerly Antenna Laboratory) Department of Electrical Engineering Columbus, Ohio 43212

TECHNICAL REPORT 1903-5 31 M a r c h 1967

Contract NSR-36-008-027

National Aeronautics and Space Administration O f f i c e of G r a n t s a n d R e s e a r c h C o n t r a c t s W a s h i n g t o n , D. C . 20546

NOTICES

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related ther eto.

The Government has the right to reproduce, use, and distribute this report for governmental purposes in accordance with the contract under which the r e p o r t was produced. To protect the p r o p r i e t a r y i n t e r e s t s of the contractor and to avoid jeopardy of its obligations to the Government, t h c r e p o r t liiay not be r e l e a s e d f o r non-governmental u s e such a s might c o n s t i t u t e g e n e r a l publication without t h e e x p r e s s p r i o r c o n s e n t of T h e 0ti io Stat e Uni v c r s ity R r s ea r c h Fou ndat i on.

I The Ohio State

REPORT

Columbus, OhioII43212

Sponsor

Netional Aeronautics and Space Administration Office of Grants and R e s e a r c h Contracts Washington, D. C. 20546

7 Contract--.--

' NSR -36-008 -027

Investigation of Subject of Report

Radar and Microwave Radiometric Techniques for Geoscience Experiments

.,'

-. Dielectric Constant of Vegetation at 8.5 GHzL

Submitted by

-. N. L. C a r l s o n '

E lectr OSc i ence Lab0r ator y

Department of E l e c t r i c a l Enginee ring

Date

31 M a r & 1967 ,

i

ABSTRACT A cavity perturbation technique has been developed to measure the complex dielectric constant of vegetation at microwave frequencies. Several types of vegetation ( g r a s s , c o r n , s p r u c e and taxus) w e r e m e a s u r e d and the r e s u l t s showed that the dielectric constant of each is roughly proportional to the moisture content. For freshly cut samples, of about 65% moisture content, t h e dielectric constant was approximately 25-J7. A s the samples w e r e allowed to dry out the die-

. l e c t r i c constant diminished t o about l. 5 for E' and 001 for E".

..

11

TABLE OF CONTENTS

I. INTRODUCTION 11. MICROWAVE CAVITY PRINCIPLES

A. The Idealized Cavity B. Cavity Coupling C. The Quality Factor and the Microwave Circuit 111. PERTURBATION THEORY IV. THE MICROWAVE EQUIPMENT V. MEASUREMENT PROCEDURE VI. CAVITY MEASUREMENT RESULTS VII. SUMMARY AND CONCLUSIONS APPENDIX A R EFERENC E ACKNOWLEDGMENTS

Page 1 1 2 4 13 17 23 25 27 33 34

39

40

iii

DIELECTRIC CONSTANT O F VEGETATION at 8. 5 GHz

I. INTRODUCTION

In the past, much work has been done on dielectric and magnetic p r o p e r t i e s of chemical compounds a n d solutions.1 However, c o m p a r a tively little is known about the dielectric properties for substances found f r e e l y on e a r t h . These include all types of vegetation and the many different kinds of rocks and s o i l s .

The p r i m a r y i n t e r e s t of this paper is with the m e a s u r e m e n t , at microwave f r e q u e n c i e s , of the complex d i e l e c t r i c constant of vegetat i o n , p a r t i c u l a r l y the l e a v e s , o r i n the c a s e of coniferous plants, the needles. Because these materials a r e quite irregular in shape, and because their dielectric constant may be expected to depend on their water content, a cavity perturbation method was used. This permits a c c u r a t e m e a s u r e m e n t of the properties of m a t e r i a l s with the v e r y l a r g e d i e l e c t r i c constant and l o s s tangent c h a r a c t e r i s t i c of w a t e r , and h a s the ,advantage of allowing samples of different shapes to be measured with the same equipment.

11. MICROWAVE CAVITY PRINCIPLES

The specimens of vegetation w e r e m e a s u r e d i n a microwave r e s o n a n t cavity made f r o m a section of X-band ( 8 . 2 to 1 2 . 4 GHz.) waveguide. One end of the waveguide was closed by a shorting w a l l and the other end w a s connected to a microwave network by a wall containing a s m a l l c i r c u l a r opening o r i r i s , Fig. 1.

The effect of the i r i s i s usually ignored in the elementary t h e o r y of cavity operation, i n which i t i s customary to a s s u m e that the cavity is lossless, and has no wall openings. Practically, this is unrealistic, since losses a r e present, and since there must be some coupling between the cavity and the microwave network in o r d e r t o m e a s u r e the effect of introducing the sample. Using a n iris t o s e p a r a t e the cavity f r o m the waveguide has the effect of replacing the s h o r t c i r c u i t of the ideal cavity with a l a r g e lumped susceptance. This susceptance will alter the conditions under which t h e cavity r e s o n a t e s and, accordingly, it will affect the r e s t of the microwave circuit.

1

P

Fig. 1. The cavity with one end wall containing an iris.

The Idealized Cavity In the absence of the iris and l o s s e s , the resonant frequency f r e s

is determined by the separation equation,

where: a, b and c a r e the cavity dimensions; p and E a r e the permeability and permittivity of the medium filling the cavity.

This equation r e l a t e s the resonant frequency of the cavity to i t s

dimensions and t o the permittivity and permeability of the m a t e r i a l

I

filling it. If the cavity dimensions a r e such that the dominant mode i s

the TE101, the resonant frequency becomes,

The fields for the T q O 1mode are:' 2

If l o s s e s a r e now considered, but still no iris i s added, a quality factor o r Q may be defined as follows:

(7)

- Q

=

wx energy stored average power dissipated

= wm

-

pd

The n u m e r a t o r and denominator of Eq. (7) may be determined from the usual expressions for stored energy

V cavity

Substituting Eq.( 4) gives, for the e l e c t r i c a l energyt

abc

c

We=' 2

Eo2 s i n 2 %a s i n ' C2 dv

where:

- -

(Io0

Wm = time average magnetic energy stored in cavity.

ye = time a v e r a g e electric energy s t o r e d in cavity.

Pd = time average power dissipated within cavity.

Evaluating the integral yields the following result,

T h e total amount of energy stored in the cavity at any time i s j u s t

3

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