Indian Journal of Radio & Space Physics Vol. 33, December 2004, pp. 399-404

Moisture dependent Ku band microwave characteristics of red soil

Vijaya Puri, S Darshane & S Shaikh

Thick and Thin Film Device Lab. , Department of Physics, Shivaji University, Kolhapur 416 004

Received 6 November 2003; revised 12 April 2004; accepted 24 June 2004

In this paper a study of the Ku band (12-18 GHz) properties of red soil obtained from Shivaji University, Kolhapur, due to moisture has been reported. The attenuation in the transmitted and reflected mode of the wetted soil has been measured using standard gain hom antenna. The effect of the angle of incidence (0° and 45°) is also reported. For very low moisture content, angle of incidence and angle of reflection dependent effects are observed. The results indicate that for very low moisture content detection in red soil, angle of incidence of 45° is more suitable and angle of reflection of 60° is most sensitive.

Key words: Ku-band, Microwave, Red soil, Soil characteristics PACS No: 92.40.Lg; 95 .75. Rs; 84.40. -x IPC Code: G 01 N 33/24

1 Introduction The type of soil is a very important aspect for

determining the yield of the crop. Depending on the geographical region the nature of the soil varies. Red soil is found in Konkan area abundantly. It is used for agricultural and horticultural purposes. Soil moisture plays a critical role in the distribution and flow of water within and between many ecosystems. The moisture in the soil also governs the type of vegetation grown in that soil. For remote sensing purposes the reflectivity of the soil due to different degrees of wetness is a very important parameter.

Microwave measurement in free space is a very useful method for non-destructive and contactless measurements'·3. The dielectric constant of the soil is influenced by the moisture content4

-8

, which affects the transmission and reflection properties of the soil. The 13-IS GHz range of microwaves is very useful, since in this range the sensitivity of microwaves towards moisture is more and the density effects on wave attenuation are minimal9

.

In this papcr, a free space transmission and reflection method, using Ku band (12-18 GHz) standard-gain-horn-antenna to measure the attenuation through the wetted red soil, is reported. The red soil was collected from the garden in the campus of Shivaji University, Kolhapur.

. 2 Experimental procedure Red soil was collected at the end of monsoon . The

soil was quite wet. It was initially dried in an oven at 3S0°C for two hours . This temperature was sufficient

to completely remove the moisture. The soil was sieved to remove the stones and other bigger particles and ground to fine size. The soil was packed tight in a plastic container of size 9.S cmx6.S cmx1.S cm. Care was taken to minimize air gaps. Initially the weight of the dry soil was measured gravimetrically . The moisture in the soil was varied by adding fixed quantity of water and mixing thoroughly and allowing time for uniform absorption. Care was taken that no soil was lost during mixing. The moisture percentage from 1 % to 2S% was calculated on wet basis by gravimetric method. For obtaining uniform moisture content throughout the entire sample, the soil with weighed quantity of distilled water was mixed thoroughly by rotating in a sealed vessel and allowed to equilibrate for three hours.

The microwave measurement system consisted of the Ku band (12-18 GHz) waveguide set-up consisting of Gunn oscillator, isolator, variable attenuator, horn antennas and detector. Two identical horn antennas were used for transmission and reception and were at a distance of 30 cm from each other. The container with soil was placed at a distance of IS cm from the transmitting antenna in between the two antennas in the path of microwaves . The experimental set-up is shown in Fig. 1.

Two types of measurements were done: (i) transmission through the soil when soil was kept at an angle of incidence of 0° and receiver antenna at 180° and (ii) transmission and reflection from soil , when soil was kept at an angle of incidence of 4So. The transmitted and reflected microwaves were measured

400 INDIAN J RADIO & SPACE PHYS, DECEMBER 2004

by moving the receIving antenna from 30° to 180°. The angle of incidence of 45° was measured as the angle between the normal to the soi l container surface and the main axis of the transmitting an tenna. The transmitting antenna was fixed and the receiving antenna was moved with reference to the normal of the soil container, the angular measurements being the angle between the surface and main axis of the receiving antenna.

Initially the empty container was characterized and it was found that it did not change the characteristics

DETECTOR HORN ANTENNA

of the antenna, except producing a decrease in output by 5-6 mY.

3 Results and discussion

3.1 At angle of incidence 0°

Figure 2 shows the output versus frequency plot of the red soi l when dry and wet. From Fig. 2 it is seen that as the percentage of moisture increases the output decreases as compared to dry soil. There is no drastic change in the shape of the curves due to moisture. The

Fig. I- Schematics of the experimental set-up

140

120

~ 100 ~.

~ ::J 80 o o

~ i 1/1 Z

g 40

20

Effect of moisture

13.4 13.8 14.2 14.6

~d~ ~1% ~3% ~6%

15 16.4 16.8 16.2 16.6 17 17.4 17.8

FREQUENCY, GHz

Fig. 2- Effect of moisture on transmission th rough red soil when angle of incidence is 0°

PURl et al.: Ku BAND STUDIES OF WETTED RED SOIL 401

soil with 3% moisture shows highest transmission in the 13.4-15.2 GHz range. There is a large difference in the output due to 3%, 6% and 10% moisture in the soil. Beyond 15.4 GHz the output decreases and becomes almost zero beyond 17 GHz even for the dry soil.

3.2 At angle of incidence 45°

When angle of incidence was 45°, for the transmitted output four angles were studied, namely, 140°, 150°, 165° and 180° and, for the reflected output, six angles were investigated, i.e. 30°, 45°, 60°, 75°, 90° and 120°. The transmitted and reflected outputs for the dry soil are shown in Fig. 3. From Fig. 3 it is seen that in the frequency range 13.4-15.2 GHz, the transmission increases as the angle of transmission increases. A large difference is observed between 180° and 140°. In the case of reflection from the dry red soil, from 30° to 60°, the reflected output

is very low in the entire Ku band. At 75° the reflection shows a dramatic increase in the frequency range 14-15.2 GHz. As the angle of reflection is increased the output decreases again . Beyond 16 GHz both the transmitted and reflected output are almost zero.

The effect of moisture on the transmitted and reflected outputs is given in Figs 4 and 5, respectively. Angle of transmission and reflection dependent effects are observed. As seen from Fig. 4 in the 1 % and 3% moisture ranges, the transmission at 180° is very high. A very interesting result obtained was that, the transmission at 140° due to most of the moisture content was higher (23.4-50 mY) than that for the dry soil (20.3 mY max) . At 6% moisture there is a drastic decrease in the transmitted output, when observed at 180°, which persists till 25% moisture. At the other angles of transmission, an increasing tendency is observed (-1 0 mY at 10% moisture to

45 .--------------------------------------------.

> e I-~

::l 0.. I-::l 0 C W l-t.> w ..J LL W II:

• 120 deg ....... 90 dug

....... 75 dug -60 dug

....... 45 dug ....... 30 dug

30

FOR DRY SOIL

15

o~~~~~~~~~~~~~~~ 13.4 14.4 15.4

FREQUENCY GHz

16.4 17.4

180,-------------------------------________________ ~

.... :J

160

140

~ 120 :J o o 100 UJ .... i 80

! 60 ....

40

20

14.4

· .180deg ....... 165d .. g

-150 deg -140 deg

TRANSMISSION FOR DRY SOIL

15.4 FREQUENCY,GHz

16.4 17.4

Fig. 3- ReOcction (upper) and transmission (lower) of the dry red soil when angle of incidence is 45°

402 INDIAN J RADIO & SPACE PHYS, DECEMBER 2004

leo leo

1% rno;.tu ... ~~tu,. -1eodeg -1Udeg

~150deg -140deg 120 . 120

eo 80

40 40

;> E ~

f- 0 ~ 0 13.4 14.4 16.4 18.4 17.4 ~ 13.4 14.4 15.4 18.4 17.4 f-~ 0 180 ~ 180 iol f-f- 10%moiatu,.. ... a%m.,.tu ... ~ 120 120

'" ~ f-

80 · 80

40 40

0 0

13.4 14.4 15.4 1a.4 17.4 13.4 14.4 15.4 18.4 17.4

FREQUENCY, GHz

Fig. 4-Effect of moisture on transmission through red soil when angle of incidence is 45°

-50 mY at 25% moisture). Beyond 15.4 GHz the output is almost zero.

From Fig. 5 it is seen that when the angle of reflection is 30° and 120° the reflected output is least. For the other angles of reflection highly moisture sensitive output is obtained in the 13.5-15 GHz range. Some interesting behaviour is observed due to 20% and 25% moisture for the angle of reflection of 90° and 75°. When moisture is 20%, the output is high for angle of reflection of 90°, whereas the output becomes high for 75° angle of reflection when moi sture is 25 %. In the reflected mode also the output is almost zero beyond 16 GHz.

The peak output and peak frequency due to moi sture is tabulated in Table 1. From Table 1 it is seen that angle of incidence and angle of observation­dependent changes are obtained. When angle of incidence is zero degree, as the moisture content increases up to 3% the peak transmission and peak frequency increases. At 6% moisture there is a sharp decrease in the peak output transmission which again increases at 20% moisture. The peak frequency

becomes minimum at 6% moisture and again increases at 10%. Due to angle of incidence 45°, in most of the observation angle, the output decreases with moisture, exception being at 60° and 165°. Here also a sudden increase at 20% moisture i- observed .

Electromagnetically soil is a mixture consisting of air, bulk soil, bound water and free water. At microwave frequencies the electric energy is strongly absorbed by water due to their dipole character iO

. The bound water interacts with the electromagnetic wave exhibiting dielectric dispersion spectrum different from that of free water molecule. At lower moisture content there are more bound water molecules in the soil as compared to free water molecules. This might be the reason for the different trends shown at 20% moisture. According to Njoku et at. I I , the real and imaginary parts of the dielectric constant of sand grains increase as the moisture content increases from 3% to 30%. Starting with the dry material, the first monolayer of added water is attached to the host material by chemisorption, whereas at higher moisture content the water is held by capillary forces 12 . The

PURl et 01.: Ku BAND STUDIES OF WETTED RED SOIL 403

80 .------------------------------------. 80 .-----------------------------------,

1% moiatul1l -- 120 deg ~!l0 deg ~711 deg 3% moiatu,.

80 -- 110 deg ~45 deg ~30 deg 80

40 40

20 20

;, e

!-< 0 0

~ 13.4 14.4 1U 18.4 17.4 13.4 14.4 15.4 111.4 17.4

"" !-< ~ 0 80 80 Q r.;1 !-< 8% moiatul1l 10"4 molatu,.

U r.;1

80 80 ~ r;.;.

~ 40

20

O~~~~~~~~ __ ~ 13.4 14.4 1U 111.4 17.4 13." 14,.4 1U 111.4 17.4

FREQUENCY , GHz

Fig. 5-Effect of moisture on reflection from red soil when angle of incidence is 45°

Table I-Data of peak output and peak frequency

Condition Peak outEut (mY) at moisture % Peak freguenc~ (GHz) at moisture % 0 3 6 10 14 20 25 0 I 3 6 10 14 20 25

AI=O°

Transmission 94.4 101.7 114.2 55.2 39.9 38.7 61.1 38.8 14.9 15.0 15.1 14.2 15.3 15.2 15.2 14.8

AI=4So

Transmission 180° 160.6 134.2 145.3 36.7 24.9 50.9 67.2 61.8 15.1 14.9 15. 1 15.4 15.1 14.9 15.1 15.1

165° 30.1 38.8 33.9 44.2 43.5 61.2 70.4 59.4 14.6 14.2 15.3 15.0 15. 1 15.0 15.1 15.1

150° 65.5 51.5 32.8 24.6 43.3 52.1 45 .5 56.1 15.1 14.3 14.4 15.1 15.1 15.0 15.1 15.0

140° 20.3 18.2 20.9 23.1 26.1 34.0 47 .7 41.9 15.1 15.1 15.1 15.1 15.0 15.0 15.0 15.2

Reflection 120° 18.2 6.4 7.0 4.6 5.3 4.5 8.8 11.4 15.1 14.9 14.9 14.7 14.2 14.2 15.3 15.0

90° 20.5 11.3 2.8 20.5 19.4 24.9 69.7 22.3 14.9 15.1 15.2 15.2 15.1 15.1 15.1 14.9

75° 40.2 39.8 26.6 32.9 32.1 22.8 16.9 32.6 15.1 14.2 15.0 14.9 15.0 15.0 14.8 15.3

60° 4.6 25 .5 46.3 11.9 37.5 38.5 33.0 15.7 14.3 14.9 15.2 15.1 15.1 15.1 15.0 15.1

45° 2.3 5.5 6.9 6.2 14.1 17.9 7.7 17.8 14.0 14.1 14.1 14.6 14.2 14.4 14.2 15.2

30° 1.2 1.9 4.3 3.7 7. 1 2.5 14.1 26.4 14.2 14.3 14.4 15.2 15.0 15.0 15.0 15 .0

Note: AI- Angle of incidence

404 INDIAN J RADIO & SPACE PHYS, DECEMBER 2004

dielectric properties of the soil depend on a number of factors like total water content, relative fraction of bond and free water, soil type, frequency etc l3

. Since the di electric constant of water is -80, variations in moi sture content make significant change in the dielectric constant of the red soil leading to changes in the output. Angle of incidence-dependent sensitivity to soil moisture has been found to be affected by surface roughness 14. Reflection measurements are also affected more by the surface characteristics . Effects due to scattering, multiple reflection s etc. also contribute to the losses in the reflection mode.

More detailed investigations for other angle of incidence and moisture content greater than 25 % are in progress.

4 Conclusions Although passive microwaves have been used for

study of soil , the use of active microwaves both in the transmission and reflection mode will provide better understanding of the water and soil relationship. Ground-based estimate of soil moisture is very important for calibration purposes. The results obtained in this work indicate that the reflection angle and incident angle-dependent data generated could be used in the low moisture content range for soil moisture studies. For very low moisture content, the angle of observation of 60° and 90° are very sensitive, whereas for higher moisture content 30° seems to be more sensitive, if attenuation observations are to be

done. If frequency change data are to be used, 45° angle of observation seems to be useful.

Acknowledgement One of the authors (VP) gratefully acknowledges

the University Grants Commiss ior~ , Indi a, for awarding her the post of research scient1st.

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