1K .Prasanth , 2K .Shashank , 3M .Ashok , 4M . AV H Krishna , 5Zachariah C Alex
1B.TECH (ECE),VIT University,Vellore,2B.TECH(ECE),VIT University ,Vellore, 3B.TECH (ECE),VIT University,Vellore,4B.TECH(ECE),VIT University ,Vellore, 5Senior professor, VIT University, Vellore.
Microwave techniques can be used to measure soil moisture content, mineral content and fertility of the soils. A microwave set up is designed for laboratory evaluation of soil moisture by measuring the output power by transmitting microwaves through soil samples. A relation between output power and soil moisture is obtained by analyzing the output power data for the soil samples. The relation is verified using known values of the moisture evaluated using gravimetric analysis. Experiments were conducted in the x- band frequency range of (8-12 GHz).Further an embedded system can be developed for the in-situ measurements of soil moisture.
Moisture content in soil is an important parameter which is useful in different fields such as agriculture, civil engineering, forestry and environmental studies. Determining the amount of soil moisture helps to determine whether a region has enough soil moisture to assist plantation. At present microwave techniques including passive microwave radiometry  and active microwave remote sensing are playing an important role in determining soil moisture. But ground truth data is required for the correct evaluation of soil moisture. Soil moisture content can be retrieved using different techniques such as gravimetric analysis, volumetric analysis, Tensiometric techniques etc. in the laboratories. Though these techniques are widely used for laboratory evaluation of soil moisture content, they are time consuming. In this paper, a microwave technique is proposed to determine soil moisture content in relation with the output power.
I I .SAMPLE ANALYSIS
To perform the experiment, soil samples were collected from different regions. Texture analysis was performed using sieve analysis. In sieve analysis , if the granular size is greater than 60 microns it is classified as sand, and if the granular size is less than or equal to 60 microns then it is classified as silt and clay. The composition of the soil samples used in the experiment are given in (Table No .1)
I I I .EXPERIMENTAL SETUP AND PROCEDURE
Different soil samples were dried in an oven at 100 degree centigrade for 24hrs. Different levels of moisture conditions were obtained by adding double distilled water to the dry samples and allowing it to stand for 24 hrs to aid settling. Gravimetric soil moisture content was calculated by using gravimetric soil analysis using the following formula.
Moisture content = wet weight (sample)-dry weight (sample)/dry weight (sample)
Klystron power supply along with reflex klystron tube acts as microwave source and operates in 8-12 GHz range. Microwave source is connected to an isolator for unidirectional propagation. Isolator connected to slotted section which is in turn connected to Directional coupler, whose coupled port is connected to a rectangular waveguide. Waveguide is filled with a known amount of soil sample and microwaves are transmitted through wave guide. Output power obtained from detector mount, mounted over the waveguide is noted with the help of a VSWR meter (as P out). Similar procedure is repeated for each sample of different textures, with different moisture levels. Soil moisture percentages are determined using gravimetric analysis. All power readings were taken at room temperature.
I V .RESULTS & DISCUSSION
Output power (db) versus moisture percentage graphs were plotted for all the 5 samples(Table No 3-7). All the equations for different soils can be made into single equation if we know the composition of sand, silt and clay...