# Introduction S with the increase in demanded service quality and data rate the conventional approach of the data transmission is getting upgraded. To achieve the demanded service compatibility various high ranges have emerged in recent past. Therefore modified approaches of microwave system are in developing process. The most important advantages of Modifies microwave system are the availability of antennas with high directive gain and large bandwidth. At such high frequencies, for example 1% bandwidth at 600 MHz is 6 MHz (the bandwidth of the single television channel) and at 60 GHz, 1% bandwidth is 600 MHz (100 television channels). But, on the other hand, at frequencies about above 10 GHz, the electromagnetic radiation starts interacting with neutral atmosphere and also with various meteorological parameters, in particular, precipitation, producing absorption of energy, and thus attenuation of signal levels. Implicit in these predictions of losses is a detailed knowledge of the physical mechanism of the various meteorological parameters and their interactions with electromagnetic radiation. Author : Associate Professor, ECE Department, St.Johns College of Engg & Tech., Yerrakota, Yemmiganur, Kurnool, A.P.India. E-mail : sudhakar_403@yahoo.co.in Author ? : Principal, Santhiram Engineering College, Nandyal, Kurnool, A.P., India. E-mail : mvsraj@yahoo.com The adverse weather causes microwave signal degradations mostly due to rain and suspended particles like fog and water vapor. Atmospheric gases cause signal attenuation through molecular absorption in certain characteristic frequency bands (Zvanovec et al., 2007). A very large number of gases exhibit resonant absorption features. But, only a few have a major impact on signal propagation through the earth's atmosphere in the wavelength range of interest. Molecular oxygen and water vapor at millimeter & sub millimeter wavelengths are the most important constituents. In order to increase transmission bandwidth, the current systems of operations are upgrading their operating frequency. Microwave signals in the new frequency band are expected to have higher propagation losses than in the 1.4-2.4 GHz (L and S bands) band due to atmospheric attenuation and terrain interference (Suen et al., 2008). The impact on microwave power link margin due to the frequency increase is been evaluated in this paper. # II. # Atmospheric Attenuation The effects of atmospheric and weather are more significant on 3-30 GHz frequency band and are not negligible as at the 1.4-2.4 GHz frequency band which the military is using now. There are mainly two types of attenuations that will affect the power margin at higher frequencies (Federici et al., 2005). One is the atmospheric gaseous absorption, while another is the rain attenuation when microwave signals pass through the rain. Additional environmental phenomena, such as, cloud, fog, ice, snow, aerosol, dust, etc., can also cause severe signals impairment as increasing operating frequency (Johnson et al., 2008). Several anomalous propagation modes (such as ducting and tropospheric scatter) also play major roles in trans-horizon interference for a very small percent time. At low elevation angle, the atmospheric scintillation and multipath fading become significant. Atmospheric absorption, clouds, fog, precipitation, and scintillation incur losses in a transmitted signal (Fiorino et al., 2009). Previously, these losses were deemed negligible at the lower frequencies. As the frequency increases, this method is not acceptable. It is necessary to identify all the propagation mechanisms and estimate attenuation that might arise in the new frequency band. # Propagation Modeling The models of the atmospheric gaseous absorption and rain attenuation for various rainfall rates were studied to modeled the propagation effect at 3-30 GHz. Atmospheric absorption and rain attenuation mainly occur at low altitudes, an area called as the troposphere. There are several models for the atmospheric attenuation calculation. They are mostly regional dependence. The principal interaction mechanism between radio waves and gaseous constituents is molecular absorption from molecular oxygen and water vapor in the atmosphere (Zvanovec et al., 2007). The oxygen volume ratio in the gases is quite stable, while the water vapor density varies a lot, with strong regional and seasonal dependence. Within the studied frequency band, there was an absorption line at 22.235 GHz (Koshelev et al., 2007). The following equations are used to plot the attenuation of oxygen and water vapor for the horizontal path, the vertical path, and different elevation angles over a specified frequency range. For oxygen, specific attenuation in the horizontal dependence is given as: ?? 0 = ?7.19 × 10 ?3 + 6.09 ð??"ð??" 2 +0.227 + 4.81 (ð??"ð??"?57) 2 +1.50 ? ð??"ð??" 2 × 10 ?3 ???? (1) Where f is frequency in GHz. For water vapor, specific attenuation in the horizontal dependence is given as ?? ?? ?0. The oxygen and water vapor equivalent heights are given as: ? 0 = 6(3)? ?? = 2.2 + 3 (ð??"ð??"?22.3) 2 +3 + 1 (ð??"ð??"?183 .3) 2 +1 + 1 (ð??"ð??"?323 .8) 2 +1 ????(5)?? ?? = ? 0 ?? 0 +? ?? ?? ?? ???????? ????(4) Using the ITU gaseous absorption model, we have calculated attenuations due to both compositions along horizontal and vertical paths. Total zenith losses and its elevation angle dependence also are calculated and plotted. The losses at 3, 6, 12, and 24 GHz are estimated respectively. Rain and other hydrometeors, such as hail, ice, and snow, can cause severe attenuation for higher frequency signals. Water drops will absorb and scatter energy from incident waves. This absorption and scattering causes the attenuation to increase exponentially as the frequency increases (kim et al., 2004). The attenuation coefficient is also strongly dependent on rainfall rate. ITU models on "Attenuation by Hydrometeors, in Particular Precipitation, and Other Atmospheric Particles" were used to plot the attenuation of rain different elevation angles and different rainfall rates over the specified frequency range. This model shows that total specific attenuation rate, ? R , is a function of rain fall rate, R, as ?? ?? = ???? ?? ???? ????/????(7) Where two coefficients ? and k are functions of signal's frequency and elevation angle and have been experimentally determined in the model. Clouds and fog can be described as collections of smaller rain droplets. Different interactions from rain as the water droplet size in fog and clouds are smaller than the wavelength at 3-30 GHz (Kim et al., 2003). Attenuation is dependent on frequency, temperature (refractive index), and elevation angle (Podobedov et al., 2004). It can be expressed in terms of the total water content per unit volume based on Rayleigh Approximation: ?? ?? = ?? ?? ?? ????/????(8) Where: ? c: specific attenuation (dB/km) within the cloud K l : specific attenuation coefficient [(dB/km)/(g/m 3 )] M: liquid water density in the cloud or fog (g/m 3 ) To obtain the attenuation due to clouds for a given probability value, the statistics of the total columnar content of liquid water L (kg/m 2 ), which is an integration of liquid water density, M, in kg/m 3 along a column with a cross section of 1 m 2 from the surface to the top of clouds, or, equivalently, mm of perceptible water for a given site must be known yielding: A = LK l /sin?, dBfor 90° ? ? ? 5° (6) Where ? is the elevation angle. In additional to the line of sight propagation, the radio wave can propagate trans horizontally through several anomalous models. Anomalous modes propagation mechanisms depend on climate, radio frequency, time percentage of interest, distance, and path topography (Hils et al., 2008). At any one time a single mechanism (or more than one) may be present. The dependence on elevation angle is then taken into account. # Global Journal of Researches in Engineering The path loss during the signal propagation is defined by the Friis Equation used to estimate distance (9) realated loss for free space or an atmospheric medium but at lower frequency (generally < 3 GHz). The effect of propagation for the developed approach is evaluated the observation obtained for the value of attenuation at different frequency of transmission is evaluated. Where EIRP is effective isotropically radiated power in dBW; and, When representing the Friis Equation in decibels (dB), we have # IV. BSERVATIONS # Conclusion The results show that for a rainfall rate of 50 mm/hour, rain attenuation at 30 GHz is about 10 dB/km, while it is only 1 dB/km at 9 GHz. Thus, the rain attenuation is the main problem at higher frequency for heavier rain. ![Journals Inc. (US) ? III.](image-2.png "") 353![km where f is frequency in GHz and ? is the water vapor density in g/m In this study we have selected a maximum value of 12 g/m 3 and an average value of 7.?? ?? = ?? 0 + ?? ?? ????/????](image-3.png "3 . 5 g/m 3 .") ![Where frequency, f, in GHz, distance, d, in km.](image-4.png "") 12![Figure 1 : Specific attenuation of water vapor (densities 7.5 and 12 g/m 3 ) for horizontal path (1-30GHz)](image-5.png "Figure 1 :Figure 2 :") 3![Figure 3 : Attenuation of oxygen and water vapor (density 7.5 g/m 3 ) for horizontal path (1-30GHz)](image-6.png "Figure 3 :") 45![Figure 4 : Elevation angle dependance of atmospheric attenuation for 3,6,12 & 24GHz(water vapor density 7.5 g/m 3 )](image-7.png "Figure 4 :Figure 5 :Global") 6![Figure 6 : Specific attenuation due to rain for an elevation angle of 90 0 V.](image-8.png "Figure 6 :") © 2013 Global Journals Inc. (US) © 2013 Global Journals Inc. (US) Propagation Power Loss Analysis and Evaluation under Variant Atmospheric Conditions ## Acknoweledgement The author thnaks Dr. M.V. Subramanyam, Principal Santhiram Engineering College, Nandyal, for his suggestions and guidance in preparing the research article. Also the author Thank the Management and the Principal of St. Johns College of Engineering and Technology, Yemmiganur for their kind cooperation and help in preparing the article. * A computational tool for evaluating THz imaging performance in brownout or whiteout conditions at land sites throughout the world STFiorino RJBartell MJKrizo SLMarek MJBohn RMRandall SJCusumano Proceedings of the SPIE -The International Society for Optical Engineering the SPIE -The International Society for Optical Engineering 2009 7324 12 * Thz standoff detection and imaging of explosives and weapons JFFederici DGary RBarat DZimdars Proceedings of the SPIE-The International Society for Optical Engineering the SPIE-The International Society for Optical Engineering 2005 5781 * Research radar for unmanned navigation DJohnson GBrooker 2008. 2008 International Conference on Radar * A W-band quasioptical homodyne Doppler radar for detection of very slow-moving targets JYSuen RSSingh ZDTaylor ERBrown 2008 * Gas Attenuation Measurement by Utilization of Fabry-Perot Resonator SZvanovec PPiksa PCerny MMazanek PPechac 2007 * 2-d Acoustic Phase Imaging with Millimeter-Wave Radiation ARedo-Sanchez GKaur ZXi-Cheng FBuersgens RKersting Microwave Theory and Techniques 2009 57 * On the development of a multifunction millimeter wave sensor for displacement sensing and low-velocity measurement SKim CNguyen Microwave Theory and Techniques 2004 52 * Terahertz profilometry at 600 GHz with 0.5 mu m depth resolution BHils MDThomson TLoeffler WVon Spiegel CAm Weg HGRoskos PDe Maagt DDoyle RDGeckeler Optics Express 16 15 2008. JUL 21 * A displacement measurement technique using millimeter-wave interferometry SKim CNguyen Microwave Theory and Techniques 2003 51 * Thz laser study of self-pressure and temperature broadening and shifts of water vapor lines for pressures up to 1.4kPa VBPodobedov DFPlusquellic GTFraser Journal of Quantitative Spectroscopy & Radiative Transfer 87 3-4 2004 * Broadening and shifting of the 321-, 325-, and 380-GHz lines of water vapor by pressure of atmospheric gases MAKoshelev MYTretyakov GYGolubiatnikov VVParshin VNMarkov IAKoval Journal of Molecular Spectroscopy 241 2007 * US) Guidelines Handbook Global Journals Inc 2013