Baluns

Balun is a balance to unbalance transformer. This is a circuit element which is used for the connection of a balanced line to an unbalanced line. This is also preferred for antennas. The most common application is in the connection of an unbalanced coaxial line to a balanced antenna. For this an ordinary tuned transformer is used which is operated at low frequencies. Here primary is unbalanced and the balanced antenna is connected to centre-tapped secondary. Adverse effects due to stray capacitances can be minimized by the usage of an earthed electrostatic shield. For high frequency operations, a bazooka balun is used. It is also known as choke or sleeve and is used for narrow band applications. Here a quarter-wavelength sleeve is placed around the outer conductor of the coaxial line.

Liquid propellants

Monopropellant and bipropellant are the two basic types in this category. Monopropellants do not require an oxidizer and produce thermal energy by decomposition either by ignition or by a catalyst. Since monopropellants cause hazards, they are not preferred. Its use is limited. It is used as a source of power to the turbine in a pump feed system. Examples of monopropellants are hydrogen peroxide, hydrazine and nitromethane. Biopropellant is a combination of an oxidizer and a fuel. On combining, this produces thermal energy. Some important combinations are hydrogen peroxide and gasoline, nitric acid and gasoline, liquid oxygen and alcohol, liquid oxygen and liquid hydrogen, liquid oxygen and hydrazine, liquid fluorine and ammonia, liquid fluorine and liquid hydtrogen, nitric acid and aniline, nitrogen tetroxide and hydrazine, hydrogen peroxide and ethyl diamene, ozone and cyanogens etc. Some combinations are hypergolic. The latest in this category is tripropellant which is a combination of metallic fuels and oxygen. The characteristics of liquid propellants are high calorific value, less toxicity, smooth ignition, high density, high reliability, negligible viscosity change with temperature, longer life span, low corrosion characteristics, easy to handle, easy to store and chemical stability.

Hydrologic cycle

The space on earth where water exists is called hydrosphere. Hydrosphere is spread over 15 km in earth’s atmosphere and down 1 km into the earth’s crust. There exists a hydrologic cycle through which water circulation is maintained in the hydrosphere. Hydrologic cycle is continuous and without any beginning or end. This cycle starts with the water evaporation from ocean surface and lands. This vapour becomes a part of the atmosphere. After some time condensation of water vapour occurs and results in precipitation, interception by vegetation, overland flow, subsurface soil flow, surface runoff etc. In the earth water is stored in oceans, groundwater (fresh and saline), ice, soil, snow, lakes, rivers, marshes and in atmosphere. Many physical parameters influence the hydrology of a region. Some examples are vegetation, topography, geology, human activities etc.