MICROWAVE FUNDAMENTALS

MICROWAVE

•electromagnetic waves with frequencies that range (500MHz – 300 GHz)• high frequencies, short wavelength•An electromagnetic wave with a wavelength in the range 0.001–0.3 m. •Microwaves are used in radar, in communications, and for heating

• Microwaves are electromagnetic waves which consists of both electric and magnetic fields perpendicular to each other and propagates at the speed of light.

• Microwaves form parts of the electromagnetic spectrum with typical wavelengths from 1 millimetre to 10 centimetres – something in between light waves and radio waves.

• Microwaves are also used in telecommunications, e.g., radars, wireless computer networks and mobile phones.

• The entire universe is filled with microwave radiation left by the Big Bang explosion of the early Universe.

Energy Sources

•Visible Light is only one form of electromagnetic energy.

•Radio waves, heat, ultra-violet rays and X-rays are other familiar forms.

•All of this energy is inherently similar, and radiates in accordance with basic wave theory.

Wave TheoryElectromagnetic radiation consists of an electrical field (E) which varies in magnitude in a direction perpendicular to the direction in which the radiation is travelling, and a magnetic field (M) oriented at right angles to the electrical field.

Both these fields travel at the speed of light (c)

Wavelength and Frequency• Wavelength is measured in metres (m) or some factor

of metres such as: ▫nanometers (nm, 10-9 metres), ▫micrometers (mm, 10-6 metres) or ▫centimetres (cm, 10-2 metres).

• Frequency refers to the number of cycles of a wave passing a fixed point per unit of time. Frequency is normally measured in hertz (Hz), equivalent to one cycle per second, and various multiples of hertz.

Wave Theory

From basic physics, waves obey the general equation:

c = v Since c is essentially a constant (3 x 108 m/sec), frequency v and wavelength for any given wave are related inversely, and either term can be used to characterise a wave into a particular form.

Electromagnetic Spectrum

•The electromagnetic spectrum ranges from the shorter wavelengths (including gamma and x-rays) to the longer wavelengths (including microwaves and broadcast radio waves).

•There are several regions of the electromagnetic spectrum which are useful for remote sensing.

Visible Spectrum•The light which our eyes - our "remote sensors"

- can detect is part of the visible spectrum.• It is important to recognise how small the visible

portion is relative to the rest of the spectrum.•There is a lot of radiation around us which is

"invisible" to our eyes, but can be detected by other remote sensing instruments and used to our advantage.

Visible Spectrum

•The visible wavelengths cover a range from approximately 0.4 to 0.7 mm.

•The longest visible wavelength is red and the shortest is violet.

• It is important to note that this is the only portion of the EM spectrum we can associate with the

concept of colours.

VIOLET: mBLUE: mGREENm YELLOW:mORANGE:mRED:m

Visible Spectrum• Blue, green, and red are the primary colours or

wavelengths of the visible spectrum. • They are defined as such because no single primary

colour can be created from the other two, but all other colours can be formed by combining blue, green, and red in various proportions.

• Although we see sunlight as a uniform or homogeneous colour, it is actually composed of various wavelengths.

• The visible portion of this radiation can be shown when sunlight is passed through a prism

Infrared(IR)Region

•The IR Region covers the wavelength range from approximately 0.7 mm to 100 m - more than 100 times as wide as the visible portion!

•The infrared region can be divided into two categories based on their radiation properties - the reflected IR, and the emitted or thermal IR.

Needs for microwaves in communication

• Microwave transmission refers to the technology of transmitting information or energy by the use of radio waves whose wavelengths are conveniently measured in small numbers of centimeter; these are called microwaves.

• This part of the radio spectrum ranges across frequencies of roughly 1.0 gigahertz (GHz) to 30 GHz. These correspond to wavelengths from 30 centimeters down to 1.0 cm.

• Microwaves are widely used for point-to-point communications because their small wavelength allows conveniently-sized antennas to direct them in narrow beams, which can be pointed directly at the receiving antenna.

• This allows nearby microwave equipment to use the same frequencies without interfering with each other, as lower frequency radio waves do.

Needs for microwaves in communication

• A disadvantage is that microwaves are limited to line of sight propagation; they cannot pass around hills or mountains as lower frequency radio waves can.

• Microwave radio transmission is commonly used in point-to-point communication systems on the surface of the Earth, in satellite communications, and in deep space radio communications. Other parts of the microwave radio band are used for radars, radio navigation systems, sensor systems, and radio astronomy.

RADIATION HAZARDS

Radiation Hazard (RADHAZ) describes the hazards of electromagnetic radiation to :1) Hazards of Electromagnetic Radiation to Personnel (HERP)2) Hazards of Electromagnetic Radiation to Ordnance (HERO)3) Hazards of Electromagnetic Radiation to Fuel (HERF)

Hazard of Electromagnetic Radiation to Personnel (HERP):

Effect only possible at 10x permissible exposure limits. Cause harmful effects to humans.

Electromagnetic radiation from antennas fed by high–powered transmitters can potentially injure personnel in the vicinity of the radiating antennas. Transmitters on aircraft, aboard ship, and at air stations are potential sources of harmful electromagnetic radiation.

The danger of HERP occurs because the body absorbs radiation and significant internal heating may occur without the individuals knowledge because the body does not have internal sensation of heat, and tissue damage may occur before the excess heat can be dissipated.

Hazard of Electromagnetic Radiation to Fuel (HERF):

Create sparks of sufficient magnitude to ignite flammable mixturesThere is a potential for accidentally igniting fuel vapors by RF–induced arcs during fuelhandling operations close to high–powered radio and radar transmitting antennas. The facilityconducts radiation surveys to determine if the hazard exists in fuel handling or fueling areas.

Extremely high power electromagnetic radiation can cause electric currents strong enough to create sparks (electrical arcs) when an induced voltage exceeds the breakdown voltage of the surrounding medium (e.g.air). These sparks can then ignite flammable materials or gases, possibly leading to an explosion.

This can be a particular hazard in the vicinity of explosives or pyrotechnics, since an electrical overload might ignite them.

This risk is commonly referred to as HERO(Hazards of Electromagnetic Radiation to Ordnance).

MIL-STD-464A mandates assessment of HERO in a system, but Navy document OD 30393 provides design principles and practices for controlling electromagnetic hazards to ordnance.

Hazards of Electromagnetic Radiation to Ordnance (HERO)

Radiation Hazard (RADHAZ)

Surveys are conducted to determine the RADHAZ distance and appropriate safety precautions to ensure personnel are not exposed to power intensities exceeding established safe limits.