Microwave Antennas and Propagation

Microwave Antennas Conventional antennas can be adapted to microwave use The small wavelength of microwaves allows for additional antenna types.

Assignment Horn antenna Slot Antenna Lens Antenna Microstrip antenna Helical Antenna Parabolic Reflectors Plasma effect Propagation effect Ground effect Refer to David M. Pozar’s book for propagation characteristics of microwave antennas.

Radiation Zones

Reactive Zone and radiative near-field zone are hazardous.

Specific Absorption Rate (SAR)

Hazard of Electromagnetic Radiation to Personnel (HERP)-potential of electromagnetic radiation to produce harmful biological effects in humans. Hazard of Electromagnetic Radiation to Ordnance (HERO)- potential of electro explosive devices to be adversely effected by electromagnetic radiation Hazard of Electromagnetic Radiation to Fuel (HERF)- potential of electromagnetic radiation to cause spark ignition of volatile combustibles such as vehicles fuels. EM RADIATION HAZARD (RADHAZ):

Caused by the thermal effect of radiated energy where the body absorbs radiation 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. Eg-if the lense of the eye is exposed to microwaves, its circulatory system would be unable to provide sufficient flow of blood for cooling and may cause cataract. HERP

Eg-the stomach, intestines and bladder are especially sensitive to thermal damage from high power microwaves. Microwave frequencies for which the wavelengths are the same order of magnitude as the dimensions of human body produce close coupling between the body and the microwave field. A large amount of heat can be generated to caused severe damage to the body. Significant energy absorption will occur even when the body size 1/10 of the wavelength.

Although the biological damage occurs mostly due to the dielectric coupling, low frequency magnetic field coupling also produce damage when exposure time is large

 Microwave energy is dangerous to ordnance like weapon system, safety and emergency devices and other equipment containing sensitive electro explosive devices (EEDs).  Radiated fields can cause unintentional triggering of EEDs  High intensity RFR fields produced by modern radio and radar transmitting equipment can cause sensitive electrically initiated devices (EIDs) classically known as electro-explosive devices (EEDs), contained in ordnance systems to actuate prematurely. HERO

 Ordnance is more sensitive than human partially because they do not have circulatory system to dissipate internal heat.  However, EEDs can more easily be protected from the effects of RF energy than humans by enclosing them with metallic enclosures which reflected back the incident microwave energy  The potential dangers to ordnance and fuels are obvious because there could be an explosive "chain reaction" by exploding; consequently, these limits are generally lower than personnel limits.

Occurs due to possibility of accidently igniting fuel vapours by RF-induced areas during fuel handling operations proximity to high level RF fields.(during fuel handling operations close to high powered radar and radio transmitting antennas. ) The probability of ignition may be significant for more than 50 volt-ampere arc. HERF

HERF precautions are of more general concern to fuel truck operators. However, some general guidelines include: Do not energize a transmitter (radar/comm) on an aircraft or motor vehicle being fueled or on an adjacent aircraft or vehicle. Do not make or break any electrical, ground wire, or tie down connector while fueling. Radars capable of illuminating fueling areas with a peak power density of 5 W/cm2 should be shut off.

For shore stations, antennas radiating 250 watts or less should be installed at least 50 ft from fueling areas (at sea 500 watts is the relaxed requirement). For antennas which radiate more than 250 watts, the power density at 50 ft from the fueling operation should not be greater than the equivalent power density of a 250 watt transmitter located at 50 ft.

 International Radiation Protection Association (IRPA) sets the Permissible Exposure Levels (PEL) for the general public  IRPA guidelines are divided into 2 categories: ◦ Occupational Permissible Exposure Levels (applies to personnel who work in the vicinity of RF for 8 hours a day) ◦ General Public Permissible Exposure Levels (applies to resident who are exposed to the RF electromagnetic environment throughout the year Radiation Hazard Limit

FCC Safety Limit Safe Hazard

MPEL (ANSI’s 1982 RF Protection Guide) Safe Hazard

Can be practised by preventing radiation from entering into the beam of the transmit antenna or from coming close to any microwave generators or propagating medium. In areas where high power radar are used, the service and maintenance personnel must wear microwave absorptive suit (made of stainless steel woven into a fire retardant synthetic fiber) Radiation Protection