CH 2: Characteristics of RF Radiation Flashcards
Radio Frequency (RF) Radiation definition
The electromagnetic radiation transmitted and received by radar systems in order to determine range, azimuth, elevation, or velocity data
Characterized by frequency, wavelength, polarization
Radio frequency definition
Frequency unit of measurement
The frequency of the transmitted signal is the number of times per second the RF energy completes one cycle
The basic unit of measurement is the hertz (Hz). One hertz equals one cycle per second. Most radars have an RF in the million of hertz or megahertz (MHz).
Wavelength definition
Wavelength unit of measurement
Frequency and Wavelength relationship
Wavelength is a measure of the physical distance between peaks of a sine wave propagated in space.
Wavelength is measured in meters but most radar signals have wavelength measures in centimeters or millimeters.
The relationship between wavelength and frequency is inverse: the higher the frequency, the shorter the wavelength.
Polarization definition
Two types of polarization
Polarization is determined by the radar antenna and refers to the orientation of the RF wave as it travels through space.
There are two types of polarization: linear and circular
Components of traveling electromagnetic energy-2 types of fields
Relationship of Field components
Polarization is defined by the orientation of which field?
Electrostatic field and magnetic field
These fields are always perpendicular to each other and perpendicular to the direction of travel
The polarization of the wave is defined in terms to the orientation of the electrostatic field ( vertical or horizontal)
Circular polarization definition
Circular polarization : right hand or left hand orientation
The direction of the electrostatic field varies with time and traces a circular locus about a fixed plane perpendicular to the direction of propagation
For a right hand circular polarized signal, the electrostatic vector appears to rotate clockwise
For a left hand circular polarized signal, the roaring is counterclockwise
Cross polarization definition
The situation when an antenna is designed to receive a particular polarization, it will have difficulty receiving a signal with an opposite polarization.
Ex: vertically polarized jet antenna may not receive horizontally polarized threat signal
Doppler Effect definition
The ‘Doppler effect’ takes advantage of the fact that the frequency of RF waves will be changed or shifted when reflected from a target moving relative to the radar.
For a stationary target the frequency of the reflected signal will equal the frequency of the transmitted signal. For a moving target the frequency of the reflected signal will be different from the frequency of transmitted signal.
f0= transmitted frequency of radar
ft=frequency of reflected RF wave from the target
Stationary target: f0=ft
Closing target: f0ft
Electromagnetic spectrum region of radar operation
Radars operate in the microwave region, normally from 0.2-200 gigahertz
RF propagation characteristics
RF propagation is affected by earth’s surface and atmospheric conditions.
The most important propagation phenomena include: refraction, anomalous propagation (ducting), and attenuation.
Characteristics and impacts of Refraction on RF waves
-The Earth’s atmosphere bends or refracts RF waves.
Refraction is caused by variation in the velocity of propagation with altitude.
- Refraction can cause an increase in the line of site (LOS) of the Radar, effectively extending the range of the radar system.
- Refraction can also induce elevation measurement errors in radar system.
Refractivity (N) definition
Refractivity relationship with altitude
Refractivity mostly affects radars at what elevation angle?
Term used for predicting the impact of refraction on RF wave propagation.
Refractivity is a scaled up expression for the index of refraction and is used to calculate the impact of refraction on actual radar systems.
Refractivity of the atmosphere decreases with increasing altitude (barometric pressure water vapor, and temp, decrease with alt). This means velocity of RF waves increases with altitude.
RF wave refraction primarily affects ground based radar systems at low antenna elevations, especially at or near the horizon. Refraction usually not a factor at elevation angles above 5 degrees.
anomalous propagation (ducting) definition
Anomalous, or nonstandard, propagation is used to describe atmospheric conditions that extend the propagation of RF waves and increase radar range.
Most common anomalous propagation phenomena is called superrefraction or ducting.
Superrefracting characteristics and impacts
Superrefracting ducting is formed when the refractivity of the atmosphere rapidly decreases with altitude.
This occurs when the temperature increases with altitude and or the vapor content decreases with altitude.
An increase in temperature with altitude is called a temperature inversion. To produce a duct, the temperature inversion must be very pronounced.
Superrefracting ducts acf like a wave guide which traps the RF wave. Thus the RF waves take advantage of decreases in refractivity and travel much farther than normal, extending range of radar system.
Radar hole definition
The extension of radar range inside a duct can result in a reduction of radar coverage outside the duct. The area of reduced radar coverage because of ducting is called a radar hole.
Due to radar holes, the extended radar range caused by ducting may result in a decrease in radar coverage along other paths of propagation.
