Week 2

Question 1

Constructive interference

Answer 1

Adding two waves with a shift of 0 degrees

2 times the amplitude

Question 2

Destructive interference

Answer 2

Adding two waves with a shift of 180 degrees

Question 3

An isotropic point source generates

Answer 3

Spherical waves

Question 4

Validity of plane wave assumption

Answer 4

For a plane wave, intensity and phase are constant across planar surfaces that are oriented perpendicular to the direction of propagation

At great enough distances from the source, the spherical wavefronts will appear approximately planar across an aperture of fixed dimension D

Phase variation across D of less than 1/16 cycle (22.5 deg) is small enough to treat the waves as planar

Valid in the far field region which is R > 2D^2/λ m

Question 5

Antenna

Answer 5

Transducer that converts bound waves in a waveguide to unbound waves in free space and vice versa

Characterized by gain and beamwidth

Question 6

Directional antennas

Answer 6

Transmit and receive EM waves over a small angular region (most radars use these)

Question 7

Antenna gain

Answer 7

The increase in power density at a given point in space when the test antenna is used in place of an isotropic antenna

Varies with angle

Question 8

If quotes as just “gain”

Answer 8

The maximum (on axis, bore sight) value is inferred

Question 9

Antenna pattern

Answer 9

The variation of antenna gain with angle

Have a main lobe and sidelobes

Usually presented as gain vs angle in a plane

Question 10

Beamwidth

Answer 10

Angular extent between the 3dB points of the main beam

Question 11

Sidelobes

Answer 11

All directional antennas have sidelobes in their patterns that are defined by nulls

The forward direction produces a main beam because of constructive interference

Radar targets can appear in the sidelobes of the antenna - gives an erroneous direction

Jammers can radiate noise and false signals into antenna sidelobes

Sidelobes can be reduced but not eliminated

Question 12

Nulls

Answer 12

Directions in which no energy is radiated

Due to destructive interference of transmissions from across the antenna aperture

Question 13

Why do we have lobes

Answer 13

Interference

Because we have a physical dimension antenna, we are going to have constructive interference

Question 14

Radar targets

Answer 14

Scatter EM waves with patterns that are similar to antenna patterns

Pattern depends on size and shape of target

Scattering of EM waves described by the RCS of the target

RCS will vary with direction

Question 15

Rayleigh scatterer

Answer 15

If target has linear dimension d &laquo_space;λ then it is more ‘point-like’ and scatters the wave in all directions (more isotropic)

Ex: raindrop with d = 6 mm seen by 10 cm radar

In the optical domain, Rayleigh scattering accounts for the blue color of the sky

Question 16

Optical scatterer

Answer 16

If target has linear dimension d&raquo_space; λ then it is more ‘large’ and scatters the wave in preferred directions (more directive, unless it is a sphere)

Ex: automobile seen with λ=3 cm (x-band radar)

Question 17

Larger targets

Answer 17

Have specific scatter patterns

Ex: flat plate a,b&raquo_space; λ (behaves like an antenna with aperture a,b; energy reflected in direction of ray reflection)

Question 18

Corner reflector

Answer 18

Has a large backscatter RCS over a wide angular range (RCS will be large)

Most aircraft are seen as several corner reflectors

Question 19

Stealth technology

Answer 19

Targets such as aircraft can be designed to have low RCS in certain directions

Key elements:

• avoid corner reflectors
• use rounded surfaces
• use flat plates to direct energy away

Question 20

RCS

Answer 20

Based on an equivalence to the reflection of radio signals by a metal sphere

Question 21

To declare that a target is detected

Answer 21

Pr >= Smin

Question 22

Smin

Answer 22

Set by the noise in the receiver

Question 23

Typical value of (S/N)min

Answer 23

15 dB = 31.6

Question 24

How high should SNR be to confidently detect target

Answer 24

10-20 dB

Want to avoid false targets

Question 25

Receiver noise

Answer 25

Rx noise is thermal in origin and is assumed to be spread evenly in frequency (constant PSD) White noise Dependence of noise power on Bn creates an incentive to make the receiver BW smalle

Question 26

Radar sensitivity

Answer 26

Being able to detect a target at greater range amounts to greater sensitivity Sensitivity increases with the energy in a pulse

Question 27

AN nomenclature for US systems

Answer 27

Developed by US military in WWII AN/xxx-nn -A = army, N = navy, nn = identifier of version of radar x-letters designate - type of installation (S: ship, T: ground, transportable, F: fixed ground) - type of equipment (P: radar) - purpose (S: search)

Question 28

Svalbard ISR radar

Answer 28

498-502 MHz Pt = 1 MW 32 m and 42 m dishes 44 dBi gain

Question 29

Over the horizon radar system

Answer 29

Utilizes ionospheric refraction (bending) on HF frequencies (3-30 MHz) to see out to enormous ranges (thousands of km)

Question 30

Multiple pulses

Answer 30

Integrating the returns from the multiple pulses improves the ability to detect the target - helps because signal tends to add up systematically while noise adds up randomly - integration can be coherent or noncoherent Receiver performs the function of adding up the returned pulses

Question 31

Coherent integration

Answer 31

Adding in both amplitude and phase Gives more improvement but requires more complicated hardware

Question 32

Integration gain

Answer 32

A boost to the S/N ratio over that expected from a single pulse Improves the S/N in the radar receiver

Question 33

Losses

Answer 33

Anything that detracts from ideal S/N

Question 34

System losses

Answer 34

``` Microwave plumbing losses Between tx and antenna, rx and antenna Waveguide run Rotating joint T-R joint (duplexes) Waveguide components ```

Question 35

Beam shape loss / scanning loss

Answer 35

Surveillance radar must rotate antenna As the antenna beam sweeps over a target the target is hit with multiple pulses The pulses strike the target at different points of the antenna pattern The hits that occur away from the axis have an antenna gain that is less than Gmax -will incur a loss as compared to having the pulses all hit at the peak of the antenna pattern Usually only count pulses that strike between the 3dB points of the antenna pattern For N>10, loss is typically 1.6 to 2 dB

Question 36

Radome loss

Answer 36

If antenna has an enclosure (radome), there will be RF loss (1.2 dB two-way)

Question 37

Rx filter mismatch loss

Answer 37

Radar pulse has sinX/X spectrum Rx bandwidth filter is ~ rectangular (causes loss since sinc function hard to implement in hardware) Typically 0.9 dB

Question 38

Reason for B ~ 1/τ

Answer 38

Larger filter to get more power through - get more noise Shrinking filter - reduces noise but choking off signal