1

RADAR UNWANTED EMMISSIONS

A personal viewJ R Holloway

All data in this presentation comes from public domain sources

ITU WP 8B Radar Seminar

September 2005 GENEVA

7

Unwanted Emission Limits Before 2003 no SE limit for radar From 2003 new radars must meet Cat A or Cat B SE limits

Cat A -60 dB Cat B -100 dB

Class B being proposed to be adopted in Europe. OOB Definition of the extent by the emission masks

Current Mask Design Aim

Status of Limits SE levels part of radio regulations Boundary part of regulation OOB mask is a recommendation Design aim for new OOB 2006/2012

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Current Unwanted Emission Limits Cat A&B

Current Unwanted Emission Mask

-120

-100

-80

-60

-40

-20

0

0.1 1 10 100

x BW-40dB Bandwidth

Po

we

r d

B

Cat A

Cat B

9

Design Aim

When the OOB Mask was introduced a design aim was also introduced.

This proposed to increase the Roll off to 40 dB/dec

If this is not agreed then the aim falls JRG is considering what should

replace the design aim

10

Design Aim Unwanted Emissions Cat A&B

"Design Aim" Unwanted Emission Mask

-120-100

-80-60-40-20

0

0.1 1 10 100

x Bw-40dB

Po

we

r d

B

Cat A

Cat B

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Problems With Current Mask

Mask perceived to be too relaxed at estimating –40 dB Bandwidth

Mask perceived to be too relaxed in terms of Roll-off for trapezoidal pulses

Magnetron Radars find it difficult to meet current mask Impossible to meet design aim

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Problems With Mask

Mask perceived to be too relaxed at estimating –40 dB Bandwidth

Mask perceived to be too relaxed in terms of Roll-off for trapezoidal pulses.

Magnetron Radars find it difficult to meet current mask Impossible to meet design aim

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Sensitivity of Equation for Bw-40 FM Pulsed

Bw-40dB gets large when tr0 Bc gets large

rc

r t

AB

tt

KB 240

15

FM Trapezoidal Pulses vs Mask

Mask 15 MHz

Value 10 MHz

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Practical Bandwidths

  Measured MHz

    Calculated 16 MHz

 

3 dB BW 20 dB BW 40 dB BW 3 dB BW 20 dB BW 40 dB BW

2.5 3.6 10 2.5 7.8 25.7

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Problems With Mask

Mask perceived to be too relaxed at estimating –40 dB Bandwidth

Mask perceived to be too relaxed in terms of Roll-off for trapezoidal pulses.

Magnetron Radars find it difficult to meet current mask Impossible to meet design aim

19

Trapezoidal Pulse

Two roll-off rates 20 dB/dec 40 dB/dec

20 dB/dec

40 dB/dec

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Problems With Mask

Mask perceived to be too relaxed at estimating –40 dB Bandwidth

Mask perceived to be too relaxed in terms of Roll-off for trapezoidal pulses.

Magnetron Radars find it difficult to meet current mask Impossible to meet design aim

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Magnetron: Difficult to meet current OOB Limits

Marine Navigation Radar X band Magnetron - Short Pulse Mode

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

8.00 8.25 8.50 8.75 9.00 9.25 9.50 9.75 10.00 10.25 10.50 10.75

Frequency (GHz)

Radar Antenna Fixed

Radar Antenna Rotating

Mask

Failure

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Coaxial Magnetron: Cat B Limits

Meteorological Radar - Ground BasedC band Magnetron

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

5.3 5.3 5.4 5.4 5.5 5.5 5.6 5.6 5.7 5.7 5.8 5.8 5.9 5.9 6.0 6.0

Frequency (GHz)

No

rmal

ised

Po

wer

Mea

sure

d in

51

1 K

Hz

Bw

(d

B)

Failure Zones

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JRG Work on New Mask Looking into how a better estimate of the reference

bandwidth. Non linear chirps Limit excessive bandwidths due to

Large Chirps Fast Rise Times

Looking into what roll-off can be practically achieved How Roll-off Relates to RB

Looking into the special problems associated with. Magnetron based radars FM CW radars

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Trade Off Reference Bandwidth vs Roll-off If the Reference Bandwidth is accurately calculated 20 dB roll-off looks achievable 40 dB roll-off looks difficult These are theoretical however in practice distortions

make things worse

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Practical Issues To Reduce Unwanted Emissions

Use High Compression ratios Use slow rise and fall times Shape pulses to remove

discontinuities Use Filters

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Practical Issues cont

Magnetrons Below rotation can use high Q filters Multi pulse length systems have to use a

filter wide enough to meet narrowest pulse Above rotation systems have limited space OOB match of filters could upset

Magnetron and cause more emissions Cost

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Practical Issues Filters

Are Lossy can contribute twice TX & RX

Can cause wild heat (active arrays) Can take up space Can cause oscillation out of band if

not well matched Can distort want signal if too narrow Limit the peak power due to arcing Costly

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Practical Issues Linear Beam Tube Transmitters

Can use moderate compression ratios Difficult to control rise and fall times Single channel systems can use High

Q channel Filters Agile systems can only use band

limiting filters See Illustration

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Practical Issues cont:

Solid State Lumped Transmitters Can use higher compression ratios Easier to control rise and fall times

(slow down) Single channel systems can use

High Q channel Filters Agile systems can only use band

limiting filters of High Q

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Practical Issues cont: Solid State Distributed Transmitters

Can use higher compression ratios Easier to control rise and fall times Agile systems can only use band

limiting filters with a moderate Q

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Practical Issues

Active Array Systems Can use very high compression

ratios Difficult to control rise and fall

times Agile systems can only use band

limiting filters of very low Q Or Low pass filters

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Illustration: Solid State ATC

Can make use off Fixed Operating Frequencies Long pulses Slow rise & fall times

Many radar applications cannot make use of all these advantages

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Solid State ATC radarCivil Air Traffic Control Radar - Ground Based

S band Solid StateKnown interference signals removed

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

2 3 4 5 6 7 8 9 10 11 12 13 14

Frequency (GHz)

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Conclusions to Date Currently there is some scope for improving the

mask Solid State systems are better than linear beam

devices and cross field devices L arger time bandwidth products

There some scope for pulse shaping in Solid State transmitters

OOB Filters are effective for fixed frequency systems Agile systems are more problematic

Limited scope for OOB control OOB control not realistic in active arrays

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END

Thank youJohn Holloway