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3.2.5.3 Radar Frequency Spectrum Definitions
The frequency spectrum can be considered in respect of three main definitions:
• Necessary Bandwidth;
• Out of Band Emission Region;
• Spurious Emission Region.
3.2.5.4 Necessary Bandwidth
Recommendation ITU-R SM.1541 (Unwanted Emissions in the Out of Band Domain)
defines the necessary bandwidth as “the width of the frequency band which is just
sufficient to ensure the transmission of information at the rate and quality required under
the specified conditions”
The pulse width of the radar defines the necessary bandwidth of the transmitted signal.
The necessary bandwidth is 20dB below the peak envelope value and is defined as the
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smaller of B N = or , where t is the pulse duration and tr is the rise time.
t. t t
r
Modified equations apply to pulse compression systems.
Radar engineers also define the 3dB bandwidth which is the reciprocal of the pulse width.
3.2.5.5 Out of band Emissions
Recommendation ITU-R SM.1541 defines out of band (OoB) emissions as “emission on a
frequency or frequencies immediately outside the necessary bandwidth which results
from the modulation process, but excluding spurious emissions”. Out of band limits are
based on the -40dB bandwidth of the spectrum of the transmitted waveform.
where K is a constant which is typically about 6.2, although it varies depending on a
number of radar parameters (see Recommendation ITU-R SM.1541). Modified equations
apply to pulse compression systems.
The extent of the out of band transmission is therefore governed by the pulse width and
pulse rise time. Spectrum efficiency is maximised by using a pulse width which is exactly
matched to the operational requirement for range resolution. In addition, increasing the
pulse width and rise times reduces the level and extent of the out of band emissions.
Tailoring the rise times is therefore one method of controlling the level of out of band
transmissions. Limiting the out of band spectrum by the introduction of filters may
introduce pulse distortion if the filtering is too severe.
3.2.5.6 Spurious Emissions
Recommendation ITU-R SM.1541 defines the spurious domain as the frequency range
beyond the OoB domain in which spurious emissions generally predominate. In the case
of pulse radars the boundary between the out of band and spurious domains is usually
taken to be the point where the OoB limits have fallen to the spurious emission level
specified in Appendix 3 to the Radio Regulations. The spurious emission levels that must
be met by all new radio determination stations and existing ones from 1 st January 2012
are detailed in Appendix 3 to the Radio Regulations.
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