IEEE - Aerospace and Electronic Systems - April 2022 - 13

Kellett et al.
In the field ofESM, it is not uncommon to have groups
of subhalf-wavelength spaced elements combined with
elements spaced further than half-wavelength. The longbaseline
elements provide high levels of angle of arrival
(AoA) accuracy (as small changes in AoA result in large
changes in phase difference across the elements) with a
large number of ambiguities, while the short baseline elements
serve to resolve the ambiguities. The exact spacings
of these elements must be chosen carefully for the frequency
bands the array will operate over.
Both dual-band and wideband array architectures are
discussed in [4].
Another problem faced by wideband antenna arrays is
Figure 5.
Comparison of HPA semiconductor technologies [3]. ß Analog
Devices.
performance metrics. Most of the amplifiers listed use
GaN on various substrates.
The maximum power and power-added efficiency
(PAE) are defined as when the device is in saturation,
where the power amplifier is most efficient. This may be
suitable for constant amplitude waveforms2 such as most
radar pulses and some communications modulation
schemes. Waveforms with a high peak-to-average power
ratio will require amplifiers to be driven in the less efficient
linear region to avoid distortion. This includes high
modulation order communications waveforms (e.g., quadrature
amplitude modulation) or transmitting multiple
waveforms simultaneously.
There exists a tradeoff space between bandwidth, power,
and PAE. Narrowband amplifiers typically provide higher
PAE for the same or lower cost. Wideband amplifiers, while
capable ofachieving the same maximum powers, suffer from
lower PAE that leads to increased cooling requirements.
WIDEBAND AND MULTIBAND ARRAYS
To fully exploit the wideband capability provided that will
be provided by MFRFSs analog components should be
designed to work over a wide bandwidth and/or over multiple
bands. This includes but is not limited to: RF filters, limiters,
circulators, antenna elements, and array architectures.
To enable ambiguity-free direction-finding elements
must be spaced at most a half-wavelength apart. As the
spacing increases grating lobes are introduced when
receiving (or transmitting) far off-boresight. Spacings significantly
less than half-wavelength use more elements to
achieve the same directivity with larger spacings. This
introduces a problem for wideband arrays as the optimum
spacing for one band will be suboptimal for the others.
2Constant amplitude as defined during the transmit cycle.
APRIL 2022
mutual coupling between elements. This effect can be detrimental
to performance and varies as a function of frequency.
There are numerous methods that can be used to
mitigate this effect that include digital processing and analog
decoupling networks. There has also been development
in materials that when built into the array reduce
mutual coupling. These include defected ground structures
[15] and corrugated truncated structures [14]. It should be
noted that in some cases mutual coupling can be exploited
to improve the wideband performance of the array [17].
The mutual coupling also limits that elements that can
receive while others are transmitting at the same time.
Technology is being developed to cancel self-interference
by up to 100 dB in the analog domain, avoiding the problem
of LNAs and ADCs being saturated [18]. Perfect cancellation
may not be achievable across all frequencies that
could result in a reduction of receive performance.
NOVEL APERTURES
DISTRIBUTEDAPERTURES
Distributed apertures allow larger apertures to be deployed
that would not be financially or SWAP practical as a single
array. In the domain of ESM, this allows for better angular
resolution, with concentrations of half-wavelength spaced
elements resolving receive ambiguities that occur from
employing large, sparse apertures. When using radar, the
benefits can be less clear, with the large side lobes illuminating
unintended areas, increasing sensitivity to clutter and
jamming.
Alternatively, transmit and receive arrays could be separated.
This could provide sufficient isolation [4] that the
MFRFS could transmit and receive simultaneously, enabling
constant ESM and use ofsome currently available (non-radar
aware) communications. Separating transmit and receive
arrays would increase the required footprint ofthe system and
add to the RCS of the platform; however, this would not necessarily
double the SWAP requirements as much ofthe transmit
and receive chains on currentMFRs are split already.
IEEE A&E SYSTEMS MAGAZINE
13
IEEE - Aerospace and Electronic Systems - April 2022

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