Instrumentation & Measurement Magazine 25-1 - 70

Metamaterials and Antennas Research Group (MARS) at Carleton
University is shown in Fig. 1b, operating from 26.5 GHz
to 40 GHz with Eravent antenna models SAR-2013-28-S2 and
SAP-28-R2 for Tx and Rx, respectively. Compared to the standard
far-field characterization, the prime benefit of a near-field
system is its compact size and ability to measure the detailed
complex transmittance of the surface. Nevertheless, the nearfields
can be used to compute the far-fields using standard
near-to-far-field transformation procedures [7].
However, it is quickly apparent that this system does not
provide much flexibility for controlling the incident field. The
designer is limited to using the specific Tx antennas which are
available to them and configuring the orientation and position
of the Tx antenna. This is dominantly due to expensive horn
antennas used in these systems in practice, where usage of
multiple Tx antennas is not always possible and is not cost-effective.
Thus, it may be that it is not possible to experimentally
produce the exact incident fields that were used in simulation,
and for which the MS is originally designed. For example, consider
a MS that was designed for a normally incident plane
wave. Using a rectangular horn antenna for illumination, it
is not possible to produce an ideal plane wave, with both uniform
phase and amplitude, as we will show. While it is possible
to approximate a plane wave by moving the horn antenna far
away from the MS, this comes at the expense of losing much
of the incident power, affecting the signal-to-noise ratio and
introducing undesired effects due to non-uniform phase distribution
across the MS.
One approach which has been taken specifically to generate
a flat phase is the use of a lens placed between the
illuminating horn and the MS [8]-[10]. This is based on the
quasi-optical approximation of the field generated by the horn
being a Gaussian beam [11]. In this case, the system can be analyzed
within the framework of paraxial optics which can be
used to design a lens that produces a beam waist (and hence
a constant phase) at the location of the MS [12]. However, the
typical spot-size generated using this lensed system is small,
of the order of few centimeters for a typical Ka-band system,
for instance, which is not sufficient to characterize larger-sized
MSs (typically several tens of wavelengths), beyond which
the phase flatness is significantly degraded. This greatly limits
the physical area that can be field scanned. In addition, a
quasi-optical approach of modeling the horn field as a Gaussian
beam reveals the inherent trade-off present-as the phase
and the amplitude uniformity cannot be optimized at the same
time. One can form this conclusion directly from the formulation
for the Gaussian beam which has a uniform phase profile
at the waist where the spot size is smallest and the magnitude
variation sharpest.
In this paper, we propose a novel technique based on just
a single Tx antenna, which does not require additional components
such as lenses and provides flexibility in shaping the
incident field, including a flat uniform phase across a large
physical area. The method involves multiple separate experiments
with different incident and scattered fields, which are
subsequently combined using superposition to produce the
desired incident and scattered fields. The application of the
superposition principle assumes a linear system, which is the
case for most MSs, including both linear time-invariant (LTI)
and linear time-variant (LTV) MSs (non-linear MSs are notably
excepted). In this work, we will numerically demonstrate the
method using an integral equation (IE) simulator [13], while
the same procedure can be carried out in a laboratory setting.
Practical Metasurface Illumination
We consider the incident fields generated by the Eravant
model SAR-2013-28-S2 rectangular horn antenna, which functions
in the 26.5 GHz to 40 GHz band. Fig. 2a shows the electric
field profile in the H plane (x-z plane), simulated using the fullwave
Ansys HFSS simulator at f = 30 GHz (simulation model
Fig. 2. The Eravant SAR-2013-28-S2 horn antenna was simulated using HFSS and is well-approximated using a Gaussian beam having the parameters
w0
= 1.00 mm, xbw = 0 cm, and zbw = −4.14 cm. (a) Full-wave horn simulation (HFSS); (b) Gaussian beam approximation; (c) Comparison at z = 40 cm.
70
IEEE Instrumentation & Measurement Magazine
February 2022

Instrumentation & Measurement Magazine 25-1

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