Comments on “Wideband Radiation Reconfigurable Microstrip Patch Antenna Loaded With Two Inverted U-Slots”

This comment investigates some missing fundamental simulation and measurement results affecting the practical performance of the antenna presented in
[1]
. In order to properly demonstrate the practical consequences of these omissions, we provide simulation results as well as experimental validation using a prototype with the exact dimensions as in
[1]
, fed with an ultrawideband coupler
[2]
, as shown in
Fig. 1
. Radavaram and Pour
[1]
have claimed 68% fractional bandwidth for a multiport antenna based only on a single-port input reflection coefficient (
S
₁₁
). Unfortunately, the mutual coupling between the antenna ports (
S
₂₁
) is not shown in their article. We have performed corresponding simulations using Ansys HFSS and found that the missing coupling coefficient between the elements within the band of interest has a magnitude close to 0 dB, which is validated by the experimental results, as illustrated in
Fig. 2(a)
. This suggests that the proposed design is operating as a wideband bandpass filter in lieu of a radiating element. The clear misunderstanding in the scattering performance arises from the fact that for dual-port antennas fed by in-phase and out-of-phase inputs, the active scattering parameters are of fundamental importance. Active scattering parameters for dual-port devices are defined as the combinations of
S
₁₁
and
S
₂₁
(some references, such as
[4]
and
[5]
, use differential and common mode scattering parameters or
S_dd
and
S_cc
). They determine the overall scattering performance when a feeding network is connected to a multiport antenna. For instance,
[3]
(see
[1, Ref. 20]
),
[4]
, and
[5]
are aimed at reducing
S
_dd11
to below −10 dB for the differential S-parameter. They use the correct definition of
S
_dd11
, as given in the following
[3, eq. (8)]
: