Comments on “Frequency Diverse Array Antenna Using Time-Modulated Optimized Frequency Offset to Obtain Time-Invariant Spatial Fine Focusing Beampattern”

In the recent articles
[1]
–


[4]
, including the abovementioned article, time-modulated frequency diverse arrays (FDAs) have been presented to obtain time-invariant spatial patterns. The presented FDAs in
[1]
–

[3]
have the feature of time-invariant spatial focusing, which means they have a constant maximum in a time duration
T
at a point with desired range
r
and angle
$\theta $
. In
[4]
, the pattern is time invariant, yet not focused. However, in this communication, it is indicated that the patterns are obtained using the incorrect definition of time in some equations. The equation system of
[3]
is explained here, which can be extended to
[1]
,
[2]
, and
[4]
, explicitly.