In this paper, four different structures of DR are proposed
for BW enhancement and for improving rad. efficiency. We
got 2.2% bandwidth and 92.7% rad. efficiency for the antenna
without DR loading. With DR loading, in the first structure
7.2% bandwidth and 99.89% rad. efficiency are achieved. In
the second structure 12.29% bandwidth and 95.7% rad.
efficiency are achieved. In the third structure 10% bandwidth
and 97.5% rad. efficiency are achieved. In the forth structure
15.5% bandwidth and 97.8% rad. efficiency are achieved.
Here maximum BW is achieved for the structure D. From
smith charts it can be seen that in the structure A and C,
one full loop (small) and half portion of second loop (large) in the
impedance locus are inside the vswr=2 circle. Means some
portion of large loop in the impedance locus is outside the
vswr=2 circle. By cutting the DRs of structures A and C
cylindrically (as shown by structures B and D), the hole large
loop is brought inside the vswr=2 circle and thus two full
loops in the impedance locus are inside the vswr=2 circle. All
these results are achieved without changing any dimension
and feed location of the antenna proposed without loading,
just additional DR is used.
Moreover, by comparing the results of structures A and C
with the results structures B and D respectively, it can be
observed that in terms of BW and rad. efficiency, it is more
advantageous to load a DR around the feed point than loading
on the feed point. Then by increasing substrate thickness to
h 0.05 , there is further improvement in BW and rad.
efficiency. BW is around 15% for structures A and C, while
around 20% for structures B and D. Rad. efficiency is above
99% for all the structures.
We designed an antenna with 30 Ohm impedance,
which can be used as a high power transmitting antenna. A
synchronous one-quarter wavelength transformer section with
characteristic impedance of 38.73 Ohm should be installed
onto the bottom end of the 30 ohm feed section in order to
connect the input terminals of the antenna to standard
transmission line impedance values of 50 Ohms.