# Design Aspects of Printed Monopole Antennas for Ultra-Wide Band Applications

Authors: K. P. Ray
Journal: International Journal of Antennas and Propagation
 (2008)

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## Abstract

This paper presents the design equations for lower band-edge frequency for all the regular shapes of printed monopole antennas with various feed positions. The length of the feed transmission line is a critical design parameter of these monopole antennas. Design curves for the length of the feed transmission line for various lower band-edge frequencies for all these regular shaped monopoles have been generated. A systematic study has been presented to explain the ultra-wide bandwidth obtained from these antennas with an example of elliptical monopole antenna.


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## Body

## 1. Introduction

Present time is witnessing a very rapid
growth of wireless communications,for which antennas with very large bandwidth are in strong demand,so that various applications are covered with fewer or
preferably with a single antenna. It will be preferred that an antenna has
bandwidth in excess of frequency range from 800 MHz to 11 GHz or even more, to
include all the existing wireless communication systems such as AMPC800, GSM900,
GSM1800, PCS1900, WCDMA/UMTS (3G), 2.45/5.2/5.8-GHz-ISM, U-NII, DECT, WLANs,
European Hiper LAN I, II, and UWB (3.1–10.6 GHz) [1]. Out of all the above-mentioned wireless
systems, ultra-wide bandwidth (UWB) wireless technology is most sought for
very high-data-rate and short-range wireless communication systems, coding for
security and low probability of interception, rejection of multipath effect,
modern radar systems, and so forth. As mentioned above, this technology uses
ultra-wide bandwidth of 7.5 GHz, ranging from 3.1 GHz to 10.6 GHz.Planar and printed monopole antennas are the good candidates for use
in UWB wireless technology because of their wide impedance bandwidth and nearly
omni-directional azumuthal radiation pattern. Many shapes of planar, also known
as planar disc, monopole antennas are reported, which yield very large
bandwidth [2–8]. Some of these
reported configurations have bandwidth in excess of that required for UWB
applications [3].  But, the planar disc
configurations are not the most preferred one for these applications, because
they are generally mounted on large ground plane, which are perpendicular to
the plane of monopole (which makes them three-dimensional structure).  Also, the large size ground plane limits the
radiation pattern to only half hemisphere. On the other hand, printed monopole
antennas (PMAs) are truly planar and have radiation patterns similar to that of
a dipole antenna. These monopoles can be
integrated with other components on printed circuit board, have reduced size on
dielectric substrate, are without backing ground plane and are easy to
fabricate. Printed antennas, commonly fabricated on FR4 substrate, are very
cost effective, which is ideally suited for UWB technology-based low-cost
systems [9–18].In this paper, design of all the regular geometries of PMA with
various feed positions is discussed. Formulae to calculate the lower band-edge(VSWR=2) frequency for all these printed monopoles are presented. For various
feed configurations, frequency-dependent design curves have been generated for
50 Ω microstrip feed line, which yields maximum bandwidth for a given lower
band-edge frequency. These design curves, which cover the lower band-edge
frequency of 3.1 GHz for UWB applications, are also valid for coplanar feed
lines. A systematic study is presented to explain as to how bandwidth increases
with increase in lateral dimension of patch giving the example of an elliptical
geometry. The theoretical study of all the configurations has been carried out
using HP high-frequency structure simulator (HP HFSS) [19]. All these results
are validated with experiments and reported data.


## 2. Design Considerations of Antennas for UWB Technology

Some of the main features required for antennas for the application
of UWB technology are as follows.(i)
It should have bandwidth ranging from 3.1 GHz to 10.6 GHz in which reasonable efficiency and satisfactory omnidirectional radiation patterns are necessary.(ii)
In this ultra-wide bandwidth, an extremely low-emission power level should be ensured. In 2002, the Federal Communication Commission (FCC) has specified the emission limits of−41.3 dBm/MHz.(iii)
The antenna propagates short-pulse signal with minimum distortion over the
frequency range.
The first point is the most important one for antenna designers, which translates
into the requirement that antenna should have impedance bandwidth ratio of
3.42:1 over which VSWR≤2. Such a high impedance bandwidth is only
realised using multiresonance printed monopole antenna, which generally exhibit
high pass impedance characteristics. For
such broadband antenna, unlike single resonance tuned dipole or monopole
antennas, some special design considerations have to be taken into account.
Instead of resonance or operating frequency, lower band-edge frequency and
total bandwidth achieved become the design parameters for these printed
monopole antennas. The lower band-edge frequency depends primarily on maximum
height of the monopole, whereas bandwidth of the antenna depends on how
impedance of various modes is matched with the microstrip or coplanar feed
line. These parameters are discussed in details for all the regular geometries
of printed monopole antennas.


## 3. Printed Monopole Antenna Design

The printed monopole antennas give very large impedance bandwidth
with reasonably good radiation pattern in azumuthal plane, which can be
explained in the following two ways. The printed monopole antenna can be viewed
as a special case of microstrip antenna configuration, wherein the backing
ground plane is located at infinity [7]. A patch is fabricated on dielectric
substrate (commonly FR4). Beyond the substrate it can be assumed that a very
thick air dielectric substrate (εr=1) exists. It makes a
microstrip antenna configuration on a thick substrate with εr closer to unity, which yields
large bandwidth.Alternatively, printed monopole antennas can
be seen as a vertical monopole antenna. A monopole antenna usually consists of
a vertical cylindrical wire mounted over the ground plane, whose bandwidth
increases with increase in its diameter. A printed monopole antenna can be
equated to a cylindrical monopole antenna with large effective diameter. This
second analogy has been used to determine the lower band-edge frequency of all
regular shapes of printed monopole antennas for various feed configurations.


## 4. Lower Band-Edge Frequency of Printed Monopole Antennas

Various regular shaped printed monopole antennas such as
printed square monopole antenna (PSMA), printed rectangular monopole antenna
(PRMA), printed hexagonal monopole antenna (PHMA), printed triangular monopole
antenna (PTMA), printed circular monopole antenna (PCMA), and printed
elliptical monopole antenna (PEMA) for different feed positions are shown in
Figure1. For different feed locations, the suffix 1 or 2, as shown in Figure 1, are put for these monopole antennas.
These antennas are generally fabricated on FR4 substrate (εr=4.4,h=0.16cm,andtanδ=0.01) with backing ground plane removed. For patch dimensions, to cover the
lower band-edge frequency of around 900 MHz, the maximum dimension of the
substrate is taken as 9 cm × 9 cm, which becomes almost half when the lower
band-edge frequency is doubled [9]. These patches can be fed by 50 Ω microstrip
line or by coplanar waveguide. For both these cases, the optimum width of the
backside ground plane in the case of microstrip feed or coplanar ground plane
is 1 cm. These data are verified through simulation for all the configurations
shown in Figure 1.Various regular-shaped PMAs with different feed configurations.
(a)(b)(c)To estimate the lower band-edge frequency of printed monopole
antennas, the standard formulation given for cylindrical monopole antenna can
be used with suitable modification [3]. The equation was worked out for the
planar monopole antennas. If L is the
height of the planar monopole antenna in cm, which is taken same as that of an
equivalent cylindrical monopole, and r in cm is the effective radius of the equivalent cylindrical monopole antenna,
which is determined by equating area of the planar and cylindrical monopole
antennas, then the lower band-edge frequency is given as [3]
(1)fL=cλ=7.2(L+r+p)GHz,
where p is the length of
the 50 Ω feed line in cm.Unlike the planar disc monopole antennas, the printed
configuration has dielectric layer on one side of the monopole. This dielectric
material increases the effective dimensions of the monopole leading to
reduction in the lower band-edge frequency. This is also confirmed by
simulation studies. Hence, more appropriate equation for the lower band-edge
frequency is given as
(2)fL=7.2{(L+r+p)×k}GHz.
With reference to various configurations in Figure 1, L and r are calculated as follows.IfS is the side length of the PSMA, then
(3)L=S,r=S2πforPSMA1,L=2S,r=S22πforPSMA2.
For PRMA, if length=L and width=W, then
(4)L=L,r=W2πforPRMA1,L=W,r=L2πforPRMA2.
For PTMA, if the side length is T, then the values of L and r of the effective cylindrical
monopole are determined for both PTMA1 and PTMA2 as
(5)L=3T2,r=T4π.
Similarly, for the PHMA with side length H, the L and r values of the
equivalent cylindrical monopole antenna are obtained by equating their areas as
follows:
(6)L=3H,r=3H4πforPHMA1,L=2H,r=33H8πforPHMA2.
For PCMA with radius A,
values of L and r of the equivalent cylindrical monopole antenna are given by L=2A and r=A/4.Finally, for PEMA with semimajoraxis=A and semiminor axis=B,the L and r of the effective
cylindrical monopole are determined as
(7)L=2A,r=B4forPEMA1,L=2B,r=A4forPEMA2.As discussed earlier, PMA can be thought
of as a variation of microstrip antenna, in which the ground plane is
considered to be located at infinity. Following this analogy, the factork in (2) can be thought of as having
similar significance as εeff. For commonly used FR4 substrate with εr=4.4 and h=0.159 cm, the empirical value of k=1.15
estimates lower band-edge frequency within 10%. Equation (2) with k=1.15 has been
validated for various reported PMA configurations. For example, a PRMA with dimension L=3.0 cm, W=2.0 cm, and p=0.2 cm with εr=4.4 in [16] has measured value of fL=1.59 GHz, whereas  (2) yields
the value of 1.77 GHz. For second reported PRMA with dimension L=1.15 cm, W=1.2 cm, and p=0.15 cm with εr=3.38, simulated fL is 4.22 GHz
[17], against the calculated value of 4.199 GHz using (2). Similarly, the
values of frequency calculated using  (2)
with k=1.15 for PEMA tally very well with measured values reported in [12, 13]. For the dimension A=8.6 mm, B=7.31 mm, and p=0.5 mm in [12], the reported measured value of frequency is
3.17 GHz, whereas (2) yields the value of 3.206 GHz. The reported measured
value of frequency for the dimension of A=10 mm, B=10 mm, and p=1 mm is 2.7 GHz [13], against the calculated value of 2.664 GHz using (2).
Thus,  (2) provides starting point
for the design of PMAs.Another important parameter, which decides the lower band-edge
frequency as shown in (2), is the length of 50 Ω feed line p. This feed probe length, the bottom contour of the printed
monopole patch and the ground plane, three together form the impedance matching
network at multiple adjacent resonances of the patch simultaneously. This leads
to an increase in bandwidth. Therefore, for different bottom shapes of the patches, the feed length p will be different.
Based on this criterion, the various regular shaped patches have been grouped
in three categories in Figure 1. In Figure 1(a), the feed transmission line sees
abrupt straight edge of the patch shape like PSMA, PRMAs, PTMA, and PHMA.
Second category, as shown in Figure 1(b), consists of patches like PCMA and
PEMAs having smooth transition between transmission line and the curved bottom
of the patch. In the third category, as shown in Figure 1(c), transition takes
place through bottom corner of the patch and feed transmission line. The value
of p, for lower band-edge frequency
with maximum bandwidth, for these three categories has been discussed
here.To generate design data for the 50 Ω feed
line lengthp, for different fL,
to obtain maximum bandwidth for all the PMAs under three categories were
designed using (2) for various values of fL starting from 0.5 GHz to 3.1 GHz. This frequency range covers lower band-edge frequency of all
the communication channels including that of UWB technology. The value of p for each case for every printed
configuration has been optimised to obtain maximum bandwidth using HP HFSS
software. The optimised value of p, for given fL and maximum bandwidth, is almost same for all the
five configurations under category of Figure 1(a). These values of p versus fL are plotted in Figure 2. For maximum bandwidth, when fL increases from 0.5 GHz to
3.1 GHz, the value of p decreases
from 12 mm to about 0.7 mm. Though these five configurations were optimised for
maximum bandwidth, the highest bandwidth ratio obtained was around 2.7:1. This
is a relatively moderate bandwidth ratio, which is attributed to the
discontinuity at the junction of a feed point, where the feed line abruptly
gets truncated by a straight base of the PMAs. Figure 3 shows the variation of p with fL for the second category of PMAs, which includes PCMA
and both PEMAs as shown in Figure 1(b). For this case also, as fL increases, feed length p reduces, but magnitude of p is an
order less as compared to that of PMAs under the first category. Here, when fL increases from 0.5 GHz to
3.1 GHz, the value of p decreases
from 1.8 mm to about 0.12 mm. These design curves on p are validated with measurements and reported results. The optimum value of p=0.3 mm reported for PCMA at fL=2.69 GHz is
exactly same as read from Figure 3 [9]. This category of PMAs gives maximum
bandwidth. The three PMAs under third categories, that is, vertex fed PMAs, as
shown in Figure 1(c), did not exhibit same behaviour with respect to one
another. However, for some cases, the value of p versus fL is
closer to those of PMAs under second categories than those of first categories.
The values of p as shown in Figures 2 and 3 are for the microstrip feed which has been found exactly the same for
coplanar waveguide feed as well.Figure 2
Variation of feed lengthp with lower band-edge frequency fL for PMAs shown in Figure 1(a).Figure 3
Variation of feed lengthp with lower band-edge frequency fL for PCMA and PEMAs.


## 5. Ultra-Wide Bandwidth of a PEMA

The PMA, as
discussed earlier, is viewed as an equivalent thick cylindrical monopole antenna.
In the radiating metallic patch, various higher order modes get excited. With
optimum feed and increased lateral dimension (i.e., larger width of the patch)
all the modes will have larger bandwidth, hence will undergo smaller impedance
variation. The shape and size of these
planar antennas can be optimised to bring in impedance of all the modes withinVSWR=2 circle in the Smith chart, leading to very large impedance bandwidth. This has been demonstrated taking the example of PEMA.The PEMAs were designed on FR4 substrate(εr=4.4,h=0.16cm,andtanδ=0.01) for fL∼1.1 GHz. The value of p for this fL as read from Figure 3 is 1 mm.The height of the PEMA is kept fixed at 2A=4.8 cm so that the fL remains
almost the same.To bring out the variation of input impedance and hence the
bandwidth with increased lateral dimension of PEMA,four different values of 2B
were chosen as 1 cm, 2 cm, 4 cm, and finally 5.2 cm.These configurations are
shown in Figure 4.These four PEMAs were analysed using HP HFSS software up to
the maximum frequency range of 5 GHz. These results in the form of impedance
loci in Smith chart and corresponding return loss plots are shown in Figure 5.
It is noted from Smith chart in Figure 5(a) that for 2B=1 cm, the PEMA behaves
as a thin strip monopole antenna, which is equivalent to thin wire monopole.
Here, impedance variation around and in between various loops, which indicates
different modes of the elliptical patch, is large, leading to very less
impedance bandwidth. As 2B increases, the impedance variation around and in
between various modes reduces, bringing smaller loops closer to each other
yielding increased bandwidth. This effect is clearly seen in impedance loci
plots of Figures 5(b) to 5(d) for 2B=2.0 cm to 5.2 cm. The increased bandwidth
can be read from the corresponding return loss plot of Figure 5(e). For 2B=5.2 cm, all the loops curl around the centre of the Smith chart, bringing them
inside VSWR=2 circle. This leads to increase in bandwidth beyond 5 GHz. For
the fourth case of 2A=4.8 cm and 2B=5.2 cm, the PEMA was analysed up to 16 GHz.Figure 4
PEMAs with2A=4.8 cm and 2B equal to (— - —)  1 cm,  (- - - -) 2 cm, (– – –) 4 cm, and (─) 5.2 cm.Impedance loci and return loss plot of four
PEMAs with2A=4.8 cm and 2B equal to (— - —)  1 cm, (- - - -) 2 cm, (– – –) 4 cm, and (─) 5.2 cm.
(a)(b)(c)(d)(e)For this optimised configuration of PEMA, the simulated surface
current distributions at four frequencies are shown in Figure6. Figure 6(a) shows
the current distribution on the patch near the first resonance at 1 GHz. The
current is distributed mainly along the edges and the feed point. Along the
periphery of the PEMA, there is one half-cycle variation of current, which
indicated the fundamental mode. On the ground plane, the current is distributed
mainly on the upper edge. This explains the importance of an optimised
dimension of the ground plane. The current distribution at 3 GHz has three
times more variation as that at 1 GHz. Similarly, current distribution at 5 and
7 GHz is also shown in Figure 6. With increase in frequency, the current
distribution has more half-cycle variations but with reduced amplitude and
confines to the outer boundaries of the patch. The gain and efficiency of this
antenna is plotted in Figure 7. The value of gain increases linearly and then
saturates, while
the efficiency decreases from 100% to approximately 80% for the frequency range
from 1 GHz to 15 GHz. The gain increases from 1.84 dB to around 7.7 dB for the
variation of frequency from 1 GHz to 6 GHz because of increase in effective
area with frequency. Later, the increase is only approximately 0.5 dB for 6 to
15 GHz because of decreased efficiency. There is less current variation and
hence impedance variation between various modes of the PEMA, which also leads
to partial filling of nulls in the radiation pattern at higher order modes.
Therefore, in complete bandwidth, the elevation and azumuthal radiation
patterns remain qualitatively similar to that of the cylindrical monopole
antenna. At lower frequencies, azimuthal radiation patterns are close to omni-directional, whereas in elevation it is a figure of eight because of the very small
ground plane.At higher frequencies, radiation patterns in both the planes remain
similar to those at lower frequencies with more variations in the elevation
plane. Moreover, these PMAs being asymmetrical configurations in two
perpendicular planes, perfect omni-directional azumuthal radiation pattern is
not achieved and also cross-polar levels are high. Cross-polar levels are
approximately 15 dB down as compared to corresponding copolar levels at lower
frequencies, which become only around 5 dB down at highest frequency of the
bandwidth. The configuration was fabricated and tested for the bandwidth.
Simulated and measured S11 plots are compared in Figure 8. There is good agreement between the two plots. The measured bandwidth is from 1.1 GHz to 13.5 GHz against the simulated bandwidth of 1.06 GHz to 14.1 GHz.Simulated current distributions of a PEMA at 1 GHz, 3 GHz, 5 GHz, and 7 GHz.
(a)(b)(c)(d)Figure 7
Variation of gain and efficiency of
a PEMA with2A=4.8 cm and 2B=5.2 cm.Figure 8
Return loss plots of a PEMA with2A=4.8 cm and 2B=5.2 cm (─) simulated, (– – –) measured.


## 6. Conclusion

Multiresonance printed monopole antennas are being used increasingly for applications of UWB technology because of their attractive features. Some of the design aspects of these antennas have been discussed in this paper. A systematic study has been presented to explain ultra-wide impedance bandwidth obtained from an elliptical monopole antenna.



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*Source: 713858-2008-04-07.xml*