integrated into other RF cir-

cuits as well as embedded

Dielectric

Substrate

into UWB devices. Figure 8

shows some typical designs

[50]­[53]. Basically, the pla-

nar radiators are etched onto

the dielectric substrate of the

PCBs. For monopoles, the

Ground Plane

ground plane may be copla-

Under Substrate

nar with the radiators or

under the dielectric sub-

strate. The ground plane may

be modified to enhance the

bandwidth. The radiators

can be fed by a microstrip

line and coaxial cable.

It should be noted that the

planar monopole or dipole

antennas feature broad imped-

ance bandwidth but some-

what suffer high cross-polar-

ization radiation levels. The

Figure 8. Typical configurations of planar antenna on PCBs.

large lateral size and/or asym-

metric geometry of the planar

radiator cause the cross-polar-

ized radiation. Fortunately, the

75

purity of the polarization

z

issue is not critical, particular-

rmax

ly for the antennas used for

Planar

50

φ

portable devices.

Radiator

?

r

ro

Roll Monopoles

The planar antennas feature

Feed

broad  impedance  band-

widths and can be easily

r = ro+αφ

50

integrated into circuits on

ro = 4 mm

α = 0.5/360°

PCBs. However, the radia-

tion from the planar anten-

nas may not be omnidirec-

tional at all operating fre-

y

1

quencies because they are

not  structurally  rotation-

mm

x

allysymmetrical. Thick or

modified cylindrical monop-

Coaxial Feed

oles or dipoles are good

options for achieving broad-

band omnidirectional charac-

teristics [54]­[57]. Structur-

Figure 9. Broadband and compact roll monopole.

ally, the thick monopoles are

situated between thin cylin-

drical and cone monopoles. In terms of bandwidth,

it is clear that the aperture between the radiators forms a

weight, and fabrication cost, they are unsuitable for

transmission structure similar to a planar waveguide.

low-cost UWB applications. Therefore, a roll monopole

Also, the elliptical radiator can be modified for the

was presented to improve the radiation performance of

reduction in size and enhancement in impedance band-

a planar monopole across a broad bandwidth [24], [58].

widths as shown in the bottom row of Figure 7.

Basically, the roll monopole is constructed by twisting

In addition, the UWB antennas printed on PCBs are

the planar radiator to a roll shape as shown in Figure 9.

more practical to implement. The antennas can be easily

68

December 2006