Design of Antennas for Handheld DVB-H ... - Lunds tekniska högskola

3 Possible solutions
This chapter describes different existing antenna types that can receive a TV
signal.
3.1 Dipole and Monopole antenna
The dipole is one of the most basic antennas. It consists of a straight piece of wire
that is cut at the centre and fed with a balanced generator. This type of antenna is
resonant at the frequency were the conductor length is a half wavelength. This
means that to be resonant at 600 MHz (λ=500mm) it needs a length of 250mm
A quarterwave monopole is a ground plane dependent antenna that must be fed
single ended, which makes it an unbalanced antenna. The ground plane is very
important, because the larger the ground plane the more efficient antenna. The
ground plane is half of the monopole antenna that has a radiation pattern like the
dipole antenna. It depends on that a quarter wave monopole “mirrors” itself in the
ground plane, much in the same way one sees their own reflection in the water.
According to that is the length of a quarter wave monopole antenna half of a
dipole antenna, at 600 MHz 125 mm [27].
3.2 Yagi-Uda
The Yagi-Uda antenna is a wideband antenna that is a very good radiator in the
frequency range 3-3000 MHz. This antenna consists of a number of linear dipole
elements, as shown in figure 3.1. The first dipole element after the reflector is
energized directly by a feed transmission line. The rest of the elements act as
parasitic radiators, the currents are induced in these elements by mutual coupling.
The parasitic elements in the direction of the beam are smaller in length than the
feed element. Typically the length of the feed element is 0.45-0.49λ and the
directors are about 0.4-0.45λ. It is not necessary for the directors to be of the same
length or to have the same diameter. The separation between the directors are also
not necessary the same for uniform design.
The most common feed element for Yagi-Uda antenna is a folded dipole. When
this folded dipole is used all elements placed behind it will act as reflectors. The
major role of the reflector is played by the one placed next to the energized
element. Very little is gained with a second reflector.
The high gain of a Yagi-Uda antenna comes from the directors, the more directors
the higher gain. Practically very little is gained by the addition of more than 12
directors, usually 6-12 are used. However, many arrays have been designed and
built with 30 to 40 elements, with a typical gain of about 15-17.5 dBi [28].
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