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AMOS

Antenna with Semicircular Radiation Diagram for 2.4 GHz

Dragoslav Dobričić, YU1AW

Introduction

n WiFi communications, antennas with semicircular radiation diagram in the horizontal

plane are often needed. Antennas with circular radiation diagrams may have gain values as

high, as it is possible to narrow its vertical radiation diagram. That's where antennas with

radiating dipoles aligned vertically are used. When a circular diagram is needed, and vertical

polarization is used, the alignment of half-wave dipoles can be carried out according to the

principle of the famous Franklin's antenna.

An open half-wave dipole fed in the middle is extended on both sides with one continuous

conductor, which is, every half wavelength, folded into a quarter-wave short-circuited part of

symmetrical two wire line. In this way, proper phasing of the half wave dipoles is performed.

This kind of wire antenna has been used, mainly in horizontal polarization, from the very

beginnings of radio on medium and short waves and is known as Franklin’s antenna, after its

author.

Horizontal Franklin antenna

The problem with impedance and parasitic radiation of phasing lines

This antenna is often used on VHF and UHF bands in vertical polarization, which yields a

circular radiation diagram. However, with the increase of frequency, problems arise with phasing

lines among dipoles, because they physically enlarge in relation to wavelength, which

consequently leads to greater impact of the radiation from this part of the antenna on the overall

diagram of the antenna. This unwanted, parasitic radiation of the phasing lines has been resolved

in many ways (by wrapping the two-wire line around the antenna axis, by the replacement of the

two-wire line with a coil or capacitor, etc.) with more or less success. However, for work on 2.4

GHz, this problem becomes significant also because of the relatively large thickness of the

antenna conductors in relation to a wavelength, and therefore the physical dimensions of two-

wire line. These dimensions not only determine the characteristic impedance, but also the

parasitic radiation of these parts, especially from the short circuit at the end of the two-wire line.

(See the figure later in the text marked with E ). The short circuit at the end of the line needs to

be physically very short, so that the great current that flows through it gives as small a level of

parasitic radiation as possible. Its shortening decreases the distance between wires in two-wire

line, and therefore decreases the impedance of the two-wire line. Even if this decreased

antenneX Issue No. 127 – November 2007

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impedance is accepted, the physical length of this part of the line can not be short enough not to

represent the significant part of a wavelength on this frequency. Therefore, the short wire

becomes a considerably efficient radiator of the electromagnetic energy. This parasitic radiation

can greatly modify the overall radiation diagram of the antenna.

In order to achieve higher gain from the antenna, a greater number of dipoles needs to be aligned

along the vertical. The increase in number of dipoles consequently increases the impedance at

the feed point very rapidly and becomes impractically high for simple and efficient supply of the

antenna by coaxial cable.

Franklin's antenna with reflector

The solution to the problem with the change of the radiation diagram

However, placing the Franklin’s line of dipoles in front of relatively narrow reflector provided

solution to these problems, but with the ˝sacrifice˝ of the circular radiation pattern, which

became semicircular. By placing the reflector near the short-circuited end of the two-wire line,

the shorting wire and the close reflector act as transmission line with the impedance of about 150

ohms. In that way, the parasitic radiation is considerably reduced. In addition, it was possible to

increase the length of that wire to achieve the wanted distance between wires and needed value

of characteristic impedance of two-wire line.

By placing the dipole in front of the conducting reflector plane, the impedance in antenna feed

point decreased to about 200 ohms, which enabled very simple and efficient feeding of the

antenna by a coaxial cable with characteristic impedanse of 50 ohms through a half wave balun

connected as a transformer with an impedance ratio 4:1. The reflector is very narrow, 0.5

wavelengths, so that it narrows the horizontal radiation diagram as little as possible. This yields

the Amos antenna.

antenneX Issue No. 127 – November 2007

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Gain of Amos antenna

Achieved calculated results

In this way, we achieved antennas with very wide radiation diagrams in the horizontal plane:

over 120 degrees for -3dB and about 180 degrees for -6dB decreases of the gain. With very

severe symmetry of the geometry of the antenna and the currents within, we achieved very clean

diagrams in the vertical plane, with the width of the main beam of 15 degrees and with very good

side lobe suppression. The high radiation resistance of the antenna gave a low antenna Q factor

and high working bandwidth that can be seen from the antenna input matching diagram. The gain

of the antenna of over 12 dBi is very close to the theoretic maximum for this kind of

configuration and is completely acceptable for an antenna with this wide radiation diagram in the

horizontal plane. This is achieved by careful optimization on the computer, using 4NEC2, a

NEC based antenna modeler and optimizer from Arie Voors.

Input matching of Amos antenna

antenneX Issue No. 127 – November 2007

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Dimensions and construction of Amos antenna

antenneX Issue No. 127 – November 2007

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Horizontal and vertical radiation diagram of Amos antenna

antenneX Issue No. 127 – November 2007

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