SMALL LOOP ANTENNA BASIC INFORMATION AND TUTORIALS

What is small loop antenna?
Small loop antenna defined.



Large loop antennas are those with overall wire lengths of 0.5λ to more than 2λ. Small loop antennas, on the other hand, have an overall wire length that is much less than one wavelength (1λ).

According to a Second World War US Navy training manual such antennas are those with an overall length of ≤0.22λ. Jasik’s classic 1961 text on radio antennas uses the figure ≤0.17λ, while John Kraus (1950) used the figure ≤0.10λ.

An amateur radio source, The ARRL Antenna Book, recommends ≤0.085λ for small loop antennas. For the purposes of our discussion we will use Kraus’s figure of ≤0.10λ.

A defining characteristic of small loops versus large loops is seen in the current distribution. In the small loop antenna the current flowing in the loop is uniform in all portions of the loop. In the large loop, however, the current varies along the length of the conductor, i.e. there are current nodes and antinodes.

The small loop antenna also differs from the large loop in the manner of its response to the radio signal. A radio signal is a transverse electromagnetic (TEM) wave, in which magnetic and electrical fields alternate with each other along the direction of travel.

The large loop, like most large wire antennas, respond primarily to the electrical field component of the TEM, while small loops respond mostly to the magnetic field component. The importance of this fact is that it means the small loop antenna is less sensitive to local electromagnetic interference
sources such as power lines and appliances.

Local EMI consists largely of electrical fields, while radio signals have both magnetic and electrical fields. With proper shielding, the electrical response can be reduced even further.

HALF WAVE DIPOLE ANTENNA BASIC INFORMATION AND TUTORIALS

What is half-wave dipole antenna?

Most antennas can be analysed by considering them to be transmission lines whose configurations and physical dimensions have been altered to present easy energy transfer from transmission line to free space. In order to do this effectively, most antennas have physical sizes comparable to their operational wavelengths.

Figure 1.12(a) shows a two wire transmission line, open-circuited at one end and driven by a sinusoidal r.f. generator. Electromagnetic waves will propagate along the line until it reaches the open-circuit end of the line.

At the open-circuit end of the line, the wave will be reflected and travel back towards the sending end. The forward wave and the reflected wave then combine to form a voltage standing wave pattern on the line. The voltage is a maximum at the open end. At a distance of one quarter wavelength from the end, the voltage standing wave is at a minimum because the sending wave and the reflected wave oppose each other.

Suppose now that the wires are folded out from the λ/4 points, as in Figure 1.12(b). The resulting arrangement is called a half-wave dipole antenna. Earlier we said that the electromagnetic fields around the parallel conductors overlap and cancel outside the line.

However, the electromagnetic fields along the two (λ/4) arms of the dipole are now no longer parallel. Hence there is no cancellation of the fields. In fact, the two arms of the dipole now act in series and are additive.

They therefore reinforce each other. Near to the dipole the distribution of fields is complicated but at a distance of more than a few wavelengths electric and magnetic fields emerge in phase and at right angles to each other which propagate as an electromagnetic wave.

Besides being an effective radiator, the dipole antenna is widely used as a VHF and TV receiving antenna. It has a polar diagram which resembles a figure of eight. Maximum sensitivity occurs for a signal arriving broadside on to the antenna. In this direction the ‘gain’ of a dipole is 1.5 times that of an isotropic antenna.

An isotropic antenna is a theoretical antenna that radiates or receives signals uniformly in all directions. The gain is a minimum for signals arriving in the ‘end-fire’ direction. Gain decreases by 3 dB from its maximum value when the received signal is ±39° off the broadside direction.

The maximum gain is therefore 1.5 and the half-power beam-width is 78°. The input impedance of a half-wave dipole antenna is about 72 Ω. It turns out that the input impedance and the radiation resistance of a dipole antenna are about the same.

ANTENNA BANDWIDTH BASIC INFORMATION AND TUTORIALS

Antennas can find use in systems that require narrow or large bandwidths depending on the intended application. Bandwidth is a measure of the frequency range over which a parameter, such as impedance, remains within a given tolerance.

Dipoles, for example, by their nature are very narrow band. For narrow-band antennas, the percent bandwidth can be written as

( fU − fL ) × 100/ fc

where

fL = lowest useable frequency

fU = highest useable frequency

fC = center design frequency

In the case of a broadband antenna it is more convenient to express bandwidth as

fU

fL

One can arbitrarily define an antenna to be broadband if the impedance, for instance, does not change significantly over one octave ( fU / fL = 2).

The design of a broadband antenna relies in part on the concept of a frequency-independent antenna. This is an idealized concept, but understanding of the theory can lead to practical applications. Broadband antennas are of the helical, biconical, spiral, and log-periodic types.

Frequency independent antenna concepts are discussed later in this chapter. Some newer concepts employing the idea of fractals are also discussed for a new class of wide band antennas.

Narrow-band antennas can be made to operate over several frequency bands by adding resonant circuits in series with the antenna wire. Such traps allow a dipole to be used at several spot frequencies, but the dipole still has a narrow band around the central operating frequency in each band.

Another technique for increasing the bandwidth of narrow-band antennas is to add parasitic elements, such as is done in the case of the open-sleeve antenna (Hall, 1992).

MICROWAVE ANTENNA BASIC INFORMATION AND TUTORIALS

What is a microwaves antenna and how to design it?

The small antenna elements at microwaves facilitate the construction of highly directive, high gain antennas with high front-to-back ratios. At frequencies below about 2 GHz, 12- to 24-element

Yagi arrays, enclosed in plastic shrouds for weather protection, may be used. At higher frequencies, antennas with dish reflectors are the norm.

The aperture ratio (diameter/wavelength) of a dish governs both its power gain and beamwidth. The power gain of a parabolic dish is given to a close approximation by:

Gain = 10 log10 6(D/λ)^2 × N, dBi

where D = dish diameter and N = efficiency. Dimensions are in metres. The half-power beam width (HPBW) in degrees is approximately equal to 70λ/D.

A microwave antenna with its dish reflector, or parasitic elements in the case of a Yagi type, is a large structure. Because of the very narrow beamwidths – typically 5◦ for a 1.8m dish at 2 GHz – both the antenna mounting and its supporting structure must be rigid and able to withstand high twisting forces to avoid deflection of the beam in high winds.

Smooth covers, radomes, fitted to dishes and the fibreglass shrouds which are normally integral with Yagis designed for these applications considerably reduce the wind loading and, for some antenna types, increase the survival wind speed.

The electrical performance of a selection of microwave antennas is given in Table 4.1 and the wind survival and deflection characteristics in Table 4.2 (Andrew Antennas, 1991).

Table 4.1 2.1–2.2GHz antennas – electrical characteristics

With shrouded Yagis and some dishes low loss foam-filled cables are generally used up to about 2 GHz although special connectors may be required. At higher frequencies, air-spaced or pressurized nitrogen filled cables are frequently used with waveguides as an alternative.

Table 4.2 Wind survival and deflection characteristics

ANTENNA BANDWIDTH BASIC AND TUTORIALS

WHAT IS ANTENNA BANDWIDTH? THE PURPOSE OF ANTENNA BANDWIDTH?

Antennas can find use in systems that require narrow or large bandwidths depending on the intended application. Bandwidth is a measure of the frequency range over which a parameter, such as impedance, remains within a given tolerance. Dipoles, for example, by their nature are very narrow band.



For narrow-band antennas, the percent bandwidth can be written as:

(fu - fl)/fc    x 100


where
fL = lowest useable frequency
fU = highest useable frequency
fC = center design frequency

In the case of a broadband antenna it is more convenient to express bandwidth as
fU/fL


One can arbitrarily define an antenna to be broadband if the impedance, for instance, does not change
significantly over one octave ( fU / fL = 2).


The design of a broadband antenna relies in part on the concept of a frequency-independent antenna. This is an idealized concept, but understanding of the theory can lead to practical applications.

Broadband antennas are of the helical, biconical, spiral, and log-periodic types. Frequency independent antenna concepts are discussed later in this chapter.

Some newer concepts employing the idea of fractals are also discussed for a new class of wideband antennas.

Narrow-band antennas can be made to operate over several frequency bands by adding resonant circuits in series with the antenna wire. Such traps allow a dipole to be used at several spot frequencies, but the dipole still has a narrow band around the central operating frequency in each band.

Another technique for increasing the bandwidth of narrow-band antennas is to add parasitic elements, such as is done in the case of the open-sleeve antenna (Hall, 1992).