1 Antennas: from Theory to Practice 4. Antenna Basics Yi HUANG Department of Electrical Engineering & Electronics The University of Liverpool Liverpool

Antennas: from Theory to Practice

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4. Antenna Basics

Yi HUANGDepartment of Electrical Engineering & Electronics

The University of Liverpool

Liverpool L69 3GJ

Email: [email protected]


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Objectives of this Chapter

• Study the basic theory of antennas• Understand how radio waves are generated

by antennas• Identify the most important antenna

parameters from both the field point of view and the circuit point of view.


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4.1 Antennas to Radio Waves

• For a single frequency case

Eliminating H, we have

Thus in a open space


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This equation gives the radiated electric field from a time varying current J (the time factor is omitted here) and is the very foundation of the antenna theory – it reveals how the antenna is related to radiowaves


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Radiation from a current source Il

η is about 377 ohms in free space


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– The electric field is always greater than the magnetic field;

– When βr >1, the ratio of |E/H| = η

– When βr <1, as the distance increases, the electric field reduces at a much faster rate (60 dB/decade) than the magnetic field (40 dB/decade).

– When r is fixed, r =1 is still an important point,

– The electric field first reduces as (or frequency) increases to the point r =1 and then changes to increase with (or frequency) after this point

– the ratio of E/H reduces first as increases to about r = 1; it then becomes a constant for r > 1.


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Antenna far field


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Major features of the far field

– Just one E field component and one H field component – a local plane wave?

– Both fields are inversely proportional to the distance r;

– The ratio of E/H is , the intrinsic impedance

– E and H fields are orthogonal to each other and the cross product of these two is the power density function which is inversely proportional to the distance square r2:


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Antenna near field

Simulated near field around a dipole antenna


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Major features of the near field

The region βr <1 for is normally called the reactive near field. The field changes rapidly with distance.

From its power density function, we can see that – It contains both the radiating energy (the real part)

and reactive energy (the imaginary part – it does not dissipate energy which is like a capacitor or inductor). The latter is normally dominant in this region.

– It has components in r and directions. The former is radiating away from the source and the latter is reactive.


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Antenna near and far fields


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4.2 Antenna Parameters from the Field Point of View

• Radiation Pattern– a plot of the radiated field/power as a function of

angle at a fixed distance which should be large enough to be considered in its far field


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– The half-power beamwidth (HPBW) of the main lobe, also called the 3dB beamwidth; or just the beamwidth (to identify how sharp the beam is);

– The 10 dB beamwidth or first null beam width (FNBW) (another one capture the main beam shape);

– The first side lobe level (expressed in dB, relative to the peak of the main beam);

– The front to back ratio (the peak of the main lobe over the peak of the back lobe, another attempt to identify the directivity of the antenna).

– Null positions (sometimes used for anti-interference and positioning).


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When the patterns are plotted in the logarithmic scale (dB plot), both the normalised field and power patterns are the same


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• Directivity– a measure of the concentration of radiated power in

a particular direction. It is defined as the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions.

Pt is the total radiated power in W, and U is radiation intensity in W/unit solid angle, linked to the averaged radiated power density Sav


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Example 4.1

The radiated power density of the electrically short current element is given earlier as:

Determine the directivity of the antenna as a function of the directional angles, find the maximum directivity and express it in decibels.


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• Gain – the ratio of the radiation intensity in a given direction

from the antenna to the total input power accepted by the antenna Pin divided by 4:

• Radiation Efficiency– the ratio of the radiated power to the input power

accepted by the antenna:


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Example 4.2

If the efficiency of the antenna in Example 4.1 is 50%, VSWR at the antenna input is 3 and the input/supplied power is 1 W, find:

a). the power gain;

b). the total radiated power


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• EIRP– Effective isotropic radiated power, or EIRP, is the

amount of power that would have radiated by an isotropic antenna to produce the peak power density observed in the direction of maximum antenna gain:

• Effective Aperture and Aperture Efficiency– The effective aperture Ae is less than the physical

aperture Ap, the aperture efficiency is defined as the ratio of these two:

and directivity


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Example 4.4

The directivity of a pyramidal horn antenna of aperture width a and height b is

Find its aperture efficiency. If the power density around the antenna is 1 W/m2, find the received power.


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• Polarisation– The antenna polarisation is the same as the polarisation

of its radiating wave: linear or circular in practice.

– Two orthogonally polarised antennas cannot communicate with each other.

• Antenna Temperature– The radiation from different sources is intercepted by

antennas and appears at their terminals as

TB is the brightness temperature of the radiation source


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• Antenna factor (AF)– the ratio of the incident electric field E to the induced

voltage V0 at the antenna terminal when it is connected to a load/cable (50 ohms by default)

• Radar Cross Section (RCS)– the ability of a target to reflect the energy back to the

radar and it is the ratio of the backscattered power to incident power density


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4.3 Antenna Parameters from the Circuit Point of View

• Input Impedance– Antenna input impedance (Za) is the impedance

presented by an antenna at its terminals or the ratio of the voltage to current at its terminals


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• Radiation Resistance– the equivalent resistance which would dissipate the

same amount of power as the antenna radiates when the current equals the input current at the terminals:

RL is the loss resistance of the antenna. To match with the source impedance, the condition is:


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• Reflection Coefficient, Return Loss and VSWR


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• Radiation Efficiency

• Matching Efficiency

• Total Efficiencysource supplied

power

This has taken the feed-antenna mismatch into account


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• Bandwidth

It can be defined by a number of things, such as the VSWRreturn loss, gain, or even 3dB beam width!


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Summary of antenna main parameters

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1 Antennas: from Theory to Practice 4. Antenna Basics Yi HUANG Department of Electrical Engineering & Electronics The University of Liverpool Liverpool