Antenna

Introduction

An antenna is a device that allows radio frequency energy to be radiated in a certain way (transmitting antenna) or that enables the capture of energy from an electromagnetic radio frequency field to be processed by a receiver (receiving antenna). ¹
Thus, antennas can be classified into:

• transmitting antennas,
• receiving antennas,
• reciprocal antennas (the same antenna used for both transmission and reception).

From the perspective of radiation direction, antennas are divided into:

• omnidirectional antennas, where radiation is approximately the same in any direction. These are primarily used in applications where the transmitter needs to serve a large number of receivers uniformly over an extensive geographical area, such as broadcasting, or in cases where the transmitter or receiver changes position (is in motion);
• directional antennas, where radiation is predominantly manifested in a specific direction. These are primarily used in applications where the transmitter serves a number of receivers located within a specific geographical area, such as television, or in cases where the transmitter and receiver do not change position (relative to the distance between the two, positional changes are insignificant). Additionally, these are also used when it is desired to determine the location of a transmission source by orienting the antenna in the direction of maximum signal strength, as in radiogoniometry or radar applications.

Depending on the orientation with respect to the ground, we encounter:

• antennas with vertical radiation;
• antennas with horizontal radiation.

Characteristics of antennas

Each antenna has a series of characteristic data necessary for assessing their qualities.

1. Impedance

It is a quantity that essentially characterizes the impedance properties of an antenna at the feed point, specifically at the connection point of the downlink cable (feeder).
Considering that the antenna is a resonant element, its impedance is frequency-dependent.
In radio frequency operation, both for transmitters and receivers, the accepted antenna impedance is usually 50 Ω. If in the case of receivers, a mismatch of the antenna impedance with the receiver can lead to poor reception, in the case of transmitters, the mismatch (depending on its magnitude) leads to a decrease in the antenna's efficiency or may cause damage to the transmitter in the case of large adaptation differences. To eliminate these impedance mismatches, adaptation devices such as loops, transformers, or tuners are used.

2. Radiation pattern (directive characteristic)

If an antenna operates as a receiving or transmitting antenna, it receives or transmits energy of varying intensities from different directions. The graphical representation of this behavior is called the characteristic or radiation pattern. For graphical representation, the so-called polar diagram is used.

3. Gain

The gain (G) of a receiving antenna is the ratio between the power (\(P_a\)) captured by the respective receiving antenna and the power (\(P_N\)) captured by a dipole at \(\frac{\lambda}{2}\), provided that both antennas are located in a homogeneous electromagnetic field and are irradiated in their main reception direction:

\[ G = \frac{ P_a}{P_N}\]

Gain is best expressed in logarithmic units (dBd)

\[ G = 10 \cdot lg \frac { P_a}{P_N} [dBd] \]

Additionally, the gain is also encountered expressed in dBi, being reported relative to an isotropic antenna. The transformation relationship between dBd and dBi is:

\[ 1 \cdot [dBd] = 2.15 \cdot [dBi]\]

4. Front-to-back ratio (RFS)

The front-to-back ratio is a characteristic that indicates the directivity dimension of an antenna at angles: \(\alpha = 0^o\) and \(\alpha = 180^o\).
By definition, the front-to-back ratio is the ratio of the voltages that appear at the terminals of the antenna when the antenna is radiated at these angles:

\[ RFS = \frac { U_0}{U_{180}}\]

or in logarithmic units:

\[ RFS = 20 \cdot lg \frac { U_0}{U_{180}}[dB] \]

5. Beamwidth

The most advantageous direction for emission or reception of an antenna is given by the main lobe of the radiation pattern. As an index, the beamwidth of the main lobe is used. The angle of \(\frac {1}{\sqrt2} \approx 0.7\) is the angle at which the normalized voltage in the radiation pattern decreases to the value; this angle applies to both the E-plane and the H-plane of the pattern.

6. Attenuation of secondary lobes

The attenuation of secondary lobes [a_s] refers to the logarithmic ratio between the maximum voltage value at the main lobe (\(\alpha = 0^o\)) and the maximum value of the respective secondary lobe.

\[ a_s = 20 \cdot lg \frac { U_0}{U_{nmax}}[dB] \]

7. Null points

Null points refer to the points in the radiation pattern where the value is 0.

8. Bandwidth

All the characteristics presented earlier are frequency-dependent, and therefore the characteristics of an antenna are only valid for a specific frequency that must be indicated each time. Thus, an antenna for other frequencies undergoes negative changes in its characteristics (in terms of quality).
However, a frequency band (frequency deviation or frequency range) can be defined relative to the frequency (central) for which the antenna was designed, where the impedance characteristic and the gain characteristic are satisfactory and do not endanger the transmitter due to significant mismatch.