RF PCB Antenna Design

Basics for PCB Antenna Design

RF, Radio Frequency

Radio Frequency is an abbreviation for all radio frequencies. The radio transmission frequencies are divided into different spectra and range from the Extremely Low Frequency Band (ELF, 3 ~ 30 Hz) to the Extremely High Frequency Band (EHF, 30 ~ 300 GHz).

Antenna

An antenna is a metallic structure used to transmit and receive electromagnetic waves. It can be passive or active. On the basis of Maxwell's equations, an exact description of the wave propagation can be made under the given boundary conditions. Antennas can operate at different radio frequencies (see RF). The characteristics of an antenna (antenna gain, antenna impedance, polarization, size, costs, etc.) must match to the application.

Dipol Antenna

The principle of a dipole antenna is based on a center-fed wire antenna, whose conductors are normally straight and correspond to half the wavelength. In the metal rods, high-frequency AC current is converted into electromagnetic waves. Taking into account the reciprocity theorem, electromagnetic waves can also be converted back into electrical energy in the dipole antenna. Thus, this antenna can be used for transmitting and receiving.

Patch / Planar Antenna

A planar antenna has all active and parasitic elements in one plane. In general, this antenna is printed onto the printed circuit boards (PCB) during the pcb manufacturing process. For example, these microstrip antennas are used in IoT environment. A planar antenna is usually excited by a coaxial feed or by a microstrip line.

Quarter-Wave Antenna

Printed PCB antennas are mainly designed as quarter-wave antennas. The ground plane below the conductor also creates a mirror image of λ/4, which leads to a behavior similar to that of a dipole antenna. PCB antennas are usually inexpensive to manufacture, but require a corresponding area on the PCB.

PIF Antenna

The so-called PIF antenna (Planar Inverted F-Shaped Antenna) is a special form of planar antenna. This type of antenna has a geometrically regularly arranged F-shaped basic pattern. In the side view the antenna looks like a lying F, in which the shape is formed from the feed line, the lateral short-circuit connection to the ground plane (feed point) and the upper antenna surface.

RFID Antenna

The so-called Radio Frequency Identification (RFID) antenna is used for wireless, i.e. contactless, identification of objects, people or animals using electromagnetic waves. RFID system consists of a transponder chip on which data is stored and an integrated antenna. Size of the RFID antenna ranges from size of a postage stamp to size of a notebook screen. Difference in size is usually an indication of reading range: the larger the antenna, the higher the gain, the greater the reading range and vice versa.

MIMO Antenna

MIMO is an effective radio antenna technology that uses multiple antennas on the transmitter and receiver at the same time to enable a variety of signal paths to transmit data. Separate paths are selected for each antenna to enable the use of multiple signal paths. MIMO antennas do not go from a single transmitting antenna to a single receiving antenna, but rather the output goes to several receivers. This makes the design of 5G antennas, for example, very complex.

MIMO Beamforming Smart Antennas

Technology of so-called MIMO beamforming is not only applied to MIMO systems, but it can be used with any antenna system. This type of antenna is used to create a desired antenna directional pattern while achieving the required performance under the given boundary conditions. Smart antennas are usually used for this purpose. Depending on the required performance and prevailing conditions, these antennas can be controlled automatically.

Smart Antenna

In general, smart antennas consist of a large number of radiating elements, a control unit and a combining / dividing network. Smart antennas are divided into two typical groups: Phased Array Systems and Adaptive Array Systems (AAS). Phased array systems are switched on the basis of predefined patterns. These patterns are used according to the desired direction. The so-called adaptive beamforming is used in adaptive array systems. This method uses an infinite number of patterns. These patterns can be adapted to the requirements in real time.

Bandwidth

The definition of bandwidth of an antenna is the frequency range between frequencies f1 and f2. These frequencies are called corner frequency, cut-off frequency, crossover frequency or half-power frequency. The range is calculated by B = f2 − f1 or dividing the center frequency by the Q factor B = f0 / Q. By definition the cut-off frequencies have -3dB (or 50%) of the maximal Amplitude of f0.

Directional Radio Pattern

The shape of the radiated energy is called the directional radio characteristic. An ideal isotropic antenna radiates the same energy in all directions, the radiation is spherical. Depending on the application, antennas have different directional radiation patterns creating several lobes. Main lobe is intentional. Side and rear lobes should be avoided, as unnecessary power is emitted here or interference is received. Antenna boresight is the axis of the maximum antenna gain of the main lobe (max. radiated energy).

Peak Gain

Gain of an antenna describes the radiation in any direction in free space. Ratio of the antenna is measured in comparison [dBi isotropic] to an ideal isotropic antenna, which radiates uniformly in all directions. Peak gain or maximum antenna gain G only applies to one frequency and direction. The gain changes with angle and frequency.

Beamwidth

The beam width of an antenna can be read from the radiation diagram. If a 3D radiation pattern is cut horizontally, result is a 2D azimuth cut, with a vertical cut the elevation cut. Opening angle indicates the angle θ at which the gain at the main lobe drops by a maximum of -3dB, i.e. at least half the power or more. With asymmetrical antennas, there is one value each for the horizontal and vertical opening angles.

50 Ohm Impedanz Antenne Smith Chart — 50 Ohm Impedance Antenna

Impedance of an Antenna

Oliver Heaviside (1850-1925) invented the coaxial cable to reduce crosstalk on telegraph lines. The "Bell Labs" found out through experiments around 1920 that 30 ohm coax is optimal for power transmission and the lowest losses occur at 77 ohms. 50 ohms is a compromise for both requirements. An antenna should be optimized so that it has 50 ohms with little or no inductive and capacitive component in the Smith Chart over the bandwidth.

Matching Network

A matching network is an impedance matching network or impedance converter. For example, the impedance of signal line is adapted to the antenna in order to minimize return loss.

Antenna Efficiency

Efficiency of antennas describes the quotient of the input power to the radiated power in% or dB (e.g. 0.5 or 50% or -3 dB). Losses in antennas come from conduction losses in the metal of the antenna, impedance mismatch and dielectric losses due to polarization effects. The losses are reflected or given off as heat. Efficiency of sending and receiving is the same for antennas. An ideal antenna is 100% efficient, but real antennas are closer to 50-60%. Due to the directional radiation characteristic, the power can be bundled in one direction.

Return Loss (S11)

Return loss is a reflection factor that describes how much energy is reflected from a discontinuity in a transmission link. It is the ratio of emitted power to reflected power, given in decibels (dB). Part of the signal energy is reflected at all inhomogeneities or impedance jumps and propagates in the opposite direction in the conductor. S parameter S11 describes the return loss (S11 = RL).

Insertion Loss (2S1)

Insertion loss is the attenuation of the signal by anything that is "inserted" into the signal path. It is therefore the sum of all losses and attenuations in the signal path. S parameter S21 describes insertion loss (S21 = IL).

EIRP, Equivalent Isotropic Radiated Power

So-called effective or equivalent isotropic radiation power is the hypothetical power that would have to be emitted by an isotropic spherical radiator in order to achieve the same (equivalent) signal strength as the actual source antenna in the main beam direction. For example, if a given transmitting antenna has a power gain of 20, power density of the antenna in a certain spatial direction is 20 times greater than that of an isotropic antenna. An isotropic antenna would have to emit 20 times as much power to achieve the same power density.

VSWR, Voltage Standing Wave Ratio

The standing wave ratio VSWR of an antenna describes the adaptation of its load to the transmission line. In the event of an impedance mismatch, part of the power can be reflected back and this leads to the generation of standing waves, which are characterized by the VSWR. VSWR is thus defined as the ratio of the maximum and minimum voltages on a transmission line. Standing wave ratio can also be described by the S parameters:
VSWR = (1 + | S11 |)/(1 - | S11 |).

Layer Stackup

The layer stack-up of a PCB describes the different layers of conductive material (e.g. copper) and dielectric (e.g. FR-4). One copper layer above a dielectric can be sufficient for antennas. If a reference plane is used, a 2-layer microstrip structure can be implemented. If the transmitter / receiver, an antenna amplifier and the adaptation network are on the same circuit board, 4-, 6- or multilayer structures may be necessary. Matching of the impedance must be observed for different structures.

Controlled Impedance

Impedance of transmission lines on a printed circuit board essentially results from the width of the conductor track, the thickness of the dielectric and its material. In order to avoid losses and reflections, RF lines should always be implemented with controlled impedance.

Tapering

A taper is the smooth adjustment of different widths of conductor structures. This can be necessary, for example, on component pads that are contacted with a narrower cable. By slowly adjusting the line width, discontinuities are reduced and thus reflections are reduced.

Clearance

HF circuit parts should be as far away as possible from other function blocks on the PCB and should also be shielded. Clearance can be defined in Constraint Manager using rules for nets and regions. With shielding functions, HF lines can be shielded by GND surfaces next to, above and below the line. These can also be automatically connected and contacted with vias.

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