Introduction Antenna design and RF layout are critical in a wireless system . The wireless range that an end-customer gets out of an RF product with a current-limited power source such as a coin-cell battery depends greatly on the antenna design, the enclosure, and a good PCB layout. It is not uncommon to have a wide variation in RF ranges for designs that use the same silicon and the same power but a different layout and antenna-design practice. Other important general layout considerations for RF trace, power supply decoupling, via holes, PCB stackup, and antenna and grounding .
An antenna is basically a conductor exposed in space. If the length of the conductor is a certain ratio or multiple of the wavelength of the signal1 , it becomes an antenna. This condition is called “resonance”, as the electrical energy fed to antenna is radiated into free space.
Types of Antenna Wire Antenna , pcb Antenna, Chip Antenna
Wire Antenna: This is a piece of wire extending over the PCB in free space with its length matched to /4 over a ground plane. This is generally fed by a 50-Ω 4 transmission line.
PCB Antenna: This is a trace drawn on the PCB. This can be a straight trace, inverted F-type trace, meandered trace, circular trace, or a curve with wiggles depending on the antenna type and space constraints. In a PCB antenna, the antenna becomes a two-dimensional (2D) structure in the same plane of the PCB
Chip Antenna: This is an antenna in a small form-factor IC that has a conductor packed inside. This is useful when there is limited space to print a PCB antenna or support a 3D wire antenna
Return loss: The return loss of an antenna signifies how well the antenna is matched to the 50- transmission line (TL), shown as a signal feed in Figure 7. The TL impedance is typically 50 , although it could be a different value. The industry standard for commercial antennas and testing equipment is 50- impedance, so it is most convenient to use this value.
Bandwidth: Bandwidth indicates the frequency response of an antenna. It signifies how well the antenna is matched to the 50-Ω transmission line over the entire band of interest, that is, between 2.40 GHz and 2.48 GHz for BLE applications
Radiation efficiency: A portion of the non-reflected power gets dissipated as heat or as thermal loss in the antenna. Thermal loss is due to the dielectric loss in the FR4 substrate and the conductor loss in the copper trace. This information is characterized as radiation efficiency. A radiation efficiency of 100 percent indicates that all non-reflected power is radiated to free space. For a small-form-factor PCB, the heat loss is minimal.
Radiation pattern: Radiation pattern indicates the directional property of radiation, that is, which directions have more radiation and which have less. This information helps to orient the antenna properly in an application
Gain: Gain indicates the radiation in the direction of interest compared to the isotropic antenna, which radiates uniformly in all directions. This is expressed in terms of dBi—how strong the radiation field is compared to an ideal isotropic antenna.
Antenna Feed Consideration Here this parameter is specifically explained for PCB antenna For the small length of PCB trace that feeds the antenna, the width requirement can be relaxed. Ensure that the antenna trace width and the antenna feed connection have the same width .
However, if it is a long transmission line approximately 1 cm from the matching network to antenna it is recommended a transmission line (TLine) type of layout, having a specific width “W” over a bottom ground plane for the feed
S = space between main RF trace and ground plane
W = RF trace width
H = Height of substrate
ER = material used Dielectric constant
T = Thickness of top layer copper
And there are different cross sections available for making transmission line, among Coplanar waveguide is in common practice as it provides good isolation from other signals . CPW is similar to Edge Coupled Microstrip
Zo even can be calculated with below equation
One can use online calculators for calculating parameters https://www.eeweb.com/toolbox/edge-coupled-microstrip-impedance/
So in above parameters T and H are dependent on stackup available with pcb fabrication house