The antenna used in the usual consumer product is very sensitive to the size of the PCB radio formation and the plastic shell of the product. The antenna can be simulated as an LC resonator, and when L (inductance) or C (capacitance) increases, the resonant frequency of the LC resonator will fall. A larger radio frequency ground plane and plastic housing increase the effective capacitance to reduce the resonant frequency.
Grounding surface
As mentioned earlier, the single-pole PCB antenna requires the ground layer to work properly. The following figure shows an example of the PCB that MIFA is placed in a PCB in the ground layer. The size range of the PCB is 20 mm × 20 mm to 50 mm × 50 mm.
By this curve, it can be understood that the larger the area of the radiographic ground layer, the lower the resonant frequency, and the ground layer is also, so the loose wave loss will be. This is a key condition in the PCB layout. The better the grounding layer provided by the quarter wavelength, the better the relationship with theoretical performance. This is a key concept in an antenna design, which can solve difficulties in providing sufficient space to the ground small module antenna.
Effect of PCB grounding layer size
Effect of housing
Similar to the effect of the ground layer, in order to quantify the sensitivity of the antenna to the product plastic housing, the experiment is carried out on the wireless mouse, as shown in Figure 26. Cypress MIFA is placed in a wireless mouse plastic housing and then measures radiation mode and returning loss.
Effect of plastic housing
By the two drawings, you can learn about the following main contents:
▪ When the antenna is placed in a place near the plastic housing, the resonance frequency will decrease.
▪ The change in resonance frequency ranges from 100 MHz to 200 MHz. The antenna must be re-debugged to obtain the required frequency band. For information on antenna debugging, please refer to the antenna placement, housing, and grounding guidelines. .
In summary, increase the ground layer size and plastic housing are in the range of the resonant frequency of the antenna to 100 MHz to 200 MHz.
Be
Antenna placement, housing, and grounding guide
▪ Always place an antenna in a corner of the PCB, maintains enough gaps with the rest of the circuit.
▪ Always follow the antenna grounding method recommended by the antenna designer / manufacturer. Commonly used PCB antennas are variants of the monopole antenna. The monopole antenna needs to be directly grounded to work.
▪ Do not place any components, planes, mounting screws or traces in the antenna of all layers. The actual disabled area depends on the antenna used.
▪ Do not close the antenna near the industrial design. Plastic has a higher dielectric constant than air. The proximity of plastic and antennas causes the antenna to see a higher effective dielectric constant. This increases the length of the antenna trace and reduces the resonant frequency.
▪ Battery cable or microphone does not pass through the antenna trace.
▪ The antenna cannot be completely covered by the metal shell. If the product has a metal housing or shield, the outer casing must not cover the antenna. Metal is not allowed near the antenna.
▪ The direction of the antenna should be consistent with the final product direction so that the radiation reaches the maximum in the desired direction.
▪ Do not have any ground below the antenna. See Figure 14.
▪ There is a sufficient space (gap) from the antenna to the ground layer, and the width of the ground layer should be minimal. See Figures 10, Figure 15, and Figure 20.
▪ Plan to provide configurations for antenna matching network because many parameters (plastics, grounding, substrate differences, and other components) of the antenna may change their impedance, so the antenna may need to be re-tuned. If the impedance of the antenna is unknown, it is best to provide a configuration for the PI or T network of three components, in which series is connected.
The component is filled with 0 ohms and the shunt components are not loaded. This helps you fill any topologies you need to match the network later.
▪ When using antenna manufacturer's matching network value, be sure to use the length of the wire from antenna to the matching network, which is specified in the manufacturer data manual or reference design.
▪ Always use the final plastic housing and the product verification antenna matching network in the typical case scenario, for example, the plastic of a mouse is placed on the hand, placed on the mouse pad, plastic, wood, metal or floor.
Our other product:
