Pattern ANTENNAS design procedure

A pattern ANTENNAS is ANTENNAS that is constructed by drawing a pattern on a substrate, etc. It can contribute to cost reduction and miniaturization of devices, so it can be said to be the mainstream of ANTENNAS.
This time, we will mainly explain the meandering ANTENNAS as a typical example.

• Pattern ANTENNAS

The simplest ANTENNAS design method is to create ANTENNAS or monopole ANTENNAS using a wiring pattern on the board.

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  Figure 1 Vertical meandering monopole ANTENNAS

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  Figure 2 Horizontal meandering monopole ANTENNAS

This section explains the necessary parameters for ANTENNAS design and evaluation.

• Voltage standing wave ratio (VSWR)

-Voltage Standing Wave Ratio (VSWR) is a parameter that indicates the percentage of power input to ANTENNAS feed point that is actually input to ANTENNAS.
-In practice, VSWR is measured using a network analyzer.
-Design and adjust ANTENNAS to obtain a resonant frequency characteristic with a VSWR < 2 or < 3 in the required band.
When -VSWR=2, the power input to ANTENNAS is approximately 89%, and when VSWR=3, the power is approximately 75%.

• Impedance characteristics (Smith chart)

-Impedance characteristics are obtained by measuring the S11 characteristics with a network analyzer.
- Impedance is expressed as Z = R + jX, where R is the real part and X is the imaginary part.
If the -VSWR value is high, the impedance generally deviates from 50 Ω even if the resonant frequency peak is in the target frequency band.
In this case, it is necessary to use a Smith chart to bring the impedance Z closer to 50 Ω (to achieve ANTENNAS matching).

Figure 3. Example of VSWR and impedance measurement

As an evaluation method, the measurement environment for directional characteristics is described.

• Directional characteristics

-The ANTENNAS 's directional characteristics are measured in a measurement environment like that shown in Figure 4.
-This type of measurement environment is called a six-sided anechoic chamber, and is a shielded room that prevents radio wave reflections in all directions.
Figure 5 shows an example of the directional pattern measurement results obtained in a -6-sided darkroom.
-Check that these directional characteristics are as designed and verify the average gain and maximum gain.

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  Figure 4. Example of measurement environment for directional characteristics

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  Figure 5. Example of directional pattern measurement results

As an example of designing a pattern ANTENNAS, we will show you an example of designing a slim meander ANTENNAS.

-Low and small ANTENNAS.
-The meander structure on both sides of the board allows for a smaller ANTENNAS.
ANTENNAS area dimensions: 6 x 10 mm
-VSWR<2 Bandwidth: approx. 320MHz

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  Figure 6. Appearance of the slim meander

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  Figure 7. Dimensional diagram of ANTENNAS element

Source: Renesas Bluetooth Low Energy Microcontroller Application Note

The VSWR and directional characteristics of a slim meander ANTENNAS are shown.

Figure 8. Example of VSWR characteristics

Figure 9. Example of directional characteristics

Source: Renesas Bluetooth Low Energy Microcontroller Application Note

ANTENNAS design procedures and points to note are shown below.

We consider specifications for the system, such as radiation characteristics, polarization characteristics, ANTENNAS area and volume, and communication distance, and select the optimal ANTENNAS.

For linear ANTENNAS (inverted L, inverted F, meander, etc.) based on monopole ANTENNAS and chip ANTENNAS, a GND of sufficient size relative to the wavelength is required to form an image effect.

Using a design example of a small pattern ANTENNAS, we model it in an electromagnetic field simulator or prototype a circuit board with ANTENNAS mounted on it.Since the GND size, ANTENNAS placement position, proximity to ANTENNAS, proximity to the housing, proximity to metal, etc. may affect ANTENNAS characteristics, we carefully consider the selection of housing material and the placement of components near ANTENNAS.

In most cases, matching adjustments are required after mounting ANTENNAS on the system board. Evaluate the VSWR and S11 Smith chart and aim for a VSWR<2 or <3.
Matching can be achieved by adjusting ANTENNAS element length or by adding a constant.
We also match the assembled system, including the housing.

After matching adjustment, the directional characteristics are measured. Maximum gain, average gain, and radiation direction are evaluated to confirm whether the intended communication distance can be achieved.
The maximum gain is also required when applying for compliance with the Radio Law.