2.4G high frequency PCB antenna design

1. The relationship between the wavelength in the PCB and the wavelength in the sky:

1) The relationship between the speed of light and the medium:

c=1/√(εμ)

among them:

Speed ​​of light c = 2.99792458 ×10^8 m/s
Vacuum dielectric constant ε =8.854187817*10^(-12) A*s/(V*m)
Vacuum permeability μ = 4π*10^(-7) N/A^2

2) The relationship between the speed of light and wavelength:

λ = c / f

3) The relationship between dielectric constant and wavelength:

λ = 1 / (f * √(εμ))

4) The relationship between the relative permittivity ε'of the PCB medium, the wavelength λ'in the PCB and the wavelength λ in the air:

λ'= λ / √(ε')

The wavelength of 2.4G is 12.5cm, and the 1/4 wavelength is 3.125cm

The relative dielectric constant of the commonly used FR-4 material is 4.6, the effective dielectric constant is 3.279 (calculated by APPCAD), and the 1/4 wavelength is 1.726cm

 

2. Design of inverted F antenna

L2 should be 1/4 wavelength. Based on this value, the effective relative permittivity of the PCB material used can be calculated to be 3.63. This relative permittivity needs to be made of FR-1 material PCB, permittivity 5.2, relative permittivity Constant 3.656

 

3. Antenna performance estimation:

Refer to TI document AN058

Reflectance: Γ = (Za-Zo) / (Za + Zo)

Voltage standing wave ratio: VSWR = (1 + |Γ|) / (1-|Γ|)

Return loss: S11db = 20log(Γ)

It is good when VSWR is 1.5 (RL = 14dB), and it needs to be redesigned if VSWR is above 2.0 (RL = 9.5dB).

 

4. Measuring method of voltage standing wave ratio:

In the antenna system, whether the antenna and the equipment are well connected or not, we often use a parameter called standing wave ratio to measure it. When the standing wave ratio is 1, it means that the antenna system is well matched and has no reflection. The larger the number, the greater the number. It means that the worse the matching condition is, the larger the reflected wave is in the system. Then how to measure the standing wave ratio of the antenna? Here I will introduce you to a relatively simple method.

To measure the standing wave ratio, a scanner is required. The connection method is shown in Figure 2-1. First, short-circuit the terminal of the feeder (near the end of the antenna system). At this time, the signal output by the scanner forms total reflection at the terminal of the feeder. Observe its total reflection waveform as shown in Figure 2-2. The maximum amplitude of the curve is a, and then connect the antenna to the terminal of the feeder. At this time, the maximum amplitude observed on the scanner in the working frequency range is b as shown in Figure 2-3. First find the reflection coefficient P=b/a, and then use the formula S=1+P/1-P to find the standing wave ratio, where S represents the standing wave ratio.

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