Introduction

The wireless systems cover a large variety of devices in different applications such as:  navigation, radars, direct broadcast TV, satellite and mobile communications. As the demand for smaller sizes of wireless devices is increasing; antennas are the most concerned to be customized to make them respond to many WLAN (Wireless local area network) applications covering all operating frequency bands. In the case of WI-FI antennas, there are five different bands used for transmissions: 2.4GHz, 3.6GHz, 4.9GHz, 5GHz, and 5.9GHz. The most widely used is the 2.4GHz expending to 2.485GHZ depending on the used antenna type ranging from Grid, Yagi, Biquad, Patch, Monopole to Dipole. Antennas vary in terms of direction of radiation and other properties. Good antenna design produces better coverage with low profile configuration; the latter means means low cost, light weight, low volume and easy integration.

For the current study, two models of WI-FI communication antennas will be investigated for the frequency band of 2.4GHz: the Biquad antenna and B-shaped dual loop antenna.

Biquad antenna [1]-[2]

Biquad antenna is a kind of loop antenna with a simple design, easy to build and offers a good characteristic of directivity and gain for point to point communications. It consists of two radiating elements made of connected squares with the side length equal to ¼ mid-band wavelength. These two squares made of copper wire, are located apart from a rectangular metallic circular plate that acts as a reflector and connected to a 50-ohm feeding coaxial cable.  

 

Part	Dimension (mm)
Wire diameter	1.5
Square side length	32
Inner conductor diameter	1.5
Outer Dielectric diameter	5.1
Reflector diameter	140
Reflector thickness	2
Coaxial cable length (Above the reflector)	18
Mid-band wavelength	125

Material	Relative permittivity	Dielectric loss tangent	Electrical conductivity (S/m)	Thermal conductivity (W/m. K)
Copper	1	0	5.96E+7	401
Aluminum	1	0	3.5E+7	237
Teflon	2.1	0.001	0	0.23

Electromagnetic boundary conditions

Wave port: The wave port boundary is applied to the dielectric input face of coaxial antenna.
Radiation: The Radiation boundary condition is used to truncate the open computation domain when analyzing antenna problems. In our case, it is applied to the outer faces of surrounding Air box.
Perfect electric conductor: a PEC boundary condition is applied to the outer lateral face of the dielectric part of the coaxial feeding cable and to the top face of the reflector.

Mesh

Results

 

The Antenna solver of HFWorks is used, coupled to the thermal case for a working frequency of 2.4 GHz. The simulation revealed the next results of Electric and Magnetic field distribution for an excitation power of Pin=1-Watt:

 

The metallic reflector reflects the electromagnetic waves back to the front of the antenna (-Z-axis), hence reducing the radiation to the back and improving the antenna gain and directivity in the forward direction. The obtained gain measures the ability of the antenna to concentrate radio frequency energy in a particular direction. It is typically measured in dB and found equal to 10.5 dB for the studied antenna design.
The Gain radiation pattern results shown by figure 5 confirms the directional criteria of the Biquad antenna in transmitting the most of its transmitted power to the desired direction better than in the other directions.

 

To evaluate the thermal behavior of the studied antenna under an applied excitation power of 5-Watt, convection BC is applied to the surrounding air box at an ambient temperature of 22°C and a convection coefficient set to 

B-shaped dual loop antenna [3]: 

Loop antennas used as single elements or in arrays have a variety of practical applications in wireless communications. They are characterized by their simple design, low cost and flexibility. They have various shapes: circular, triangular, elliptical, etc..

The adopted loop antenna design is made of a half of dual connected loops over a conductor plane, as depicted in figure 7. The power feeding is made through a low loss 50-Ohm coaxial cable.

 

Part	Dimension (mm)
Wire diameter	1
Loop radius	20
Inner conductor diameter	1
Outer Dielectric diameter	3.35
Substrate dimensions	220 x 100 x 1.6
Mid-band wavelength	125

The used metallic wire is made of copper with Teflon material for the dielectric coax part. A glass-fiber FR4 substrate is employed with dual metalized sides.

Material	Relative permittivity	Dielectric loss tangent	Electrical conductivity (S/m)	Thermal conductivity (W/m. K)
FR-4	4.6	0	0	0.36

Electromagnetic boundary conditions

Wave port: The wave port boundary is applied to the dielectric input face of coaxial antenna.
Radiation: The Radiation boundary condition is applied to the outer faces of surrounding Air box.
Perfect electric conductor: a PEC boundary condition is applied to the top and bottom face of the substrate part.

Mesh

Results

For a range of frequency of [2GHz-3GHz], the simulation revealed the next results at 2.4 GHz: The first figure shows the distribution of electric field:

The next figure shows the 2D plot of return loss results versus frequency, with nominal frequency 2.4GHz. As shown, the bandwidth achieved by the B-shaped loop antenna for S11< -10dB is 420 MHz

 

A comparison between HFWorks and measurement results -Table 5 - confirms the good agreement between them.

Results	Measurement	HFWorks simulation
Magnitude of S11-2.4GHz	20 dB	21 dB
Gain -2.4GHz	6.31 dB	5.98 dB
	40%	42%

The radiation pattern is defined as 3D/2D graphical representation of the far field radiation properties of the studied antenna at a specified frequency, as a function of the direction of propagation of the electromagnetic (EM) wave. A radiation pattern can represent several quantities, such as gain, directivity, electric field, or radiation vector. For the studied case; the results of E-field radiation pattern  and  at 2.4 GHz are shown below:

 

Finally, to evaluate the thermal behavior of the studied antenna under an applied excitation power of 5-Watt, convection BC is applied to the surrounding air box at an ambient temperature of 22°C and a convection coefficient set to 

Figure 12 - Temperature distribution across the copper part at 2.4GHz.

Conclusion

In this work, a dual antenna analysis was performed with two different designs for WIFI connectivity applications. The obtained results proved their good performance for wireless systems in the 2.4GHz band. Both designs are simple to build and offer good directivity and gain characteristics. Furthermore, the performed thermal coupling analysis allowed to confirm that the studied antennas are not suffering from high heat loads and thermal stress effects under the applied excitations.

References

[1]. Singh, Bablu Kumar and Amandeep Singh. “A Novel BiQuad Antenna for 2.4GHz Wireless Link Application : A Proposed Design.” (2012).
[2]. www.sjsu.edu/people/raymond.kwok/docs/project172/Omni_and_Biquad_antenna_2009.pdf
[3]. Chamorro-Posada, Pedro, et al. "A plug’n’play WiFi surface-mount dual-loop antenna." HardwareX 1 (2017): 46-53.