Building a Wireless Internet Receiver with a Biquad Antenna

A biquad antenna uses two square-loop elements to create a directional radiator. With an appropriate reflector and feed arrangement, this geometry can be used around bands such as 2.4 GHz. The result depends on more than bending a wire into a familiar shape: dimensional tolerance, impedance, polarization and feed-line loss all affect practical performance.

Resonance and Geometry

The side length of each square is on the order of a quarter wavelength:

lambda = c / f

The actual optimum dimensions can shift with conductor diameter, corner geometry, reflector spacing and surrounding dielectric effects. At microwave frequencies, millimeter-scale construction errors become meaningful.

For that reason, copying fixed dimensions without considering the intended frequency and materials is less reliable than calculating the geometry and then validating it.

Reflector and Directionality

A conductive reflector reduces radiation in the rear direction and reinforces a forward main lobe. Reflector size and element spacing affect gain, front-to-back ratio and impedance.

Directionality can improve a link budget, but it also increases alignment sensitivity. A high-gain directional antenna that is pointed incorrectly can perform worse than a lower-gain antenna with suitable orientation.

Feed Line

Coaxial cable, connectors and solder joints form part of the RF path. Loss in a long or unsuitable cable can consume much of the gain obtained from the antenna.

The transition between an unbalanced coaxial feed and the antenna geometry can also allow common-mode current; an appropriate choke or balun may be needed depending on the implementation.

Measurement

Software signal-strength indicators are not a complete antenna measurement. Where possible, return loss or VSWR can be checked with a VNA, and link throughput or packet loss can be measured repeatedly across orientations.

Any transmitting use also has to respect the applicable frequency, power and antenna regulations.

This page preserves an old practical antenna experiment while separating the construction idea from claims that require RF measurement.

References

• **[1]** Constantine A. Balanis. (2005). Antenna Theory: Analysis and Design, Third Edition. Wiley.
• **[2]** John D. Kraus; Ronald J. Marhefka. (2002). Antennas for All Applications, Third Edition. McGraw-Hill.