Improved Axial-Mode-Helical-Antenna Impedance Matching Utilizing Triangular Copper Strip for 2.4-GHz WLAN

In this paper, impedance matching for a PVC-tube helical antenna operating at the frequency of 2.4 GHz is experimentally investigated. The helical wire is wound around a polyvinyl chloride (PVC) pipe having the diameter of 34 mm. The PVC pipe, in effect, shifts down the onset of axial-mode radiation to lower value of normalized circumference of the helical antenna, compared with an air-core helical antenna. Impedance matching is accomplished by replacing a section of helical wire near the feed point with a triangular copper strip having the height of 15 mm. The length of the matching strip is identical to that of the helical wire being replaced. Overall return loss is below -20 dB across the entire 2.4-GHz WLAN frequency range for the strip length varying from 35 mm to 50 mm with the optimum at 40 mm. Such wide variation relaxes the restrictions on the length of impedance-matching strip so long as the strip length is not excessively away from its optimum. This, in effect, simplifies the impedance-matching process, making this technique attractive. In addition, the result shows that return loss of approximately lower than -20 dB can be obtained across the frequency range from 2.0 GHz to 2.7 GHz when the strip length is equal to 40 mm.