A study on axial‐mode helical antennas with spiral strip arms

Abstract

An axial-mode helical antenna with a spiral strip arm excites circularly polarized plane waves propagating along the axis of a drill string bore. This antenna extends the transmission distance of downhole signals in drill strings, thereby increasing the depth of possible measurements. The half-power lobe width and directivity of the axial-mode helical antenna matched the microwave channel of the drill string bore. The helical radiator in the antenna was optimized to ensure that center frequency was within the optimal operating frequency band of microwaves in the drill string bore. The reflector in the antenna was optimized through the creation of gas vents, which reduced its cross-sectional area. The axial-mode helical antenna reduced the effects of drill string twisting on the microwave channel, reduced reflection in the walls of the drill pipe, and reduced the loss of microwave signals. Compared to linear antennas, the axial-mode helical antenna had superior performance because S21 increased to about 45 dB in the 2.4 GHz frequency band. The use of axial-mode helical antennas in drill string bores during microwave measurement while drilling (MMWD) helped in monitoring downhole safety risks and expanding the scope of gas drilling. Therefore, axial-mode helical antennas have practical applications and require further research.