  A Bradbury-Nielsen gate (BNG) is often used to modulate ion beams. It
consists of two interleaved and electrically isolated sets of wires with
uniform tension, which can keep parallel, equidistant, and coplanar over a wide
temperature range, making the BNG reliable and robust. We have previously
analyzed the non-uniform problem of wire tensions with sequentially winding
method, and developed a template-based transfer method to solve this problem.
In this paper, we introduced a progressively reduced pretension method, which
allows directly and sequentially winding wires on the substrate without using a
template. Theoretical analysis shows that by applying proper pretension to each
wire when fixing it onto the substrate, the final wire tensions of all wires
can be uniform. The algorithm and flowchart to calculate the pretension
sequence are given, and the fabrication process is introduced in detail.
Pretensions are generated by weights combination with a homebuilt weaving
device. A BNG with stainless steel wire and a printed circuit board substrate
is constructed with this method. The non-uniformity of the final wire tensions
is less than 2.5% in theory. The BNG is successfully employed in our homemade
ion mobility spectrometer, and the measured resolution is 33.5 at a gate
opening time of 350 {\mu}s. Compared to the template-based method, this method
is simpler, faster and more flexible when making BNGs with different
configurations.
