Metric equivalent of 1/8 inch diameter bolt to use as monopole probe on cantenna

An 1/8-inch diameter bolt is 3.175 mm in metric.
The nearest standard metric size is:

✅ M3 bolt (3 mm diameter)

• Slightly smaller than 1/8″ but the closest common metric size
• Available in 60 mm length (M3×60)

Alternatives

If you need something closer to the exact 3.175 mm diameter, there is no standard metric bolt, so M3 is almost always the correct substitute.

Summary

Buy: M3 × 60 mm bolts