A well-placed antenna is essential for achieving good RTK positioning results.
You can place your smartphone near the window in your house and it will obtain GPS coordinates after some time, but for RTK such environment won't be sufficient. For RTK to work there are special requirements for antenna placement.
An antenna needs to have a clear sky view 30 degrees above the horizon. There should be no obstacles that could block the view like buildings, trees, cars, humans, laptops, etc.
Examples of good environments for antenna placement:
top of the hill
Examples of bad environments for antenna placement:
close to the ground
Electronic devices may produce RF noise that could affect the reception of the GPS signal. Keep all electronics (including the Reach M2 itself) as far as possible from the antenna.
Helical antennas do not require ground planes. They are robust and demonstrate good performance.
Still, a ground plane can improve observation quality significantly. In some cases, it might be reasonable to mount a helical antenna on a ground plane for safety redundancy.
A ground plane is a conductive plate that reduces multipath, provides shielding, and improves signal reception. It may be a piece of metal, a roof of a car, a metal roof of a building, etc.