How to Extend YL-69 Soil Hygrometer Humidity & Soil Moisture Detection Sensor

Hi,

I'm working on a project using a YL-69 Soil Hygrometer Humidity & Soil Moisture Detection Sensor but it's not covering the whole area.

My question is: How can I extend the reading range of this sensor?

Does anyone have experience with this particular problem? I've tried with normal copper cables, but it doesn't work.