... electric field, through special amplify method it just need 20 meters receiver pole to complete survey.Need 3-5 people or more ... 't be lost when power off.5. It can storage 20 curve measuring data of3 frequency and 30 frequency.6. Curve ... point for fixed-pointmeasurement, measuring the following 30 frequency data:20,30,40,50,60,70,80,90,100,110,120 ...