For RF detector experts

4G Mobile Phone detector is a device for detecting if there is a radio signal transmitted to a base station from a cell phone in the vicinity.

Cell phone signal propagation is not as straightforward as people think. It is not as ideal as 1 ＋ 1 ＝ 2. We need to be familiar with some basic RF properties to well operate a cell phone detector.

At the same location (a distance from a cell phone), received signal strength can vary with respect to time.
At the same time, received signal changes with respect to Distance.

On average (over tens of seconds):

At outdoor area, doubling the distance will attenuate the signal strength by 6dB. In other word, the strength will be reduced down to only ¼.
In indoor area, doubling the distance will attenuate the signal strength by 4.5dB; i.e., reduced down to 1/3 of the strength.

In Radio Frequency (RF) world, Signal Strength is usually expressed by dBm, in order to cover a gigantic variation range.

Generally speaking, cell phone should receive signal strength from a base station at about -85 dBm, or 0.0000000038 mW. The cell phone, on the other hand, will transmit signal with strength strong enough to be captured by a base station at about -85dBm too.

In summary:
On average (over several tens of seconds):

At outdoor area, every doubling of a distance reduces the signal strength by about 6dB.
In indoor space, every doubling of distance reduces the signal strength by about 4.5dB.

The signal transmitted from a cell phone will reduce from about 20dBm to -50 or -60 dBm at 10 or 20 meters away. In other words, it reduces to only 1 millionth of the original strength.

At the same moment, the received signal reduces with respect to distance but not following a smooth curve.

Actually, it can create a situation that at certain time, or at certain location, if people are nearby or not, the detector can not detect a signal transmitted from a cell phone happens to transmitted at the frequency that has serious cancellation. Cell phone detector can not function properly under this condition.

In addition, cell phone antenna is somewhat directional. The user usually is not stationary; signal transmitted toward the detector can therefore vary to a degree that can cause trouble for detector to detect. As a Counter Surveillance Experts, you need understand radio frequency characteristic and experience in using the detector.