Here is an ideal wireless network cable tester to check whether RF signal is present in the network cable or not. It is a non contact version tester and can detect the presence of RF signal from a distance of two feet. It can be used to check all kinds of signal cables like Ethernet cable, Modem cable, Dish TV cable etc. The circuit lights an LED at the moment it accepts RF energy around the cable.
The Network Cable Tester circuit has two parts. The front end has an RF detector followed by a signal amplifier. Coil L1, diode D1, capacitor C1 and resistor R1-R2 forms the signal detector section. L1 is a 0.5 mm inner diam coil made up of 10 turns of 22 SWG enameled copper wire. D1 is the high frequency Schottky signal diode. C1 decouples the signals. The signal amplifier is built using the popular BiMOS operational amplifier CA3140. It has gate protected MOSFET inputs.
The IC has low input current requirements as low as 5pA. It is a high speed device and can work between 5 to 36 volts. When the antenna (A telescopic antenna used in pocket radio) come close to the cable, coil L1 accepts the RF energy and the fast recovery diode D1 passes this energy into the inverting input of IC1 . IC is designed as an inverting amplifier with feedback. Resistor R3 determines the degree of feedback. When IC1 gets a positive pulse, its output becomes high lighting the LED. This indicates the presence of RF signal in the cable.