Field strength meter has always been one of my favorite circuits that I loved to construct. This one is a passive field strength meter I had made recently. Field strength meter (FSM) is also called as the signal strength meter. Field strength meter when used in a radio receiver it is called as the S-meter. This instrument is used to measure the electromagnetic field around your place. The frond-end of this circuit can be modified to receive selectively whatever field that we are needed to measure. It can be tuned to the radio frequency spectrum to receive radio frequency signals that are around your place. It is a simple circuit to construct and also an useful one. The unit of measure of field strength is volts per meter.
The circuit consists of a tank circuit, a diode detector, and a digital readout meter. The input radio signal is collected by the use of a small piece of wire. A small coupling capacitor is used to sample a part of this energy to be fed to a tank circuit or a tuned circuit. The tank circuit consist of an LC, which is an inductor and a capacitor in parallel. The tank circuit can be tuned to any frequency range by changing the capacitor value or inductor value using variable inductor or variable capacitor. A combination of LC inductor and capacitor connected can also be used to select the different frequency ranges. The hot end of the the LC is connected to the detector diode at the anode end which is placed in forward direction. The diode acts as a detector to detect the incoming radio signals which is an alternating current (AC) into direct current(DC). A second diode is also used in the reverse direction from the negative side for isolation and polarity.
The output direct current from the diode is filtered using a filtering capacitor of 0.01 microfarad. The RF component in the signal is further filtered using a radio frequency (RF) choke connected to the output of the diode (cathode side). This output of the radio frequency choke may be devoid of any high RF signals and further filtering may be done if necessary to relieve off any stray RF signals using a second bypass capacitor of 0.01 microfarad connected at the other end of the RF choke. The signal here is pure DC, which can be attenuated using a suitable resistor combination or a variable resistor to be fed to a microvoltmeter or to a digital multimeter. The readout of the signal strength is got from the digital multimeter with the selector switched to the microvolt range.
The FSM is mostly used for the purpose of detecting any powerful radio signals around the place and to determine the signal strength of the transmitter. It can also used for testing purposes in measuring the output of the oscillator, tune the IF stages of the transmitter, testing the output of the transmitter stages, and to tune the output of the transmitter power amplifier. Sometimes it can also be used to detect the mobile phone radiation from your hand-phone.
The output direct current from the diode is filtered using a filtering capacitor of 0.01 microfarad. The RF component in the signal is further filtered using a radio frequency (RF) choke connected to the output of the diode (cathode side). This output of the radio frequency choke may be devoid of any high RF signals and further filtering may be done if necessary to relieve off any stray RF signals using a second bypass capacitor of 0.01 microfarad connected at the other end of the RF choke. The signal here is pure DC, which can be attenuated using a suitable resistor combination or a variable resistor to be fed to a microvoltmeter or to a digital multimeter. The readout of the signal strength is got from the digital multimeter with the selector switched to the microvolt range.
The FSM is mostly used for the purpose of detecting any powerful radio signals around the place and to determine the signal strength of the transmitter. It can also used for testing purposes in measuring the output of the oscillator, tune the IF stages of the transmitter, testing the output of the transmitter stages, and to tune the output of the transmitter power amplifier. Sometimes it can also be used to detect the mobile phone radiation from your hand-phone.
