Simple RF Field Strength Meter Circuit

This circuit shows you how to build a tool to measure radio waves!

Radio waves are invisible signals that carry things like radio broadcasts and cellphone signals.

This tool can help radio lovers, engineers and hobbyists see how strong these signals are in a certain area.

They can use it to improve their radio antennas, make sure their radios are working well and see how strong the signal is from different radio stations.

Circuit Working:

Parts List:

This uncomplicated circuit serves to gauge the strength of a transmitted signal from a transmitter.

It is frequently employed by model airplane constructors to verify the effective transmission of radio signals from their transmitters.

By employing this FS meter one can swiftly identify whether the issue lies with the transmitter or the receiver module.

The sole active component in this circuit is a single transistor, denoted as T1.

This transistor is configured as a controlled resistor, constituting one segment of a balanced bridge circuit.

The base of T1 is directly linked to the receiving antenna.

As the radio frequency voltage at the transistors base increases the transistor conducts and disrupts the equilibrium of the bridge circuit.

This imbalance directs currents through R3, the meter and the emitter collector junction of the transistor.

Instructions for use: Prior to activating the transmitter calibrate the meter to zero by adjusting P1.

Formulas:

From the above circuit below is the formula for common emitter amplifier.

A common emitter amplifier is a simple transistor amplifier arrangement in which the input signal and the output circuit share the same transistors emitter.

This is a thorough rundown of all the formulae and factors to take into account while building a common emitter amplifier:

Voltage Gain Av: The following may be used to estimate the voltage gain of a common emitter amplifier in terms of resistors Rb, Rc and Re:

Av ​= −R_c /​ R_e

where,

• This calculation makes the assumption that, for AC signals the bypass capacitor is big enough to short circuit R_e.

Input Impedance Z_in: The typical emitter amplifiers input impedance is roughly:

Z_in​ = R_b

where,

• Primarily, this is dictated by the base resistor Rb.

Output Impedance Z_out​: The transistors inherent impedance and the collector resistor Rcs design determine the output impedance:

Z_out​ = Rc ​/ β + 1

where,

• where β is the current gain of the transistor.

Voltage Gain with Bypass Capacitor Av​: if a bypass capacitor C1 is placed across Re, hence shorting Re for AC signals.

Av ​= −Rc ​/ Re

• In comparison to the non bypassed scenario, this design considerably enhances the voltage gain.

Choosing the right resistor values, making sure the biasing is correct, and examining the AC and DC characteristics are all necessary steps in the design of a common emitter amplifier in order to get the required voltage gain, impedance matching, and signal quality.

How to Build:

Building the RF field strength meter circuit for measuring the strength of a radiated signal involves assembling the components and connecting them according to the provided circuit diagram.

Prepare Components:

• Identify and gather all the necessary components listed above.

Connect Transistor T1:

• Place the transistor T1 on the PCB.
• Connect the base of T1 directly to the receiving antenna.

Configure T1 as Controlled Resistor:

• Set up the transistor T1 as a controlled resistor in one part of a balanced bridge circuit.
• Connect additional components to create the balanced bridge which may include resistors and capacitors.

Connect Resistor R3:

• Connect resistor R3 in series with the meter and the emitter collector junction of transistor T1.

Install Potentiometer P1:

• Integrate potentiometer P1 into the circuit.
• Adjusting P1 will calibrate the meter to zero before transmitter activation.

Set Up Meter:

• Connect the meter to the circuit, ensuring proper polarity.

Finalize Bridge Circuit:

• Complete the balanced bridge circuit using the necessary components to maintain equilibrium when there is no signal.

Calibration:

• Before turning on the transmitter, adjust potentiometer P1 to set the meter to zero.
• This step is crucial for accurate signal strength measurement.

Power Supply:

• Provide a stable power supply within the operational range of the components.

Test:

• Test the circuit by activating the transmitter and observing the meters response.
• The meter should indicate variations in signal strength.

Fine-Tuning:

• Fine tune the circuit as needed based on testing results.
• Adjust component values or configurations for optimal performance.

Additional Considerations:

Safety Precautions:

• Follow safety guidelines when working with electronic circuits.

Component Values:

• Choose resistor, capacitor and potentiometer values based on your circuit design and specific requirements.

Antenna:

• Ensure the antenna is correctly connected and positioned for effective signal reception.

Grounding:

• Pay attention to proper grounding to minimize interference.

Note:

• If you are not experienced with electronics consider seeking assistance from someone with expertise in circuit design or consulting relevant resources for additional guidance.

Conclusion:

These meters are valuable tools for activities such as aligning antennas, locating sources of interference, and ensuring compliance with RF regulations.

The design of the circuit can vary depending on the desired frequency range the sensitivity required and the intended application.

As with any RF related device proper handling and safety precautions are essential when working with RF field strength meters.

References:

Field strength meter