Simple RF Field Strength Meter Circuit

The construction of a radio wave measuring device is explained through this circuit!

Both radio broadcasts and cell phone transmissions use radio waves which are undetectable signals.

This application is useful for radio lovers, engineers and newcomers to find out the strength of particular signals in certain regions.

In order to improve their radio antennas and check their radios are functioning correctly they can use it to measure the signal strength from different radio stations.

Circuit Working:

Parts List:

This simple circuit measures the intensity of a signal that is sent from a transmitter.

Model airplane makers often use it to confirm that radio signals from their transmitters are being sent successfully.

One can quickly find out if the problem is with the transmitter or the receiver module by using this FS meter.

This circuits only active element is a single transistor represented as T1.

This transistor forms one part of a balanced bridge circuit in the form of a controlled resistor.

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

The transistor conducts and disturbs the balance of the bridge circuit when the radio frequency voltage at the transistors base rises.

Currents flow through resistor R3, the meter and the transistors emitter collector junction as a result of this imbalance.

Operating instructions: Adjust P1 to zero the meter before turning ON the transmitter.

Formulas:

The common emitter amplifier formula comes from the circuit above.

The input signal and the output circuit share the same transistor emitter in a common emitter amplifier which is a simple transistor amplifier design.

Below is a detailed explanation of every formula and consideration to make while constructing a common emitter amplifier:

Voltage Gain Av:

A common emitter amplifiers voltage gain in terms of resistors Rb, Rc and Re is estimated using the formula below:

Av ​= −R_c /​ R_e

where,

• This calculation considers that the bypass capacitor is large enough to short circuit Re for AC signals.

Zins input impedance:

The input impedance of a common emitter amplifier is roughly calculated:

Z_in​ = R_b

where,

• This is mainly influenced by the base resistor Rb.

Zout the output impedance:

The design of the collector resistor Rcs and the basic impedance of the transistor decide the output impedance:

Z_out​ = Rc ​/ β + 1

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

• This design greatly improves the voltage gain as compared to the non bypassed condition.

The AC and DC properties must be taken note of while constructing a common emitter amplifier.

To get the required voltage gain, impedance matching and signal quality one should choose the correct resistor values and be sure the biasing is right.

How to Build:

To build a Simple RF Field Strength Meter Circuit below mentioned are the connection steps to follow:

Get the components ready:

• Identify and collect each of the above mentioned components.

Connect the Transistor T1:

• On the PCB place the transistor T1.

• Connect T1s base directly to the receiving antenna.
• Set up T1 to function as a controlled resistor.
• Design the transistor T1 in a single part of a balanced bridge circuit as a controlled resistor.
• Connect more parts such as resistors and capacitors to form the balanced bridge.

Connect the R3 resistor:

• With the meter connect resistor R3 and transistor T1s emitter collector junction in series.

Install Potentiometer P1:

• Connecting potentiometer P1 to the circuit is necessary.
• P1 should be set to zero to adjust with the meter before turning ON the transmitter.

Set up the meter:

• One should check that the circuit to which the meter is connected which should have the proper polarity.

Finish the bridge circuit:

• Complete the balanced bridge circuit with the necessary components because to keep things balanced even if there is no signal.

Modification:

• Remember before switching ON the transmitter adjust potentiometer P1 to set the meter to zero.
• This step is important for correctly identifying signal strength.

Power Source:

• One should provide a steady power source within the components operating range.

Examine:

• Turn ON the transmitter and watch how the meter reacts to test the circuit.
• Changes in signal strength have to be displayed by the meter.

Adjusting:

• Based on the testing results adjust the circuit as necessary.
• For best results change the settings or values of the components.

Safety Measures:

• Follow safety rules specially when handling electrical circuits.

Values of Components:

• Choose the values of the resistor, capacitor and potentiometer according to the circuits design and particular needs.

The antenna:

• To ensure good transmission of signals be sure the antenna is positioned and connected properly.

Grounding:

• To reduce interference be sure that the grounding is done correctly.

Take note:

• If one lacks electronics skills think about asking help for someone with experience for circuit design or looking for extra information from reliable sources.

Conclusion:

These meters are useful instruments for tasks including antenna position, identifying interference sources and checking compliance to RF laws.

The main purpose of this circuit is its use, the needed sensitivity and the desired frequency range that can all affect the circuits design.

Working with RF field strength meters requires careful handling and safety measures just like any other RF related device.

References:

Field strength meter