Summary of RF Field Strength meter using AVR microcontroller
This article describes a battery-powered, microcontroller-free RF field strength indicator probe. It uses a small antenna to detect radio frequency energy and convert it into millivolt signals for a DVM. The circuit features hot melt glue for thermal stability and component protection, ensuring low current drain and automatic shutdown for extended battery life.
Parts used in the RF Field Strength Indicator Probe:
- Hot melt glue
- Three transistors (MPSH34 or 2N2222)
- Battery holder
- Small antenna (15 cm insulated wire)
- Digital Volt Meter (DVM)
The hot melt glue that covers the circuit serves multiple purposes: It helps to keep
the temperature even among the three transistors (to minimize thermal drift), it protects
the components from physical damage, and it holds the battery holder on the board.
As I used this probe last nigh to determine if a 384 MHz oscillator was really working or not, I remembered email I received a while ago, asking how to make a field strength indicator without the microcontroller. Thus this page. If you want the auto zero version, which is this circuit with an auto-zero integrated circuit, use the circuit shown on this page for details.
This broad band probe has a small antenna (about a 15 cm length of insulated wire). Radio Frequency energy coupled to the antenna is detected and made available to drive millivolt level signals to the input of a DVM (Digital Volt Meter). Its battery powered for convenience with very low current drain and automatic shutdown for long battery life.
the temperature even among the three transistors (to minimize thermal drift), it protects
the components from physical damage, and it holds the battery holder on the board.
As I used this probe last nigh to determine if a 384 MHz oscillator was really working or not, I remembered email I received a while ago, asking how to make a field strength indicator without the microcontroller. Thus this page. If you want the auto zero version, which is this circuit with an auto-zero integrated circuit, use the circuit shown on this page for details.
This broad band probe has a small antenna (about a 15 cm length of insulated wire). Radio Frequency energy coupled to the antenna is detected and made available to drive millivolt level signals to the input of a DVM (Digital Volt Meter). Its battery powered for convenience with very low current drain and automatic shutdown for long battery life.
You can use 2N2222’s for the transistors if you want.
The MPSH34 has two things going for it: low input
capacitance, and I have a lot of them on hand.
The MPSH34 has two things going for it: low input
capacitance, and I have a lot of them on hand.
I’ve used the circuit shown below to check the output of transmitters at 4 MHz, 35 MHz, 55 MHz, 100 MHz, 384 MHz, 900 MHz, a cell phone, and a microwave oven. It really is broad band, and I am sure the response varies considerable with frequency. Since the collectors and emitters of the detector transistor are both at RF ground, choice of transistors isn’t all that critical.
For more detail: RF Field Strength meter using AVR microcontroller
- What is the primary function of the hot melt glue in this project?
The glue keeps the temperature even among the three transistors to minimize thermal drift, protects components from physical damage, and holds the battery holder on the board. - Can I use 2N2222 transistors instead of MPSH34?
Yes, you can use 2N2222s for the transistors if you want. - How does the probe handle power consumption?
The device is battery powered with very low current drain and includes automatic shutdown for long battery life. - What frequency range has been tested with this circuit?
The circuit was used to check outputs at 4 MHz, 35 MHz, 55 MHz, 100 MHz, 384 MHz, 900 MHz, a cell phone, and a microwave oven. - Does the choice of transistor matter critically for this design?
No, since the collectors and emitters of the detector transistor are both at RF ground, the choice of transistors isn't all that critical. - How does the probe generate signals for the DVM?
Radio Frequency energy coupled to the antenna is detected and made available to drive millivolt level signals to the input of a DVM. - What length of wire is recommended for the antenna?
The probe uses a small antenna which is about a 15 cm length of insulated wire. - Is there an auto-zero version of this circuit mentioned?
Yes, an auto-zero version exists which uses this circuit with an auto-zero integrated circuit.
