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
