analogfsi

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.

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. A low base-collector capacitance will enhance the VHF and UHF sensitivity. All transistors should be of the same type and thermallly coupled to one-another to minimize thermal drift. The DC gain of the detector is about 25X (estimated by multiplying the voltage drop across the collector load by 38). Assembly is not critical and mine was built on punched fiberglass board without a ground plane. The 10k pot is a the offset adjustment.

The circuit is powerd by a single 1.5 volt AA cell. The current drain is so small, about 60 microamps, I didn't bother with an on-off switch -I just slip a battery in for the day, and hope I remember to remove it when I'm done. The supply voltage can be increased up to the breakdown voltage of the transistors to increase the sensitivity, but beware - the sensitivty to thermal drift will increase as well.

How it works: RF voltage coupled by the antenna is applied to the base of one transistor, and this current causes an increase in collector current of the transistor (the transistor on the far-right of the schematic), increasing the voltage drop across the 39k resistor on its collector, which results in a reduction of the collector voltage. That resistor, along with the capacitor from the collector to ground makes a 1 kHz low pass filter. The transistor in the middle provides a reference voltage for the voltmeter. With no RF filed applied, the 10K pot is used to make the voltages on the two collectors equal, making zero volts across the voltmeter's terminals. If you are the kind of person who likes to see the meter read zero when there is no signal present, I suggest using a 10k pot with a knob instead of a trimpot, or better yet, building the auto zero version.

The transistor on the far right of the schematic generates a bias voltage for the other two transistors. Since the base-emitter voltage of these transistors need to track each other, its a good idea to put them under a cover or glue them together, or both.

The circuit is intended to drive the input of a high impedance DVM. Making the circuit battery powered gives the additional advantage of being able to float the circuit so that one of the DVM terminals can be grounded, if necessary.

I use this detector to drive a DVM set to the 200 mV scale since the signals I am interested in seeing result in an output of a few tens of millivolts. The flexible wire antenna provides flexibility in use in that it can be bent and shaped to control the sensitivity. The wire antenna can also be formed to make a "sniffer" probe to help in finding areas with highest RF levels.

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