RF Field Strength meter

Posted on Apr 8, 2012

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.

Click here to download the full size of the above Circuit.

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),...

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