An Audible Field Strength Indicator for Radio Direction Finding

Posted on Feb 5, 2014

An audible field strength indicator (AFSI) lets you listen to changes in radio signal strength so that you can use your eyes to look for concealed antennas. Using inexpensive, easy-to-obtain parts, the following design combines a sensitive amplified field strength detection circuit, and a variable frequency audio oscillator. Its main features incl

An Audible Field Strength Indicator for Radio Direction Finding
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ude: Combining the AFSI with a directional antenna, such as the 2-element collapsible delta quad, makes a powerful sniffing tool that lets you home in on a signal source quickly. One caveat: Most field strength meters (including this one) aren`t very selective. That is, they don`t do a very good job of distinguishing between two or more signals on the air at the same time, even when those signals are widely separated in frequency. Despite the LC tuned circuit resonant at 146 MHz used in this design, the AFSI will respond with surprising sensitivity to paging transmitters at 158 MHz, nearby 440 MHz transmitters, and even cellular phones! So use the AFSI with a certain amount of skepticism in areas where you suspect there could be other RF sources. A schematic diagram for the AFSI is shown in Figure 1. The signal arriving from the antenna is fed into the tuned circuit formed by L1 and C1. To prevent the antenna system from loading down the tuned circuit, the signal is fed into L1 at a tap located one-half turn from its grounded side. The signal leaving the tuned circuit is taken several turns from the top of L1, also to prevent detuning the tuned circuit. Diodes D1 to D4 prevent extremely strong signals from overdriving U1. C2 couples the RF from the tuned circuit to D5, which rectifies it, creating a DC voltage that varies with changes in RF signal strength. The DC voltage is fed into a simple integrator (R1-R3 and C3) that...

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