Based on a field strength meter design by Tom Bruhns, K7ITM.
By Charles Scharlau, NZØI
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 include:
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 smoothes the DC voltage and shapes its decay curve. The DC level across C3 is connected to the input of a logarithmic DC amplifier formed by U1, D6, and R4-R6. The amplified output goes to R7 and C4, which limit how rapidly the DC voltage can swing. Finally the voltage is coupled by R8 to the input of the variable frequency audio oscillator (Q1, U2, and the R's and C's attached to them). R12 serves as the volume control for the audio output.
- 9-volt Battery
C1 - 5 - 65 pF trimmer (Digi-Key #SG3009-ND)
C2 - 2000 pF ceramic
C3 - 0.01 uF ceramic
C4, C7 - 10 uF, 16V, aluminum electrolytic
C5 - 100 uF, 16V, aluminum electrolytic
C6 - 0.1 uF ceramic
D1 - D4, D6 - 1N4148 diodes (5) (Radio Shack #276-1122)
D5 - Germanium Diode (Radio Shack #276-1123)
L1 - 40 nH inductor (C1 and L1 chosen to resonate in band of interest)
Q1 - 2N2222 small signal transistor
R1, R5, R8 - 100K ohm, ¼W
R2 - 22M ohm, ¼W
R3 - 1M ohm, ¼W
R4, R10, R11 - 1000 ohm, ¼W
R6 - 100K ohm trim pot
R7 - 10K ohm, ¼W
R9 - 10K ohm trim pot
R12 - 1K ohm potentiometer audio taper
S1 - SPST switch
U1 - CA3130 MOS op amp (Digi-Key #CA3130E-ND)
U2 - NE555 timer (Radio Shack #276-1723)
This field strength indicator is simple enough to be built “dead bug” style, or it can be built on a small circuit board if you prefer. Enclosing the AFSI in a metal chassis is recommended, especially if you plan on using it with a directional antenna. The shielding provided by the chassis will help prevent signal pickup through routes other than the direction finding antenna.
The most difficult part of building this project is getting the LC tuned circuit (L1 and C1) to resonate at the desired frequency. Winding your own L1 seems to be the best way to go. Wind several inches of #14 copper wire around a size-AAA battery serving as a coil form, and remove the battery from the center of the coil leaving you with an air-core inductor. The number of turns you use will depend on your choice of C1. Six turns is about right for the value of C1 shown above. Be prepared to make some adjustments to L1 in order to get the circuit to resonate. Your efforts will be rewarded by superior signal sensitivity.
Once you have the circuit built you are ready to align it. The only essential tool for aligning the AFSI is a weak signal source: a 2-meter transmitter capable of putting out 100 mW or less, or a more powerful transmitter operating into an attenuator or dummy load.
Step #1: After attaching a speaker or earphone to the AFSI, power it on.
Step #2: Set R6 to approximately mid range.
Step #3: With no nearby radio signals on the air, adjust R9 until once-per-second (1 Hz, approximate) pulses are heard coming from the speaker.
Step #4: Turn R6 fully clockwise, then fully counterclockwise. You should find that turning R6 in one direction results in the pulses coming from the speaker getting slower, or staying the same. Turning R6 in the opposite direction should cause the pulses to get much faster. Starting with R6 turned so that the pulses are slowest, turn R6 slowly until the pulses just begin to get faster.
Step #5: Connect to the AFSI the antenna you intend to use. Then turn on a weak nearby signal source operating in the frequency range at which you intend to use your AFSI. Bring the AFSI in close proximity to the signal source until the pulses coming from the speaker become obviously faster. While taking care not to jostle the AFSI and its antenna, adjust C1 for the fastest pulse rate (highest tone pitch) coming from the AFSI speaker. There should be a definite peak as you tune C1. If the peak occurs when C1's plates are fully meshed (maximum capacitance) you will need to increase the inductance of L1 and repeat calibration step #5. If the maximum occurs when C1's plates are fully unmeshed (minimum capacitance) you will need to decrease the inductance of L1 and repeat calibration step #5. If the peak occurs when C1's plates are partially meshed, you have done a good job of choosing L1 and C1 for resonance!
Step #6: With the signal source turned off, re-adjust R9 until the desired "no-signal" pulse rate is heard coming from the speaker; approximately 1 Hz is suggested.
Operating the AFSI is straightforward. When it's time to sniff out a nearby transmitter, whip out your AFSI, connect its antenna, and power it up. Move or rotate the antenna in the direction of highest tone pitch, and follow your ears to the hidden T!
A device of similar design has been featured in . Circuit boards, and kits were available at the time it was published in QST.
 "The NVARC FoxFinder", Reif,
Swick, and Pozerski, QST, Apr 2001, p. 35
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