Detecting RF signals above approx 30MHz, this may be used as a relative signal strength meter or as an RF probe to detect sources of EMI.

The unit consists of an antenna, detector, amplifier and moving coil meter. The antenna used should ideally be a 1/4 wavelength, however, with a wideband detector such as this, that is not really practical. A small telescopic antenna, or just a length of stiff wire approximately 700mm long works very well.

Capacitor C1 together with choke L1 forms a high pass filter. Detection of the signals is performed by D1. D2 and D3, connected in inverse parallel, protect the input of the op-amp against large RF signals.

Op-amp IC1B converts the current through D1 into a voltage which drives the small moving coil meter M1. The unit is powered by a 9V alkaline battery, IC1A being used as an active voltage splitter.

Layout is not particularly critical, but lead lengths around the RF stage at the input to IC1B should be kept as short as possible. A possible layout for a PCB is included. This layout was created using Proteus Lite from Labcenter Electronics. The original layout file may be downloaded as a small zipped file.

R1 - 10M R2 - 1k R3 - 100R R4,5 - 100k C1 - 10pF C2 - 100nF C3 - 100pF C4,6 - 10uF/16V radial

C5 - 10nF L1 - 2.7uH D1 - BAT82 D2,3,4 - 1N4148 IC1 - TLC-272 M1 - 50uA (2k7-3k) moving coil meter Misc - Telescopic antenna, 9V alkaline battery, case, on/off switch