This simple circuit helps you sniff out RF radiation leaking from your transmitter, improper joints, a broken cable or equipment with poor RF shielding. The tester is designed for the 2-m amateur radio band (144-146 MHz in Europe). The instrument has a 4-step LED readout and an audible alarm for high radiation voltages.
The RF signal is picked up by an antenna and made to resonate by C1-L1. After rectifying by diode D1, the signal is fed to a two-transistor high gain Darlington amplifier, T2- T3. Assuming that a 10-inch telescopic antenna is used, the RF level scale set up for the LEDs is as follows:
When all LEDs light, the (optional) UM66 sound/melody generator chip (IC1) is also actuated and supplies an audible alarm. By changing the values of zener diodes D2, D4, D6 and D8, the step size and span of the instrument may be changed as required. For operation in other ham or PMR bands, simply change the resonant network C1-L1. As an example, a 5-watt handheld transceiver fitted with a half-wave telescopic antenna (G = 3.5 dBd), will produce an ERP (effective radiated power) of almost 10 watts and an e.m.f. of more than 8 volts close to your head. Inductor L1 consists of 2.5 turns of 20 SWG (approx. 1 mm dia) enameled copper wire. The inside diameter is about 7 mm and no core is used.
The associated trimmer capacitor C1 is tuned for the highest number of LEDs to light at a relatively low field strength put up by a 2-m transceiver transmitting at 145 MHz. The tester is powered by a 9-V battery and draws about 15 mA when all LEDs are on. It should be enclosed in a metal case.