circuit coll

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=3.5dBd), 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 20SWG (approx. 1mm dia) enameled copper
wire. The inside diameter is about 7mm and no core is used. The
associated trimmer capacitor C1 is tuned for the highest number of LEDs
to light at a relatively low fieldstrength put up by a 2-m transceiver
transmitting at 145 MHz. The tester is powered by a 9-V battery and
draws about 15mA when all LEDs are on. It should be enclosed in a metal
case.