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If you have a

ham radiolicense and a

transmitter, it

really helps tocheck if your

signal is comingout of the

antenna! This

extremelysensitive field

strength meterwill show

whether or not

something isbeing

transmitted and indicates any changes in power output or antenna design. It does NOT indicate

whether the signal is free of spurious unwanted and illegal emissions " for that a tunable fieldstrength meter wavemeter is needed. Nonetheless I find that this unit comes in very handy for

checking a transmitting system. # big advantage is the wide range of meters that can be used forthis pro$ect% almost anything that can be found will work. I threw this design together myself with

what I had on"hand at the time and use it fre&uently.

'ow it works( )* signals entering through the antenna are detected by voltage"doubler diodes

+-+. # /0 I1 op"amp configured as a non"inverting amplifier increases the signal by a factordetermined by ) and ) according to the formula

2#IN 3 4 ) - ) and finally a micro" or milli" ammmeter, shunted by resistor )5 to read "volt full"scale, displays

the amplified signal strength.

1onstruction( 6uild on a prototyping board first and start with ) and ) values shown, with

appropriate )5 value for the meter you have see 7 in parts list below. #pply power. The circuitwill probably need ad$usting to 8ero the meter( add resistance in increments of about 9: of )5 in

series with 7 to bring the meter needle to 8ero " this will take some trial"and"error. #lternatively, a

8ero"ad$ust potentiometer can be inserted between 7 and ground. *or more gain, increase the ratioof ) to ) and re"8ero the meter. ;se a walkie"talkie, remote or cellular phone, toy car remote

control " anything that emits an )* signal " to confirm that itxtend antenna, power up, and approach )* source. #d$ust sensitivity control 9? potfor desired reading. 7y version, built with the values given in the schematic, pins the needle when

brought to within @ feet of my dipole antenna fed with the 7ighty 7ite transmitter powered at 0volts.

A#)T= BI=T) " C.@?"ohm resistor see text

) " 99?"ohm resistor see text)5 " resistor appropriate for meter used see 7 description below

7 " any meter from 99u# to 99m# fullscale. ;se Ohm

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99u# C?

C99u# ?m# ?

Cm# 99"ohm

9m# 99"ohm Cm# 09"ohm

C9m# 9"ohm 99m# 9"ohm

Ohm-I% in this application(

)esistanceDinDohms 3 voltsDdesiredDfullscale - meterDcurrentDinDampsDfullscale

If you have a C9u# or less meter, =#E> IT for a different pro$ect!

9?"ohm potentiometer

/0 op"amp integrated circuit F"pin I1 socket for above

N50# germanium diodes, or e&uivalent " =chottky diodes such as the NC/ are fine.

common GglassG silicon diodes such as the NH0 will also work,but with greatly reduced sensitivity

9.99"u* ceramic disc capacitor6,6 " H"volt transistor"radio batteries

= " +ouble"Aole, =ingle"Throw =witch or use - of +A+T switch...1ollapsible portable"radio replacement antenna

or use about an FG of wire as an antenna, box, knob, etc.