Diode Probe

The diode probe is used to detect low levels of RF voltage in a circuit. From the diagram below the input is coupled with capacitor c1. Diodes d1, d2 rectify (voltage doubler circuit )  The rf is converted into a dc value with a ripple voltage . C2 smoothes the hf  ripple from the diodes  R1 and C3 and acts like a low pass filter. The choice of the value C3  is important to determine which type of output you require. If C3 is a large (10 uf )  value this will remove the ripple from the waveform giving a DC value output .  If the RF signal you are trying to detect is amptitude  modulated then make the value of c3 0.27uf  This gives a dc output directly in proportion to rf amptitude and also gives an audio output as well.   By adding a series capacitor   (10u f to block the dc and let the ac through)  couple this into  the audio amplifier where you will hear the audio output.   I put the dipole probe into a syringe and  made c3 a fixed value of 0.27uf.  If  you just want a dc output add an external capacitor of  10uf across the c3.   Also note that if you use a high impedance voltmeter across c3 and c3a  this will charge up to 2.828 * peek rf voltage.  The capacitor c3 and c3a will take a  long time to discharge as capacitors are discharged through your high impedance voltmeter  (time to discharge  t (sec) =(1.4*R*C)   R is impedance ofvolt meter ( typical impedance for high impedance voltmeter 10M ohms) .

And C =10uf then t=(1.4*10*10^6*10*10^-6)=140 sec.  If you add a 10k across the value c3 and c3a  this will reduce the  0.14sec.

R1 1K Ohms 1/8 w

C1  0.001uf  Ceramic capacitor

C2 0.001uf  Ceramic capacitor

C3  10uf electrolytic note the + goes R1 If  you want am on signal make 0.27uf

D1   OA91 Germanium diodes or  BAT46 Schottky diodes

  The germanium diodes will give most sensitivity but they are a little leaky in reverse direction  and limited to 20 v PIV.   Their forward voltage is about 0,3 v  you will  detect rf voltage down as low 0.1 v. The schottky diodes will be a little less sensitive than the germanium diode, forward voltage about 0.4v  and will detect RF down as low as  0.2v. They will also have a higher PIV voltage than the germanium diode and  a lower Leakage current in reverse direction. 

I made the circuit board to fit the syringe.  Keep the lead length to minimum as this will increase the frequency range and detect rf voltages.  This is the layout  I used.

Remove the plunger from the syringe and put this to one side then remove the needle and throw  it a way. Make sure that it is correctly disposed of.    The needles are very sharp and can hurt you so be careful with  it  Now make a hole in the side of the syringe as shown in diagram.

Push a thick piece of wire in the hole where needle was, ensure wire is pushed right through to the plunger end.  Push a thin piece of  wire through the small hole on the side, ensuring wire is pushed right though to plunger end.  Solder wires on to  the board, make sure thin wire is connected to the earth on the board, and the thick wire is connected to C1 on the board.  The connect two insulated wires to the output on the board which is across C3. Now gently pull thick and thin wire in the syringe so the board slides inside the syringe. Make hole inside plunger and slide output wires through the hole exiting at the plunger .Tie knot to stop wires being pulled out, put plunger back into syringe use glue gun or wax to  permanently fix plunger in place. I used 22mm copper pipe fixed over syringe as a RF shield and earth point. Tape may be necessary to adjust diameter of syringe for a tight fit. Then solder earth wire to copper tube. Fix jubilee clip around copper pipe, fit insulated piece of wire under jubilee clip, fit a croc clip to the end of the wire.