Hi,
I don't know much about that stuff. Maybe it needs a bandpass filter or something. Maybe the wire antenna needs to be a loop or a spiral. Maybe a one-turn trimpot is virtually useless for that kind of circuit if that's what has been used. Maybe your circuit just needs to be in an enclosure. Maybe the second diode that isn't in the schematic but appears on the PCB is confusing without an explanation as to what it's doing there, if it's a diode.
The writing in the photo is out of focus. It says something about VHF. Maybe that JFET can't work at the required frequency. Maybe the JFET in the schematic has a VGS(off) which is very different to the one you have used.
Without knowing the results being obtained nor useful background info. beyond "My circuit doesn't work. Why?" I would guess the cap and inductor are integral parts of the circuit.
In dealing with RF the golden rule is: Keep connections as short as possible, particularly in the case above, cut off the extra length of the components and mount them close to each other.
I doesn't work even inductor and cap, I have no signal
The Million-Dollar Question: Are you measuring the signal strength of what kind of transmitter? You did not mention anything about the device/circuit under test.
as you can see in the video it can easily detect cellphone
The stuff shown at the video is not a cell phone, it is a FM transmitter.
Anyway, are you making tests with your cell phone when a call is made or in idle state?
Firstly - ALL the components must be fitted. If any are omitted it simply wont work.
Secondly, your photo shows two diodes but there should only be one.
You will have to accept that a design like that is very insensitive, it needs quite a lot of signal to operate it. Typically it is called a "field strength meter" and used close to a transmitting antenna for purposes of tuning it to resonance.
The JFET does not operate at the signal frequency, it is a DC amplifier with high input impedance and the potentiometer is to set the cut-off voltage so that it is at the sweet spot of just conducting with minimal signal but can be used to DE-sensitize it under strong signal conditions. You should find the meter reads very little at one end of the range then suddenly starts to rise at some point as it is turned. The optimal setting is just as it starts to rise.
The antenna should just be a short length of wire, not a loop and not grounded at the other end. The diodes must be germanium types although a small signal Schottky diode would probably work as well.
Be aware that with the potentiometer wiper at the ground end there is nothing to limit the JFET current, it would be wise to add a resistor of say 220 Ohms in series with the drain pin.
Brian.
Please show a picture of the top and bottom of the board and state what type of JFET and diodes you used.
It should work, the meter should operate even when there is no RF present.
Brian.
I still do not see all the components. There is a missing resistor from the JFET gate to ground and the coil is not connected properly.
Is the center pin of the potentiometer wired to anything?
What type of JFET have you used, it was designed for a 2N3819 but it looks like you might have used something else.
Brian.
Have you tried to connect a ground? (Battery negative rail to mains earth) This will improve sensitivity dramatically. I trust that you know what you are doing in connecting to the mains earth.
No, they all are connected well, pot center also connected on the other side of circuit, as I said before resistor didn't Improve anything so I removed It, it didn't change anything and there wasn't really high current
You should have a working circuit before you try to "improve" it. You only need the resistor, the JFET, the potentiometer and the meter for the basic circuit. If you can't adjust the meter reading with those components, the wiring is incorrect or a component is broken. When the basic circuit works, you can add the coil, diode, capacitor and antenna. You should not remove the resistor, but you can increase the value.
Just have a couple questions about your circuit. It looks like you have connected coil to the end of your antenna wire. And the ends are just twisted together. The coil is also not very neatly wound. And the individual turns are widely spaced. I don't know if any of these things matter. Just thought I'd mention it and maybe the gurus could comment on it. Hopefully in the end it helps with your problem.
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