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RF antenna picking up something but what?

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boylesg

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I have the following circuit implemented.

X1 is actually a neon bulb and 74HC14D_6V is actually a SN74HC14 chip.

The floating wire attached to C1 is currently about 6m of wire coiled up. I am intending to connect this to a short length of wire beneath a tesla coil as an antenna to pick up the tesla's RF. The coiled up wire will merely be a means of connecting the antenna to the below circuit and I will eventually trim it, although I am not sure what length yet as the connecting wire will probably function as part of the antenna.

Currently the circuitry is picking up something as the LED is flashing irregularly. It doesn't seem to be mobile signals as puting a mobile phone near the coiled up wire doesn't seem to effect the flashing of the LED.

My question is: what is my antenna likely to be picking up at present?
 

U1A is a CMOS device so the current drawn into it's input pin will be extremely low, probably only a few nA. The Neon lamp is effectely open circuit until it has striking voltage across it so there is almost no load on the antenna input wire and it 'floats' at whatever voltage is around it. What you are seeing is diturbances in the electrical field around the antenna. Some of this will be radio pickup but most will be from interference from electrical loads being switched on or off and from atmospheric discharges (lightning flashes). Mobile phones transmit on much higher frequencies than these logic gates can switch so it's not suprising they don't operate it although you might find that a the flashing stops when an operating phone is nearby because it will induce DC offsets into the circuit which will to some degree stabilize the logic levels.

Incidentally, the neon will not work as protection as it is at the moment because any signal greater than the supply to the ICs will conduct current into them and destroy them will before the neon strike voltage is reached. You should consider much higher values for R1 and R2, at least 1M each to protect the ICs. You can also remove C2, R3 and R4 and connect the output of U2A to the transistor base through a 10K resistor to simplify the circuit.

Brian.
 

betwixt said:
U1A is a CMOS device so the current drawn into it's input pin will be extremely low, probably only a few nA. The Neon lamp is effectely open circuit until it has striking voltage across it so there is almost no load on the antenna input wire and it 'floats' at whatever voltage is around it. What you are seeing is diturbances in the electrical field around the antenna. Some of this will be radio pickup but most will be from interference from electrical loads being switched on or off and from atmospheric discharges (lightning flashes). Mobile phones transmit on much higher frequencies than these logic gates can switch so it's not suprising they don't operate it although you might find that a the flashing stops when an operating phone is nearby because it will induce DC offsets into the circuit which will to some degree stabilize the logic levels.

Incidentally, the neon will not work as protection as it is at the moment because any signal greater than the supply to the ICs will conduct current into them and destroy them will before the neon strike voltage is reached. You should consider much higher values for R1 and R2, at least 1M each to protect the ICs. You can also remove C2, R3 and R4 and connect the output of U2A to the transistor base through a 10K resistor to simplify the circuit.

Brian.
Click to expand...
Sorry Brian I have neglected to add the 2 zener diodes that are clipping any input signal to +-4.7V.

I have updated the image.
 
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    bmb_10

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Ok, sort of makes sense but you shouldn't be using Zener diodes. Use normal signal diodes, both with cathodes facing the + supply. That will clip any signal to no more than 0.6V below ground or 0.6V above supply.

Brian.
 

betwixt said:
Ok, sort of makes sense but you shouldn't be using Zener diodes. Use normal signal diodes, both with cathodes facing the + supply. That will clip any signal to no more than 0.6V below ground or 0.6V above supply.

Brian.
Click to expand...

Why shouldn't I use zener diodes?
 

Well, you CAN use Zeners but there's no advantage in doing so. The top diode does nothing anyway because Zeners work as ordinary diodes below their knee voltage so any voltage more than 0.6V below ground will make the bottom diode conduct current to ground. Similarly, anything above Vz of the bottom diode will be clipped by it's Zener action. Two normal signal diodes will work just as well if you 'aim' them in the same direction with cathodes toward the + supply.

Apart from the higher costs of Zener diodes, they have one other property which may reduce performance, they have higher capacitance than signal diodes so they load the incoming signal more. In your application that may not be a problem but at higher frequencies it could reduce the signal levels and hence sensitivity.

Brian.
 

betwixt said:
Well, you CAN use Zeners but there's no advantage in doing so. The top diode does nothing anyway because Zeners work as ordinary diodes below their knee voltage so any voltage more than 0.6V below ground will make the bottom diode conduct current to ground. Similarly, anything above Vz of the bottom diode will be clipped by it's Zener action. Two normal signal diodes will work just as well if you 'aim' them in the same direction with cathodes toward the + supply.

Apart from the higher costs of Zener diodes, they have one other property which may reduce performance, they have higher capacitance than signal diodes so they load the incoming signal more. In your application that may not be a problem but at higher frequencies it could reduce the signal levels and hence sensitivity.

Brian.
Click to expand...
I think I can see what you mean about capacitance and the effect on signal strength.

I did as you suggested and changed the zeners to 1n4148s and now the LED is flashing noticeably brighter.
 

Just as an experiment - try listening to a long-wave or medium wave AM broadcast radio, tuned off any station and see if the flashes coincide with crackles of interference.

Brian.
 

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