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I am a complete beginner with radio and electronics in general.

I did some research on antenna designs and found that, with dipole antennas, at least from my understanding, the second antenna will act as a ground, balancing them. I have built the following circuit and it does not produce static, as I thought it would.

enter image description here

I would like to know:

  1. If this circuit should at least produce static through the speakers in theory.
  2. If not, how can I use a dipole antennae properly to do so.

Apologies if this question is really dumb, and the answer is obvious, but I've tried googling everything related to this and it seems like it should work.

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    \$\begingroup\$ google crystal radio \$\endgroup\$ – jsotola Jan 13 at 4:03
  • \$\begingroup\$ You will need to be able to tune in a carrier and then you will need a way to integrate out the carrier with a capacitor (and probably a resistor, too) low-pass filter. If you can find a very, very sensitive headset, then you might get away without much more. (These fancy headsets are very hard to find, these days. But you can make them, if you want.) But as @jsotola says, you should really just google up "crystal radio" and see what you find. Lots of good stuff that way. \$\endgroup\$ – jonk Jan 13 at 8:42
  • \$\begingroup\$ For AM, it is close. \$\endgroup\$ – MadHatter Jan 13 at 13:49
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That circuit unfortunately wouldn't work even if the neighbour were an AM radio station. There's no closed circuit. The rectified AC voltage would charge the capacitance between the antenna branches and then the current would stop. The diode prevents discharging.

A working version of the minimum receiver has the diode in parallel with the speaker. The antenna is at the ends of the diode. It would work well if the speaker was sensitive enough having much more turns in the coil than ordinary speakers today have. Magnetic or piezoelectric headphones would be a good replacement for a speaker.

The next link shows how those sensitive old style magnetic headphones look: http://www.crystal-radio.eu/en2000ohm.htm There's also application guidance. Typically those headphones had so much wire in their coils that their DC resistance was 2000 Ohm.

FM reception would be impossible without parts which convert frequency to voltage. I guess the antenna is not selective enough, but if there's a strong AM station in the town something can well be heard with the fixed version. The antenna wires should be several meters long for substantial listening wolume even with headphones, but that depends on the distance and the power of the transmitter. A few watt strong 27 MHz CB transmitter with AM mode ON can generate within few meters so strong field that 50 cm long antenna wires catch a hearable signal.

Not asked:

The normal crystal receiver has a tunable resonance circuit because it makes possible to select between more than one transmitters. In addition it's resonance makes the voltage higher if there's not too much load. That reduces the attenuation caused by the voltage drop in the diode. If you try a crystal receiver, get a germanium diode. Its voltage drop is much lower than the drop in silicon diodes.

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The idea is correct, but it will not work in practice.

If you had an ideal diode (conducts current in one direction with no loss and no voltage drop) and a perfect speaker then you would probably pick up some noise.

A real diode, however, requires a certain minimum voltage to work. This can be difficult to reach with small antennas.

The required voltage is lower if the current through it and the speaker can be kept very low. This requires a special earphone that operates on very little current. You can buy magnetic earphones with very high impedance coils. There are also special earphones with a piezoelectric crystal inside that will work properly.

If all you want is to receive the ambient "static" radio noise, then two very long wires (tens of meters,) a germanium diode, and a piezoelectric earphone should work.

To actually receive an amplitude modulated radio broadcast, you would need to use a filter made of an inductor and a capacitor (either or both of which may be tunable) to select the radio frequency you want to receive.

At that point, it is probably better to buy a crystal radio kit. It will include all the parts you need to build an AM radio receiver. If you tune it in between stations, you would hear the expected "static."

The problems with your attempted "receiver" circuit are as follows:

  1. The antenna wires are too short to receive enough power.
  2. The diode requires a higher voltage than you can achieve with your antennas and the speaker.
  3. The speaker requires too much current to operate, which drags down the available voltage. At the same time, the current that could flow would cause the diode to require a higher voltage to be able to conduct.

Under the correct conditions, your receiver would work.

I worked with a fellow who grew up in West Berlin during the Cold War. There were several extremely high power amplitude modulated short wave radio stations in West Berlin at that time. Depending on who you ask, they were there to provide news from the rest of the world to those living behind the "iron curtain" or to broadcast capitalist propaganda to dishearten the loyal comrades of the soviet state. Whatever.

At any rate, this person who grew up in West Berlin told of "building" his first AM radio receiver. It was much like yours, but simpler. Just hold one pin of a diode in your hand. Touch the other pin to one terminal of a speaker, and touch the other terminal of the speaker with your other hand.

In many parts of West Berlin, that was all you needed to listen to those powerful broadcasts.

If you take your circuit to someplace close (hundreds of meters or less) to a high power (tens of thousands of watts of transmitted power) broadcast station then you would probably hear something in the speaker.

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This is as simple as it gets. You need a tuned circuit consiting of a variable capacitor and a coil. For long and medium wavelengths the coil is usually wound on a ferrite rod enter image description here

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    \$\begingroup\$ I remember hearing somewhere that the tuned circuit is unnecessary if you have a very strong radio signal from one AM station. You want to add the tuned circuit if you want to be able to pick a station instead of hearing them all mixed together. \$\endgroup\$ – user253751 Jan 13 at 16:26

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