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I have been on the look out for AM radio circuits on the internet.

The ones I have come across are circuits using monopole antennas and LC tuned circuits.

However, I require a circuit using a quartz crystal for the tuned circuit and a dipole antenna. The antenna may be balanced or unbalanced. No amplifier is required.

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    \$\begingroup\$ Why do you specifically want to use a crystal in a simple AM receiver? \$\endgroup\$
    – JRE
    Jul 1, 2023 at 23:09
  • \$\begingroup\$ I need a quartz crystal driven radio receiver with dipole antenna. Not just a RLC circuit, which I must balance with a trimmer capacitor for a stable frequency. \$\endgroup\$
    – user321220
    Jul 1, 2023 at 23:11
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    \$\begingroup\$ @JRE I believe they mean a diode, these type of receivers were known as ‘crystal sets’ because the early ones used a galena crystal as the detector. Later ones used germanium diodes, silicone diodes like the 1N4148 aren’t as sensitive. \$\endgroup\$
    – GodJihyo
    Jul 1, 2023 at 23:21
  • \$\begingroup\$ A good one would be maybe a 1n34a diode. It should be a germanium one, which is used in radios as the detector. You know 1n4148 diode has a need of 0.7v, that would be cut off from antenna voltage. Correct me, if I am wrong. \$\endgroup\$
    – user321220
    Jul 1, 2023 at 23:29
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    \$\begingroup\$ You're getting terms mixed up. A crystal radio doesn't use a quartz crystal. As @GodJihyo explained crystal radios are named after their galena crystal detectors. (They retained the name after moving to germanium diode detectors.) So there is no quartz crystal radio circuit to show you. \$\endgroup\$
    – Graham Nye
    Jul 2, 2023 at 0:55

4 Answers 4

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MOST important elements here are antenna and ground...both are required.

Get as much wire suspended outside above ground as you can for the antenna. The far end is not connected to anything. If you have a dipole, electrically tie one arm to the other, and bring that into the receiving diode with a wire. The antenna wire needn't be heavy gauge, just strong enough to stay up and be visible to birds.

For ground, I used the electrical distribution in the house - it is a modern 3-wire system that includes ground. Metal plumbing could also be used. If no ground is available, use another long wire laid on the ground, hopefully in the opposing direction to the air-suspended antenna.

As @GodJihyo says, the "crystal" refers to the semiconductor diode, not a quartz crystal. These are two completely different devices. If you get the antenna and GND right, almost any diode will give some audio. Of all diode types available, a Germanium diode is somewhat better. I've even used a silicon power rectifier diode, giving somewhat attenuated audio to headphones.

The most simple AM receiver possible has four components:

  • Antenna (monopole)
  • Earth
  • diode
  • audio transducer

schematic

simulate this circuit – Schematic created using CircuitLab

Headphones of high impedance are best for the audio transducer. Loudspeakers are not high-impedance transducers, and ear buds, along with most headphones are not much better. Few headphones have impedance above a few thousand ohms. This will be the most difficult component to source.

An alternative is the 2nd circuit, which adds an audio step-down transformer so that more common low-impedance headphones appear to the diode to be much higher impedance.

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  • \$\begingroup\$ Without an LC tuned circuit then the "radios" would pickup lots of mains hum and all nearby AM stations at the same time. \$\endgroup\$
    – Audioguru
    Jul 2, 2023 at 17:48
  • \$\begingroup\$ @Audioguru Mains hum hasn't been a problem in my experience. But yes, you get a mixture of audio from multiple transmitting stations, depending on their signal strength at your antenna location. Selecting one station with LC circuits gives no end of problems for many inexperienced constructors. Best to start with simplest possible circuit as described. \$\endgroup\$
    – glen_geek
    Jul 2, 2023 at 17:54
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Here's a research paper 'A New Family of Passive Wireless RF Harvesters based on R-C-Quartz Oscillators' that presents Resistor-Capacitor-Crystal (R-C-Quartz) oscillators for use as passive amplification in RF energy harvesters.

enter image description here

Your concept for a crystal radio is quite similar but appears to require an amplifier. Secondly, the extreme high Q of a quartz crystal and the difficulty of obtaining one very close to the frequency of the station to be received, will pose problems in achieving the desired results.

There are, however, simpler schemes to realise a working crystal radio.

Here's the schematic of a fixed-tuned crystal radio. The term 'crystal' refers to a germanium diode and not to a quartz crystal.

enter image description here

The moulded inductor, measuring 8 mm diameter x 10 mm long, series-tunes the capacitive, 'short', monopole antenna to resonance at the desired frequency.

enter image description here

This low part count design was used to receive a local AM radio station at considerable volume.

For the 60m long antenna, M0UKD's 'Loaded Quarter Wave Antenna Inductance Calculator' gave a loading coil inductance of the order of 200 μH at 612 kHz (the frequency of the 200 kW AM radio station located 20 km away).

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  • \$\begingroup\$ Just wait and listen. I think I was not able to clarify the problem, I was talking and writing about. My subject was about making an AM receiver with 8 MHz quartz oscillator on it and making use of a dipole antenna to receive. All the guys here are writing eagerly about something, I was already informed. My goal is not the diode, which is sure necessary in circuit, but the quartz. I got answered the question by my own, because I tried it out and I know that it works. The last picture on this post is just how my experiment was working. Not more. Thank you, guys, for all the postings. \$\endgroup\$
    – user321220
    Jul 3, 2023 at 17:53
  • \$\begingroup\$ Would just someone make an upvote the answer below. I am getting nervous, when someone downvotes anything, that deserves an upvote. Sure, you are the readers, who judge here. Anyway. \$\endgroup\$
    – user321220
    Jul 3, 2023 at 18:03
  • \$\begingroup\$ Hi lastime, My answer has been edited. \$\endgroup\$
    – vu2nan
    Jul 4, 2023 at 4:22
  • \$\begingroup\$ Thanks for feedback. \$\endgroup\$
    – user321220
    Jul 4, 2023 at 8:01
  • \$\begingroup\$ Anytime, lastime! \$\endgroup\$
    – vu2nan
    Jul 5, 2023 at 17:46
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I experienced the learning by doing at home and I soldered an 8 MHz quartz oscillator between the legs of the dipole antenna. The necessary diode has been also added between legs. After all, I switched in a current amplifier with a headset at the end. I transmitted with a shortened 8 MHz antenna, some music from my mp3 player and there was sound that I could hear by the headset. The only thing that makes me thinking is that the grounding is immense important while receiving. All I had to do, was to ground the positive leg with a chassis of my choice bound with a crocodile cable. The picture below is just for information.

pic

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  • \$\begingroup\$ I have still 4 hours to make my answer valid. Do not forget to upvote. Someone has already downvoted and I do not know the reason for that behaviour. Please, before you downvote anything or use a flag, give me the reason for that, so I can better clarify, why it could be reasonable or not. \$\endgroup\$
    – user321220
    Jul 3, 2023 at 18:11
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However, I require a circuit using a quartz crystal for the tuned circuit and a dipole antenna. The antenna may be balanced or unbalanced. No amplifier is required.

Using a dipole antenna will switch the incoming signal between own legs. Getting a proper AM modulated signal requires firstly a diode, which makes the traffic of this transmission clearly. The one leg for incoming signal and the other leg for earthing. After this, the incoming signal must be filtered from other signals. A quartz crystal or tank circuit will do this job. An oscillator is also necessary to select and amplify the filtered signal.

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