# Why does this AM radio circuit not work?

I have been trying to make an AM radio over the last couple days and am very confused as to why this circuit is not working. I found the circuit on this website and I saw this video on this design. I have tried to copy it but it is not working. I have the output leads connected to a Sony speaker. It is a Sony VGF-WA1.

Is the circuit diagram correct?

Is there a better AM radio circuit diagram that you know works?

simulate this circuit – Schematic created using CircuitLab

• What does "not working" mean? Mar 12 at 1:58
• @KevinWhite what I mean by not working is that I can’t hear any radio stations, only static. Mar 12 at 5:52
• I'm surprised you can hear static, as there is no amplification Mar 12 at 11:22
• It won't work without a resistor across the output, something like 10k is appropriate. Normally the diode would be connected to a tapping on the inductor, maybe 1/4 of the way from ground. Mar 12 at 14:10
• "I have the output leads connected to a Sony speaker." To the aux input, I hope. Mar 12 at 20:39

With components of 1 mH and 20 pF, the minimum frequency you'll be able to tune is about 1.125 MHz. And that's neglecting the self-capacitance of the inductor, which is probably quite significant.

For the US AM broadcast band, 535-1620 kHz, the de-facto standard part values are 240 µH and 40 to 365 pF. You might start by pulling these parts out of an old pocket radio to use in your circuit.

• The capacitor is 20pF - 70pF so wouldn’t that get the right range? Mar 12 at 5:54
• By minimum, do. you mean maximum? :) Added C will lower the frequency. With 70 pF that takes it down to 600 kHz. Mar 12 at 7:45
• Normally, when only a single value is shown for a variable capacitor in a schematic, it is the maximum value. I was assuming it could only go down from there. Mar 12 at 11:35
• That makes more sense. Talking about added parasitic capacitance made me think you were saying "too much capacitance". I guess you were suggesting that the additional capacitance reduces the range of the variable capacitor. Mar 12 at 15:26
• @DaveTweed sorry for the confusion I changed the circuit diagram. So would these values be ok and give the right frequency range (20p - 70p)? Mar 12 at 15:53

I made this back in 6th grade, about 1969. Make sure the diode is germanium. Add a 10k resistor as a load if no headphones in use. Use a long wire (several yards if possible) for antenna. When working good it may pick up many stations at same time.

Consider adding a earth grounded wire, it can greatly increase signal strength. Use high impedance headphones if possible (>32 ohms). You may need to add a 10uf capacitor in series with the headphones so only sound is output, not the tiny bit of dc the diode creates. Read @Dave Tweed answer as the variable capacitors range and inductor value is very important.

• I will try to add the 10k resistor later today. As for the headphones, is the Sony speaker a good thing to use as the output for the audio? Mar 12 at 15:58
• I am not sure the name matters. Of great importance is that the load be of high impedance due to weak drive signal. For speaker you might be able to find old 70 volt to 8 ohm transformer used in PA systems. I used ancient 600 ohm headphones. Check ArmyNavy stores. Mar 12 at 16:19

Here's a conventional schematic.

A 60' long, 30' high horizontal wire for the antenna and a buried water pipe for the earth connection, are a must. So is a germanium diode.

The coil may be wound on a 2" former (on-line calculators would be of help in determining the number of turns). The easier option would be an off-the-shelf 240 μH moulded inductor.

An air-spaced variable capacitor or a 'PVC gang capacitor' may be used for tuning.

Good results may be obtained using sensitive 'balanced armature' or standard 'high-impedance' phones.

A local medium-wave AM radio station would be of help in tweaking the setup.

• For my fifth-grade science fair project, I investigated antenna designs for this sort of radio. Simply running a thirty-foot wire across the ground was good enough to receive most of the local stations; a five-foot stub was sufficient to pick up a nearby 50 kW station.
– Mark
Mar 12 at 22:00

There was too many signals interfering with the radio. I was very close to the internet router. When I moved to a different room it worked.

• Tinfoil is very effective at shielding RF noise, although if your router has WiFi, that may not be viable. Mar 13 at 11:01
• Many audio amplifiers can act like unintentional AM receivers, and pick up annoying sounds from mobile phones that are nearby. An guitar amp wouldn't have frequency response that extends anywhere near that high, but differential clipping on the front end can result in unwanted amplitude demodulation of signals that are orders of magnitude higher than AM broadcast stations. Such signals are many orders of magnitude stronger near the source than any broadcast AM stations would be in most environments, and a passive LC tuner might not be able to boost the broadcast over the noise. Mar 14 at 15:24

It will "work"- in the sense that an AM radio station will be heard- if the RF signal is large enough.

Use a good antenna, and live close to a radio station. And tune to the station.

Is there a better AM radio circuit diagram that you know works?

Suggest you have a look at:

From the title you can see that booklet is out of the dark ages.

For this kind of radio, you need a BIG antenna**, try for 100 feet of wire as high as you can get it, perpendicular to power lines if possible. Also, you need a good ground. Multiple earth ground rods at least 3 feet apart will do a good job. The antenna will serve you well when you graduate to shortwave, get a ham license, and start transmitting.

I built a similar radio in the 7th grade in about 1952. Mom and dad never suspected that I was listening to late night radio. In those days the diode was a little lead sulfide crystal that you had to poke with the whisker of a cat.

** Take appropriate lightning protection measures with the antenna and position it to avoid possible contact with utility power.