Electronic – Trouble tuning am radio

That's a very very very very minimalist circuit and very 'laid back' construction. It will work with a suitably high impedance ear-piece and proper construction and enough transmitter power and close proximity and ... ie It MAY work but it's not surprising if it doesn't as there are too many potential pitfalls.

• The earpiece looks like it is low impedance - either 8 ohms or perhaps a few hundred ohms.

  Without a tuned circuit you need all the sensitivity and lack of loading that you can get. Use of a very very high impedance "crystal earpiece" will make a significant difference.

• Your general construction is excessively rough for what you are trying to do. You MAY make it work but one bad join may doom it and stability is unlikely.

• Your wiring lengths and stray capacitance and inductance are long for use at 145 Mhz.

• As MikeJ-UK noted in a comment - if your transmitter is FM (frequency modulated) then you will probably not hear anything as the diode acts to "recover" or "demodulate" the amplitude variations in the carrier wave of an AM (amplitude signal).

• Adding a tuned circuit at the frequency of interest - as seen in almost all crystal set designs you will see, is probably the single best improvement that you can make.

• Clip leads of the sort you are using usually use crimp construction. They can be high resistance or open circuit or have a thin skim of oxidation at the wire to clip contact so that you need a certain level of current flow before they will conduct.

  Soldering the core of the circuit with short lengths of wire is far preferable.

  Clip leads of the sort shown should ALWAYS be soldered at the wire to clip join. Failure to do this will give you many hours of harmless and unproductive fun. When I buy leads like this I put a loose knot in the wire and remove it once the lead has been soldered.

• The diode MAY be a 1N34a. That's a Germanium small signal diode. Rare and not cheap. More usual would be a 1N4148 or 1N916. Most small glass diodes are probably OK. Do NOT use a low Voltage power diode.(1N400x family etc)

• The "rabbits ears" antenna leg is OKish but a length of wire is just as good

• Power outlet ground MAY be safe as safe and MAY kill you when you put the earpiece in your ear.

Your basic strategy to use a piece of wire for the antenna, into a L-C resonant circuit tuned to the frequency of the desired station, amplify that, then connect to a speaker is on the right track but misses at least one important step. That is to detect the AM signal, or to demodulate the amplitude modulated radio signal to get the original audio signal.

After the demodulator, you should also add a stage to drive the 8 Ω speaker with. That will be a stage meant to produce power, not necessarily amplify the voltage. To skip this, you can make just a line level audio signal and feed it into some ordinary audio amplifier, or powered speaker like you would hook up to a PC sounds card. The powered speaker is really just a power amplifier and speaker in a single box.

The simplest AM detector is just a diode. The output of the diode effectively rides the peaks of the RF signal, and goes up and down with its amplitude. You set up the impedance driven by the diode such that the output decays slowly enough so that it doesn't change much in the gap between carrier cycles, but does change fast enough to follow the modulated signal. Fortunately this isn't too hard since there is a large frequency ratio from audio to carrier. If the highest audio frequency is 5 kHz and the carrier is 1 MHz, then you have 200:1 difference.

A even better way to make a detector is to use a bipolar transistor as emitter follower. You can think of this as a powered diode in this application:

This audio out signal can probably be fed directly into a PC "powered speaker".