Crystal Radio

[anaoum]

Hi,

When you're makeing a crystal radio set, what type of coil should you use, and how does the coil have an effect on what you recieve?

Thanks

 

[stevez]

The coil is the inductor part of an inductor-capacitor pair that is intended to be resonant at the frequency of interest. As an example, a 100 pf capacitor and 100 microhenry inductor should resonate at about 1700 kHz if I am reading the table right.

A coil is made up of multiple turns of wire, often on a form but a form is not required. More turns is more inductance so what is usually done is a coil is wound with maximum turns required then tapped so that less turns can be easily employed.

As a starting point a machine wound coil, 2 inches in diameter with 10 turns per inch will deliver about 25 microhenries - so a 4 long inch coil would give you about 100 microhenries.

Pair this with a variable capacitor from an old radio - might be 50 pf to 365 pf. That pair will give you coverage at the upper end of the AM broadcast band here in the US.

Advise if you need more info. I am sure there are good websites. I know National Radio Club has a lot of stuff like this. All of my info is printed in handbooks.

 

[TillEulenspiegel]

Boy this brings me back in time (;

The first coil form I used was a paper towel cardboard tube. As I became more proficient at coil winding , I use to use large thread skeins ( the little plastic thingies that thread is wrapped on ) about 1 - 1 1/4 inch.

Actually in the very old days they used to call these sets "cat whisker" radios. That was because they really used a cats whisker in place of the diode detector!


good luck.

 

[stevez]

Somewhere in my archives (paper) I have a description of a project that is nice - it's a simple one tube crystal radio. According to the author greater fidelity can be obtained from a diode detector if the RF voltage is higher than might otherwise appear from an antenna. The tube is fairly common and serves as an RF amp - the coil, variable capacitor and diode are typical of crystal sets. 10-9 volt batteries are connected for a 90 volt supply and an lantern battery supplies the filament voltage. Resulting audio is supposed to be nice - not loud but nice. The current at 90 volts is supposed to be miniscule so the batteries last a while.

This post reminds me of that and how simple some fun stuff can be.

 

[Nigel Goodwin]

Somewhere in my archives (paper) I have a description of a project that is nice - it's a simple one tube crystal radio.

If it's got a valve (tube) in it, it's not a crystal radio, it's a valve radio :lol:

Pretty well all AM radios work in that way, amplify the incoming signal, then detect it using a diode - a superhet is just the same, but simply converts the incoming signal to a common frequency first.

 

[stevez]

I was sufficiently inspired by the post to find the article on the one valve (tube) not-crystal radio last night. The article described the coil, which ought to be prefect for the crystal only radio. It's supposed to be wound on a 2" diameter piece of pipe. The coil is 2" long made of 22 ga enamelled wire with turns right next to each other though no indication of the number of turns. I am making some crystal radios for the children of some friends so I need to experiment - starting with the coil described. Once I get the number of turns so that the broadcast band is covered I'll post here. Almost any coil coming close to that description will allow reception of something however in an RF noisy enviroment my goal is for the kids to receive a local AM station or two - more would be nice.