For better selectivity I have used two coupled inductors (not shown). The second col is wound on a cardboard tube of smaller diameter (try plastic or aluminum wrap rolls) that slips inside the larger toilet roll coil. Then I can adjust the desired coupling by sliding one coil over the other. The diode is connected to the second coil. The selectivity is superb. I can easily pick up four stations at 900, 1010, 1150 and 1280kHz.

How are the coupled coils connected? I.e., what is the actual schematic look like?

 

You seem interested in the fundamentals of what's happening so just to give you an idea what you are working with:

Thanks a lot, Tez!

This gives me some useful resources for a few things I am interested in.

 

I have a schematic somewhere but I have to search for it.

The primary coil is connect in parallel with the tuning capacitor and to the antenna as show before. One end of he secondary coil is connected to GND. The diode is connected to the other end of the secondary coil.

 

Hello,

My first crystal radio set used an AMROH 402 coil.
Here is the story of radio Bob that also uses the same coil:
http://www.dos4ever.com/bob/bob.html

Bertus

 

I have a schematic somewhere but I have to search for it.

The primary coil is connect in parallel with the tuning capacitor and to the antenna as show before. One end of he secondary coil is connected to GND. The diode is connected to the other end of the secondary coil.

Does this capture the essense (my symbol library is very limited right now)?

Notice that I have not connected the secondary coil to ground. Is that necessary? It would seem like it shouldn't be. My understanding is that you need a solid ground for the antenna circuit in order to get a good signal, but it would seem like the secondary side of the circuit could float.

 

Yes, that is the essence. I suppose that it is not necessary to connect the secondary to GND but I have always done that.

 

Here is a photo of what the coils look like, one inside the other:

 

To get some estimate of the impedance at the antenna, assume a 10m vertical wire, 4mm dia. At 1MHz the input impedance over ground is about
0.44-j2150Ω,

(to see how this is calculated:
http://www.ece.rutgers.edu/~orfanidi/ewa/ch22.pdf eqn 22.3.24
this is for a dipole, so a monopole over ground has half the impedance)

or in parallel terms 74pF || 10.5MΩ.

Shorter wires will have higher impedance, longer lower. Slant wires will be different but still of this order.

I was worried about my estimate of the antenna impedance, but it's been a long time since I've done any MW analysis. The real part of the impedance seemed too small given my experience with crystal sets and the designs I have seen (in particular - if the real part of the parallel impedance is of the order of MΩ then why don't all crystal sets simply connect the antenna to the top of the tuned circuit. Many 'better' designs connect it to a tap down the coil, or loosely couple it in on a smaller winding.).

My calculation is correct, this is the radiation resistance of the antenna, but the input impedance of the antenna is dominated by losses, so the real part is much higher than this and what you end up doing is matching to the combined (mainly loss) resistance. For the actual antenna - the piece of wire - it is likely that copper losses will dominate, so there is a tendency to use thicker wire. In this type of antenna system though the ground resistance tends to dominate, typically the ground resistance is 10 - 20Ω even with a good ground (it can be much higher), so there is a limit to how much improvement you get in going thicker than hookup wire.

So if we assume that the antenna has a reasonable ground (ground resistance of 20Ω) and say 2Ω of copper losses in the wire, the input impedance becomes 22.4 - j2150 which in parallel form is
206kΩ || 74pF

and for a less efficient ground connection this impedance is lower.

Remember that for a loaded Q of around 100 we needed a total load impedance on the top of the tank of around 150kΩ. A poor ground connection will load the tank if connected directly to the top and give poor selectivity. This explains why it may be preferable for the antenna to be tapped down, or loosely coupled to, the tank circuit

I should have remembered this, as one of the key parameters for the siting of AM broadcast transmitters is the ground conductivity, if it is poor then much of the AM transmitter's power is dissipated in the ground resistance (i.e. warming the surrounding grass). Typically copper radials are buried out to λ/4'ish.

It's amazing how much physics and engineering is buried in a design using so few components.

 

Thanks.

I may get to give it a try this weekend, afterall. My crystal earphones arrived! I have a list of other things that HAVE to get done first, so I may or may not get a chance to play.

 

So here's another question -- maybe I should start another thread, but this will do.

For my ground, the cold water pipes in the house might or might not be the best ground. We have well water and I don't know the kind of pipe going from the house to the well, so I don't know for sure that the water pipes in the house are grounded by anything other than the water itself. I suspect they are since I imagine code requires it.

But, be that as it may, while they are accessible in many places in the house, they aren't convenient to where I need them.

Is there any problem with using the earth ground connection in one of the household electrical outlets as the radio ground for the antenna circuit? I can't think of any.

 

So here's another question -- maybe I should start another thread, but this will do.

For my ground, the cold water pipes in the house might or might not be the best ground. We have well water and I don't know the kind of pipe going from the house to the well, so I don't know for sure that the water pipes in the house are grounded by anything other than the water itself. I suspect they are since I imagine code requires it.

But, be that as it may, while they are accessible in many places in the house, they aren't convenient to where I need them.

Is there any problem with using the earth ground connection in one of the household electrical outlets as the radio ground for the antenna circuit? I can't think of any.

From my experience as a kid I had great success with water pipes, but all this meant was that my pipes were well connected to the ground. For pipes to work they have to be conductive (as you note) and buried in ground with reasonable conductivity. There is no guarantee, try it and see. I'd avoid the mains earth given your daughter will be playing with it, just in case some combination of faults was to make it live. Experimentation is half the fun, if you can't find a convenient bit of buried metal, you may need to drive in an earthing stake.

Page 7 here http://www.ccmr.cornell.edu/education/modules/documents/CrystalRadio.pdf
gives some interesting anecdotes on ground connections.


Also, if you use an outside antenna, you should fit a lightning arrestor.

As far as your comment about the convenience of your water pipes (or any other ground), don't forget that at 1MHz the wavelength is 300m, so a 5m run of wire to the ground point is electrically short.

 

Hello,

The attached PDF will show you circuits that use a coupling coil for the antenna.

Bertus

 

Thanks alot, Bertus. Looks very interesting and makes a nice progression in complexity.

 

Did you measure the impedance of an earpiece from an old telephone?

 

Did you measure the impedance of an earpiece from an old telephone?

This would involve having an earpiece from an old telephone to measure the impedance of.

In point of fact, I think I DO have one, but it is buried in The Greater Wall of Stuff.

 

Well I will do that for you, because mine isn't.

 

My rotary dial phone speaker measures 24 Ohms when disconnected.
Funny I have two different types of crystal earpieces, and I can't measure them.
Are they out of range of multimeters or something?
I just measured a 50K pot, so I know it works.

 

My rotary dial phone speaker measures 24 Ohms when disconnected.
Funny I have two different types of crystal earpieces, and I can't measure them.
Are they out of range of multimeters or something?
I just measured a 50K pot, so I know it works.

I think crystal or ceramic earphones are capacitive at DC, which is why you need to shunt them with a 33kΩ resistor, give or take, to provide a DC path for the diode current.

 

Hello,

A crystal earpiece is also called a ceramic earphone.
It uses a piezo electric crystal.
See this wiki page for more info:
http://en.wikipedia.org/wiki/Crystal_earpiece

Here is a PDF to make your own speaker.
When the wire is thin enough the impedance will be usable for the crystal set.

Bertus

 

Thanks.

Making a speaker is definitely on the list of items to do. I'll add this to the plans I've already collected.