Crystal radio question
 

Hello, i built one of those cheap 80's crystal radio kits. It picks up my 2 local stations decently. I am only using 15 feet of wire. If i add more wire the volume goes down. Why is that? I thought the more antennae the better.

Don't know, maybe it's the stray capacitance of the wire?

How is the wire arranged physically?
Do you have a good ground for the set?

The ant. wire just goes up and to the left around tools on the peg board and then down towards the floor. For a crystal set does it matter which way the diode points? The instructions aren't clear on that.

Ground...gies to the breaker box...
Then that goes to ground.

Ground sounds OK.

Not sure what to suggest about the antenna except maybe running it as straight as possible without bending back. When I had a crystal set as a kid in Chicago I threw the antenna wire out the upstairs window and into a tree. It was mostly horizontal and maybe 20 feet long and got a few Chicago stations.

Diode direction should be immaterial, just changes the phase of the audio by 180 degrees.

Just had a thought - how long is the ground wire? Does the ground wire run in the opposite direction from the antenna or more or less the same direction?

Ground wire 18 inches, and same direction as ant wire

I don't think one that short could be a problem - out of ideas for now.

Hmm maybe adding longer antenna changes how it recieves??

Is the volume lowering OR is the station not being picked up as strong? (That would make voluime seem to go down also)

Did you read the first post? He said using a longer antenna changes reception.

In a crystal set, the volume is entirely dependent on the received signal strength. It's not either/or.

More questions:
When you lengthen the antenna, do you have to retune the station?
What is the tuning mechanism? Variable coil length or variable capacitor?
Can you post a schematic?

If i lengthen the ant the volume gets weaker and does seem to move on the coil. This is tuned by changing coil length. Could a very cold environment affect the earphone? It is 20 here and this is in my garage.

I cant get the diagram pic to load correctly

My camera's pictures are too big file size....wise to load here.

Doubt the temp has much to do with the efficiency unless it affects the diode somehow? Even if it did, it would not account for the difference with antenna length.

You can email me the schematic and I'll post it.

wayne bretl "at" cox "dot" net (no space in name)

Upload your picture here buddy and share the link with us :)

http://pixhost.to/classic-upload/

I uploaded them...hope you have better luck than me!

568991232_1000000212

Here's a page with a diagram of antenna impedance vs. length that assumes a 0.1 wavelength wire antenna that gives an equivalent of 300 pF resonating with the coil.

https://electronics.stackexchange.co...ystal-radio-be

It seems in your case, you have a 15 foot optimum antenna length (about 0.01 wavelength), which would be about 30 pF, if I'm thinking about this right.

Suggested experiment: put a small capacitor (maybe 30 to 60 pf) in series with your antenna wire, and then lengthen the antenna and see if it works better.

I will did through my tub of stuff and get the cap in question and try this out this weekend..

Don't be too dissapointed if my idea doeasn't work. It could be that you need a different coil to work with a longer antenna.

It was kinda interesting...with the cap and a longer ant. the 2 local stations swapped positions. 940 am was on the left side of the coil and 1150 was in the middle. Now 1150 is towards the left and 940 is more in the middle. Funny!
I will try different ant. lengths and a variable cap and see what happens.

When you say towards the left, do you mean fewer windings in the circuit (left in image xxxxx213 that shows the physical picture)?

I think 1150 (higher frequency) to the left (less resonating coil) is the way it should be, so you are going in the right direction by adding the series cap.

I think having the station positions reversed initially was an indication that the antenna capacitance was loading the coil too much.

Antenna loading can be reduced by using a second, smaller coil for coupling the antenna to the main coil.

https://www.youtube.com/watch?v=mh5KxCpbQ5U

Yes, fewer windings...I will check out the video you linked.

The local stations are only 2 or 3 miles away, if this makes a difference.

I watched the video, if I saw it correctly there is not an ant. connection to the main coil.....how does that work?

The antenna tuning coil is inductively (magnetically) coupled to the main coil. Since the antenna coil is lower inductance, it is affected less by the antenna capacitance. Then the transformer coupling to the higher inductance means that the main coil sees, in effect, a higher impedance (lower capacitance) antenna load, and because of the turns ratio, also outputs a hgher voltage to the diode and earphone.

You could use a lower inductance main coil in the first place to make the antenna capacitance less of a problem, but that would reduce the voltage output to the diode and earphone.

I am self taught....there is not an iron core in the video, so it is proximity of the 2 coils that causes induction of signal?

Yes - air coupled.