Troubleshooting Crystal Radio: Tips for Building and Improving Reception

[davenn]

I must have misunderstood that adding tin to copper alters (conduction) characteristics.

yes it does and it is for the worst. Brass, isn't as good as copper.
Was also reading that the conductivity depends on the zinc content ... more zinc less conductivity, which is kinda interesting

 

[darth boozer]

It's not easy to tell from your picture, but the wire on the coil looks as though the insulation is still present preventing the slider from making contact. Also the number of turns appears excessive. A coil with about 80 turns on a cardboard tube from a toilet paper or kitchen paper roll tuned with a 365 pF variable capacitor across it works fine for the AM band (531 - 1602 kHz), using a circuit like that posted by Davenn.

 

[Mark Palmer]

Thanks again, all.

I am obviously new to this, and my "research" into the area comes up with a lot of variables. The conductivity of brass is interesting. The wiper bar I am using is brass, soldered to a brass ID tag which is spring loaded for tension. The question for me is whether I should be using something other than brass for the wiper bar. Most of the instructions I have seen just say to use wire hanger or bike spoke or such.

Also, I am still curious about the construct of the coil, to darth boozer's comment. I used schedule 40 pvc pipe, size 1.5" for the coil body, and I know I wound a bunch of wire on it. A couple of questions on the coil:

1. Does the heavy wall pvc pipe work any differently than a paper tube?
2. Does the excess winding degrade the radio tuning in some way?
3. I used fine grit sandpaper to clean the coating from the wire on the wiper path, brushing out the residue. Is this sufficient?

Thanks, all.

 

[Mark Palmer]

I have another question on the diode. In reading up on diode technology, I see it is directional. Does this apply to the crystal radio? The right side of the coil is connected directly to the diode.

 

[jim hardy]

1. Does the heavy wall pvc pipe work any differently than a paper tube?

i used PVC for my TV antenna. I googled and so far as i could tell it's electrically pretty near transparent.

I put a piece in the microwave for a minute and it didn't get warm to the touch
so i think it doesn't absorb significant RF energy.

 

[darth boozer]

PVC pipe is fine, but cardboard tubes are often used because they are cheap or free. Sanding the wire insulation is OK, but make sure that adjacent turns cannot short to each other. Because you are not using a capacitor with the coil to form a tuned circuit, you are relying on unknown, stray, capacitance so the signals you pick up could possibly be in the lower part of the HF bands and be subject to fading, or the stray capacitance is changing as you move your arms etc. and thus tuning away from the station. The usual recommended circuit will tune to the AM band and your local stations should be strong and steady as well as being recognisable.

 

[davenn]

I have another question on the diode. In reading up on diode technology, I see it is directional. Does this apply to the crystal radio? The right side of the coil is connected directly to the diode.

View attachment 238993

I'm not sure if it really makes a difference, some one else may comment...
Normally it would go in the other direction as in my post on page 1 of this thread. That is the black bar on the diode should be towards the earpiece

The usual recommended circuit will tune to the AM band and your local stations should be strong and steady as well as being recognisable.

yes agreed, as per the circuit and comments I posted n page 1 of the thread

Dave

 

[Tom.G]

I'm not sure if it really makes a difference, some one else may comment...

Nope, no difference at all.

 

[darth boozer]

A little more reading about the subject could be helpful.
https://www.electronics-notes.com/articles/radio/modulation/amplitude-modulation-am.php
https://www.electronics-notes.com/a...tude-modulation-am-demodulation-detection.php

 

[Mark Palmer]

Thanks again, all.

Another question related to tuning.

The slider or wiper bar is apparently a crude tuning method. I think I will modify this build to install a tuning capacitor. I have this one rated at 365 pF, intended for crystal radio use. It has two solder terminals on each side.The wiring diagram posted early in the thread shows the capacitor connected to both sides of the coil. How exactly should I connect to the terminals?

 

[darth boozer]

The grounded side of the coil should connect to the metal body of the capacitor and the antenna side to the side terminals which connect to the fixed plates (the bottom ones in your picture). At the frequencies involved, a few inches of wire to make the connections will be fine. As a temporary measure for testing, the connections could be made with alligator clips.