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 Post subject: Re: First time crystal radio build
PostPosted: Apr Mon 26, 2021 9:18 pm 
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Joined: Jan Sat 25, 2020 5:23 am
Posts: 699
Location: Colorado Springs, CO 80917
Quote:
Would you mind posting a picture of your setup? I think I might have to redo My Copper wire coil Again.


Here you go.


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 Post subject: Re: First time crystal radio build
PostPosted: Apr Mon 26, 2021 9:39 pm 
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Posts: 521
Location: New Hampshire
Thank you, I am thinking I will try the coils closer together and see what happens.
And I am a member of the nearc and was planning on going to the meet on the 15th, but I have not been to a meet yet so I wanted to meet some of the members first.
But until then I will keep experimenting and will post if I find anything.


Henry


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 Post subject: Re: First time crystal radio build
PostPosted: Apr Mon 26, 2021 10:49 pm 
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Joined: Jan Sat 25, 2020 5:23 am
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Location: Colorado Springs, CO 80917
Keeping the coil turns close together, as I did, increases the coil effectiveness (more microHenries) versus being spread out.


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 Post subject: Re: First time crystal radio build
PostPosted: Apr Wed 28, 2021 7:53 pm 
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Location: New Hampshire
sorry to bother you guys again,
I rewound another coil tightly using the same wire ( about 100 turns) as my last coil and it still makes a hum as it did last time.
Wayne, you mentioned that you had to make sure your germanium diode was good before you used it, can you try using a bad one and tell me if it hums or is just 100% dead?
thanks,
henry


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 Post subject: Re: First time crystal radio build
PostPosted: Apr Wed 28, 2021 8:06 pm 
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Location: Texas
whfh99 wrote:
sorry to bother you guys again,
I rewound another coil tightly using the same wire ( about 100 turns) as my last coil and it still makes a hum as it did last time.
Wayne, you mentioned that you had to make sure your germanium diode was good before you used it, can you try using a bad one and tell me if it hums or is just 100% dead?
thanks,
henry


Test your diode with a multimeter. Most meters have a diode setting, but if yours doesn't use low ohms setting. It should read 30-80 ohms in one direction, practically infinite in the other direction.

I expect that without an adequate antenna, all it's going to pick up is the stray AC fields. In addition to a good antenna, you may need to move the radio out of the basement and use a long ground wire. That's not the first choice, but necessary if you are surrounded by hum.

Another possibility is that you may have a very noisy environment that is actually swamping out stations you'd otherwise hear. Tune a portable AM radio to an unused frequency, walk around the house and yard and see where the noise is loudest. In my case it was my neighbor's defective computer power supply that wrecked all my AM reception. He couldn't believe it when I held the radio near and the hum got extra loud. Lucky for me, the power supply was very hot and I was able tell him if he wanted to avoid a fire he should replace it, which he did. Every time I turn on my plasma TV, I can't listen to AM radio. So better check that.


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 Post subject: Re: First time crystal radio build
PostPosted: Apr Wed 28, 2021 8:24 pm 
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Joined: Jan Sat 25, 2020 5:23 am
Posts: 699
Location: Colorado Springs, CO 80917
Quote:
Wayne, you mentioned that you had to make sure your germanium diode was good before you used it, can you try using a bad one and tell me if it hums or is just 100% dead?


I didn't test the diode (like with an ohmmeter) but tried four different diodes & used the one that worked. The ones that didn't work made no sound at all, not even buzzing noise.

My setup, and probably yours too, is very very very marginal, because it really has no antenna. I've noticed that my setup goes dead at night when the local station reduces power output. So, without a decent antenna, it's maybe surprising that mine picks up stations and maybe not surprising yours does not.

Silicon diodes have a ~0.7 V forward (conducting) drop. That means that any RF signal has to be more than 0.7 V just to get thru the diode & make a noise in the headphones. Germanium diodes have about half that forward drop & thus will more easily detect radio stations. So, basically, I was trying diodes until I found a germanium one.

Even from a pile of germanium diodes, one can usually find one or two that work a little better than others. One of the pastimes of crystal radio fans is trying different diode materials.

By the way - I tried a few transistors (like you started out) that I though were germanium but they didn't do anything. I may need to look closer at what I was using, though.

It seems your setup was picking up radio noise, so the next step for you, I think, is to work on how to bring a really long antenna wire to the setup. Is there perhaps a basement window you can bring an outside wire through temporarily?


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 Post subject: Re: First time crystal radio build
PostPosted: Apr Wed 28, 2021 9:43 pm 
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Location: New Hampshire
thanks, guys,
I will be taking a walk with one of my radios this afternoon around my house then I am going to test my diode.
I would agree with you on the antenna though I don't think you understand my radio is in front of my basement door and I am using wire running it down to a pipe. not exactly practical given you can't move much without bumping into wires, but it's ok for now I like to call it my "crystal radio testing ground" 8)

just curious what detector has the lowest voltage drop? germanium?

henry


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 Post subject: Re: First time crystal radio build
PostPosted: Apr Wed 28, 2021 10:28 pm 
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whfh99 wrote:

just curious what detector has the lowest voltage drop? germanium?

henry

Schottky Diodes generally have a forward voltage drop in the range of .15 to .46.

https://www.mouser.com/Semiconductors/D ... ?P=1yuobkp

For your purpose right now though, the black Diode you harvested from the PCB should work just fine. Measure it's resistance both ways, as described above, to make sure that it's valid.

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 Post subject: Re: First time crystal radio build
PostPosted: Apr Wed 28, 2021 11:50 pm 
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Location: New Hampshire
interesting,
will measure the diode later today.


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 Post subject: Re: First time crystal radio build
PostPosted: Apr Thu 29, 2021 4:21 pm 
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Location: New Hampshire
my diode tested 30 ohms in one direction and infinite in the other, will take a walk around my house with a radio later.




henry


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 Post subject: Re: First time crystal radio build
PostPosted: May Sat 01, 2021 8:34 pm 
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sorry for the delay bad weather was holding me up.
but the good news is that I got to walk around with my radio outside;
I tuned it to a local station walking around my house, it did not like the computer room or in the basement, too much electrical interference I am guessing.
when I put the radio down outside my basement door where my crystal radio components and loop were originally, it sounded fine very clear.
but when I put my higher up were my antenna was I could not hear it well.
I am thinking that my antenna is in a bad spot, however, unless I run more wire from my radio to my ground connection I don't have a good spot to hand my wire antenna.
but since you managed to use your radio without an antenna and just a ground...there might be something wrong with my radio.


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 Post subject: Re: First time crystal radio build
PostPosted: May Sat 01, 2021 9:30 pm 
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whfh99 wrote:
I tuned it to a local station walking around my house, it did not like the computer room or in the basement, too much electrical interference I am guessing.


The idea is to determine how much of a problem man-made noise is where your crystal radio operates. First identify any sources of noise, like hum or raucous static, and then turn off whatever device(s) cause it so the noise doesn't swamp out the weaker station signals you are tying to capture.

A more revealing test would be to tune the radio not to a station, but a blank space in the band. Then all it will pick up is noise. What you hope to hear is natural noise like atmospheric noise or far away background noise. These will sound like a hiss or rushing sound with occasional pops. What you don't want to hear is static with a pattern that's just as loud or louder than received stations. To hear the distinction, tune as described, then put the radio near a light dimmer, large TV, computer power supply. Those give you an idea what the undesirable noise sounds like. Then walk around your house and record the quietest spots.

True, there are places in the house where the transistor radio will perform better, especially if you rotate is between 0-90 degrees. But if you have debilitating noise sources, they can render a crystal set pretty much worthless.

Let us know what you learn with this more sensitive test.


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 Post subject: Re: First time crystal radio build
PostPosted: May Mon 03, 2021 9:10 pm 
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Location: New Hampshire
Doing what you described I tune in my radio To a blank spot On the dial.
Then I held it up to a florescent light In my basement And other hi power spots.
When Put the receiver Where my crystal radio was, It sounded like natural sounds Same thing happened when I put it Where my antenna was.
I am really glad I'm not using it in my basement anymore! There is Tons of interference down there, but the weird thing is that it made the same noise Down in the basement as it did outside...


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 Post subject: Re: First time crystal radio build
PostPosted: May Sun 16, 2021 1:24 am 
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Had a good time at the the NEARC swap meet this morning, met some real nice people including a man who had a table full of crystal gear.
He was selling galena crystal sets, and almost bought one. But decided I would rather build one myself sometime.

I think I have a good plan for a antenna, bought a arrl handbook and might be building a ham antenna anyways.
But for now I will try some long wire hung up on my roof.

Will post results.

Henry


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 Post subject: Re: First time crystal radio build
PostPosted: Aug Sat 07, 2021 12:40 pm 
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Hi whfh99,

You may find relevant information at

https://nandustips.blogspot.com/p/crystal-radio.html

Also specific information on using a Germanium transistor as a detector at

https://nandustips.blogspot.com/2014/05 ... t-fed.html

Regards,

Nandu.

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 Post subject: Re: First time crystal radio build
PostPosted: Aug Sun 08, 2021 4:39 pm 
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Location: New Hampshire
Great website Nandu! A lot of valuable info. I especially liked your QRP page.

Since my last post in May, I have around 20 brand new diodes and more parts. I set up a 50 ft dipole-type antenna on the side of my barn coming through a window on my workbench. I have also built this untuned circuit download/file.php?id=356047&mode=view see picture below of my exact build.


Since I have a proper antenna I just need proper grounding, unfourtanaly there are no copper pipes that are close to the workbench/antenna without running an excessive amount of wire in the yard.


I do have a very large metal stake that I could try, however...


Henry


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 Post subject: Re: First time crystal radio build
PostPosted: Aug Sun 08, 2021 7:33 pm 
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Location: Santa Rosa, CA
Quote:
Silicon diodes have a ~0.7 V forward (conducting) drop. That means that any RF signal has to be more than 0.7 V just to get thru the diode & make a noise in the headphones. Germanium diodes have about half that forward drop & thus will more easily detect radio stations. So, basically, I was trying diodes until I found a germanium one.


It's true that Ge diodes are more sensitive than Si, but the forward drop numbers really don't relate to radio signal levels.

A good crystal set will detect signals in the milliVolt range... maybe even microVolts. You would have to be close to a powerful station to get 0.3 Volts (300 mV) of signal. So don't give up on any Germanium diodes until you are sure you have a good working circuit and then can compare results.

I have asked semiconductor R&D PhDs to explain this, but no one seems to worry about detection these days and no one is designing new Ge diodes. Detection isn't the same as rectification.

Old timers added a bias battery to the detection circuit to get the operating level closer to the forward voltage point. I never got dramatic results with added bias.

Rich

PS: You can also used the Base-Emitter junction of an old Ge transistor as a detector diode.


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 Post subject: Re: First time crystal radio build
PostPosted: Aug Tue 10, 2021 7:05 am 
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Posts: 90
Location: Bangalore
whfh99 wrote:
Great website Nandu! A lot of valuable info. I especially liked your QRP page.

Since my last post in May, I have around 20 brand new diodes and more parts. I set up a 50 ft dipole-type antenna on the side of my barn coming through a window on my workbench.

I have also built this untuned circuit download/file.php?id=356047&mode=view see picture below of my exact build.

Since I have a proper antenna I just need proper grounding, unfourtanaly there are no copper pipes that are close to the workbench/antenna without running an excessive amount of wire in the yard.

I do have a very large metal stake that I could try, however...

Henry

Thank you so much for your compliment, Henry!

Should your dipole have a coax feeder, you would need to use either the core or the braid for the antenna connection. A buried metal water pipe would serve for the earth connection.

Image

Connecting the coax core at the feed point and grounding the braid is a no-no. The weak signal would definitely be lost in the coax.

Have fun!

Regards,

Nandu.

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