Building a Crystal Radio first time

[edteach2]

I am building a crystal radio with my nephew. I followed the instructions and I believe I have a proper circuit. We actually built two and a loop antenna. We also made a 16 foot antenna out of 8 gauge wire. Ends were soldered in small loops and we hang it out side by plastic cord so its insulated. We put a variable capicator on the Loop antenna and both antennas bring in static and funny enough the one local FM station. I can pick that up even with the wiper not touching the coil. Just wondering if I could get any suggestions or if I have something wrong. We are building a third set with the antenna 25 wraps then 90 wraps for the radio coil. Our nearest AM radio station is about 35 to 40 miles away. Thanks for any help in this project. We are learning about radio together. Its a great way to spend time with my young nephew and learn together.

 

[danadak]

What schematic are you working off ?

Detector diode, what are part number are you using ?


Regards, Dana.

 

[edteach2]

On the radio set I used this lay out off of YouTube.

I used a 2N34A ITT Crystal Radio Gold Bonded Germanium point contact diode. and a 100PACK 10K Ohm 5% tolerance 1/2 W carbon film resistor. The coil is 127 turns on a 2 1/4 inch PVC pipe of 22 gauge magnet wire. The smaller one is a coil I bought already done off of Amazon. https://www.amazon.com/dp/B09LTC2B64?psc=1&ref=ppx_yo2ov_dt_b_product_details

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[rjenkinsgb]

It appears the antenna and coil are in series through everything?

The coil and tuning capacitor should be in parallel between antenna and ground, then the diode and earpiece connected across the coil.

In the most basic form, something like this:

Or better, tap the antenna and detector diode at points part way down the coil for better matching and higher Q; it will almost certainly also need a tuning cap across the coil. as above:

(Plus the resistor across the earpiece, for a crystal type).

 

[edteach2]

Thanks a million I will check that out.

 

[edteach2]

What is C2? A second capacitor? Or the wiper for the coils?

 

[danadak]

C2 filters REF out of the audio, help to just recover the low freq audio.

L wiper is a way of tuning antenna to frequency of interest. To help maximize
signal into detector.

Some ref material you can download :

Crystal Receiving Sets Jones : Free Download, Borrow, and Streaming : Internet Archive

crystal radio book.

archive.org

https://www.lessmiths.com/crystal/.handbook02.pdf

Crystal Radio Handbook

An advanced handbook for Crystal Radio intended for the serious beginner to the veteran user. This book contains project ideas and notes interspersed with technical discussions on the theoretical basis for different aspects of the radio hobby. Discussions introduce mathematical formulas and...

www.lulu.com

https://www.arrl.org/files/file/QST%20Binaries/LloydCrystalRadio.pdf

Regards, Dana.

 

[edteach2]

I am also making a third coil with the antenna built into the coil. The schematics I saw said 25 turns of 22 gauge magnet wire and then start a new coil with the same wire wound in the same direction with 90 coils, with a 1/8 to 1/4 inch gap between them. So there would be 4 leads from the tube, two for the antenna and 2 for the radio coil. Other schematics have 130,150 even 190 turns. Is there a specific number of turns or is that not important? My tube is 2.25 inch PVC so closer to 2.5. Photo attached of the schematics

inchOD.

 

[danadak]

The turns, hence inductance, of the coil, and capacitor determine what range your
set will tune to. Here is a calculator for coil inductance, and with min and max value of C,
corresponding to highest and lowest frequency, you can find the tuning frequency range :

Air Cored Inductor Calculator – M0UKD – Amateur Radio Blog

If your cap is fixed then you can use calc with its value, to find range, with slider, effectively
1 turn to max turns you have on coil.

Regards, Dana.

 

[ZipZapOuch]

Make sure you have a crystal radio earphone - not lower impedence headphones.

 

[edteach2]

I have three sets of ear and head phones. Two are the Piezoelectric ear pieces. They work because I can pick up the local FM station. So far that is all I have gotten but for static and a buzzing that comes and goes. The head phones are a vintage set. Bakelite ear pieces.

 

[danielj]

On those AM stations being 30 to 40 miles away, unless you have 100 feet of long wire for an antenna it would be hard to receive those stations. Even the 100-foot wire would be a challenge. At night however, you can receive a world of stations, so nighttime would be the best bet on using the crystal set, with a 100 feet long wire.

 

[edteach2]

On those AM stations being 30 to 40 miles away, unless you have 100 feet of long wire for an antenna it would be hard to receive those stations. Even the 100-foot wire would be a challenge. At night however, you can receive a world of stations, so nighttime would be the best bet on using the crystal set, with a 100 feet long wire.

what size wire do you recommend?

 

[Nigel Goodwin]

what size wire do you recommend?

Crystal sets need pretty substantial aerials, unless you're receiving a high power station nearby - but AM is pretty well dying out now. They were also made for much less 'busy' times, and their lack of selectivity means that you're likely to get loads of different signals wiping out what you're hoping to receive - it's a really bad sign if you can hear FM stations breaking through.

I presume the old headphones pictured are 2000ohm types?, which is what would have been used back in the crystal set days.

 

[danadak]

what size wire do you recommend?

For what you are doing main concern is strength when strung up and loaded with snow if you are in that region. Some folks use outdoor galvanized steel clothesline cable, strong and works great. Far end anchored
in tree with an insulator.

Antenna handbook :

Internet Archive: Digital Library of Free & Borrowable Texts, Movies, Music & Wayback Machine

archive.org

Many folks just run it out over the lawn.

AM band at night pretty active. Day and Night reception can best be explained :

https://www.arrl.org/files/file/LabHandbook/RLH%20Unit%202.pdf

https://www.arrl.org/files/file/Technology/tis/info/pdf/8312011.pdf

Internet Archive: Digital Library of Free & Borrowable Books, Movies, Music & Wayback Machine

archive.org

Regards, Dana.

 

[rjenkinsgb]

Some folks use outdoor galvanized steel clothesline cable, strong and works great.

That may work initially, especially if it's insulated, but definitely not ideal for long term use.

The metal surface resistance is important with antennas due to skin effect; normal insulated copper wire is likely to work far better - any bare metal will quickly degrade, as an antenna, as it tarnishes or corrodes.

Insulated, Copper coated steel is a possibility; some types are used for telephone cables between poles.

A common type in the UK has some copper cores and some high tensile steel cores.

You can get excellent antenna wire from here, if you are in the US:

https://thewireman.com/

 

[edteach2]

So I took your advice and went to the wireman and bought 100 feet of 18 gauge copper clad steel insulated antenna wire. I also bought two ceramic insulators and a couple of steel end loops.