Crystal Radio Antenna Preamplifier – could this be improved?

by Stephen Hobley on October 12, 2010

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I’ve been working on adapting one of my failed Tesla coils into a crystal radio set to show the neighborhood kids. I’ve run into the #1 problem everyone faces – not enough antenna to generate a strong signal. I read somewhere that a “proper” AM antenna should be between 100 – 500 ft for good reception (technically known as quarter wavelength). I don’t think anyone living in a city can install an antenna of that size.

So I managed to push about 60′ of wire into the air, and I was getting OK results from it – but thinking that maybe a simple pre-amplifier circuit would be a good thing to squeeze a bit more performance. So I found this schematic from the Antique Radio Forums.

I quickly put it together in Eagle, and generated a single sided PCB mask. (Click for the big picture)

Schematic

Layout

Mask

This was designed for a 10′ antenna and does work quite well. I’m thinking about adding an extra stage of amplification through another transistor – does anyone know if this is a good or bad thing? Obviously I will be amplifying the noise too, so maybe an audio amplifier would be better, installed after the tuning coil.

I really want to avoid adding too many amplifiers as the “magic” of a crystal radio is that it works without external power.

UPDATE : Since posting yesterday, this article has generated quite a bit conversation behind the scenes – one thing that has come up is that replacing the transistor with a 2N5179 and then adding a class B complementary emitter follower on the output would be a good strategy. I’ll be working on this and posting a follow up to show what happens.

…and finally, I found this article on building an efficient (loud) Crystal Radio from 1945

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{ 2 comments… read them below or add one }

Adam E. September 18, 2012 at 6:25 pm

I am very impressed you took a simple transistor such as the 2N3904 and made a RF amplifer. I have been using the other transistor called the MPF102 to make a preamplifer, but this one you made does not require a fixed choke like the other circuit I have built to improve AM reception. I cannot wait to try this one out for to improve AM and long wave reception. Thanks!

Adam E. November 8, 2012 at 7:15 pm

Steve, You should try to use the 2N5109 NPN transistor since it’s a RF transistor and it will provide better results.

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