
My foxhole radio - jgrahamc
http://blog.jgc.org/2012/02/my-foxhole-radio.html
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jgrahamc
Funny what tickles the fancy of Hacker News, folks here might also enjoy my
coffee can antenna that lets me leech WiFi from a hotel 1/4 mile away:
[http://blog.jgc.org/2012/02/long-range-wifi-antenna-from-
ill...](http://blog.jgc.org/2012/02/long-range-wifi-antenna-from-illy.html)

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jrockway
Is the coathanger antenna cut to a particular length?

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jgrahamc
No, and it's of no importance. It's just something that will make an
electrical connection with the coil and stay there.

Edit: I answered the wrong question here, thought he was talking about the
coat hanger I used in the crystal radio set. In the case of the WiFi antenna
it matters a lot!

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ja27
It certainly does matter, especially on the transmitting side. It's just hard
to predict how long it needs to be without breaking out antenna modeling
software.

Here's a site with a few different wifi antenna designs and some antenna
modeling: <http://www.lincomatic.com/wireless/homebrewant.html>

This site has a calculator for cantenna dimensions:
<http://www.wikarekare.org/Antenna/WaveguideCan.html>

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jgrahamc
Sorry, I was answering a different question (the original question was a bit
unclear). The antenna length certainly does matter as you point out, I thought
the parent was asking about the piece of coat hanger that I use to sweep along
the coil to find stations in my crystal radio.

My mistake for misinterpreting the question.

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jeffcapers
I made one with a hacksaw blade and no tuning arm a few years ago - could pick
up a single nearby station (just about).

Have you looked into creating an earpiece/speaker? it's the "weak link" in
such a basic setup.. always wondered if something could be cobbled together.

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linker3000
Full marks only if bare copper wire was used and he cotton covered it himself!

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ja27
And built the speaker. That's one thing I haven't done in a crystal set. Just
a coil, magnet or even scrap steel, and the right paper.

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tocomment
How does the diode work?

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jgrahamc
Bluing steel turns the coating of the steel in magnetite, Fe3O4, which is a
semiconductor. The point of the pencil is graphite and the result is
essentially a Schottky Diode: <http://en.wikipedia.org/wiki/Schottky_diode>
They have a very low voltage drop which is vital for a crystal radio because
the voltages involved are tiny and any lost voltage must be minimized.

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tocomment
Thanks for the informative answer. Now my next question. Why does a radio need
a diode to operate?

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ajuc
AM radio works by modulating carrier wave (of constant, high frequency), by
voice (small, variable frequency). You want to get the voice, and if you just
connected signal from antenae to the speaker, it'll be vibrating too fast for
you to hear the voice.

So you need to filter out the carrier part of the signal. Diode only allows
current to flow one direction, and the rest of circuit allows it to discharge
slowly (it has some capacity).

When there is unmoduled carrier wave (silence on radio), there is balance
between the current that flows thoruhgt diode, and the current of discharge,
so the voltage on speaker is more or less constant (probably not on 0 level,
but it doesn't matter).

When carrier wave is modulated to half the amplitude, speakers gets less the
voltage.

So speakers vibrate with voice wave frequency, not carrier wave frequency, so
you can hear the voice.

Imagine you want to measure the daily sea level, and it's very windy (big
waves). You can't just measure it once a day, because depending on the moment
you'll measure high or low level of wave, and you want the average level. So
you make a container that is filled by waves throught one-way pipe, and you
drill small hole in the bottom of that container, so it leaks at a constant
rate. When you tune the size of hole in the bottom, and the size of one-way
pipe correctly, you can measure level of water in this container, and it will
be proportional to the average level of water in the sea.

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almost
That's a remarkably clear explanation. I learnt something new, thank you very
much for that!

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Craiggybear
If you put a high-value resistor (between 220K-470K) in parallel across the
crystal earpiece the audio will be much stronger/clearer.

OR you can use the high impedance earpiece form an old telephone instead of a
crystal earpiece which will give a fantastic, clear and loud output.

My grandfather used to put his "cans" from his crystal set in a pudding bowl
and it acted like a speaker -- several people gathered around could hear it at
the same time.

A 300pf variable capacitor across the coil will also give you quite effective
tuning.

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jgrahamc
Thanks for the additional advice. I was planning to make a capacitor out of
foil and newspaper interleaved. One day when I've finished all the other
things I want to play around with...

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ja27
There are lots of ways to homebrew variable capacitors. A sheet of glass and
two pieces of scrap copper does well.

I have a "crystal" radio (uses a modern diode) all built on a paper towel
tube. The coil is wound on the tube like usual but at the other end of the
tube there's also a layer of foil and a smaller tube with a layer of foil that
slides inside that to make a variable capacitor.

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ul5255
I once found this site with lots of interesting projects:
<http://www.sparkbangbuzz.com/>

