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!
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.
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.
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.
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.
Awesome, when building crystal radios out in the sticks as a kid, getting my hands on a suitable crystal diode was one of my problems. The other problem was getting a crystal earpiece. I guess a rather complicated way around that would be to make some kind of DIY vacuum tube amplifier and make an electromagnetic speaker.
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.
Roughly, when you're receiving an AM signal, you use the rectifying effect of a diode coupled to a low-pass filter. The idea is that the envelope is all you really need to pass to the speaker, if the signal itself is powerful enough.
This material can also have an interesting negative resistance ("kink") characteristic as well, like so many natural semiconductors.
You can even make an oscillator from some of these negative resistance materials by just biasing the crystal with the right DC voltage and connecting a parallel LC tuned circuit. The basis for a simple transmitter!
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.
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...
Newpaper will be 'fun' to work with as its moisture content will vary with environmental conditions, making its dielectric constant not very..erm..constant - unless you impregnate it with wax or a lacquer.
(Made plenty of crystal sets during my teen years in the mid 70s-80s)
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.
Also, I find that inserting a ferrite rod into the coil (or winding the coil around the ferrite rod to begin with) makes tuning much more effective (sharper) and makes the audio signal much stronger. A steel or iron 6" nail also works if you want to dispense with the ferrite rod.
I live very close to the BBC regional broadcast transmitters (100KW apiece) and since boyhood I've made loads of crystal sets as well. A few feet of wire often being quite sufficient as an antenna and the radiator pipe as an earth connection. A good earth connection is very important to the effectiveness of a crystal set.
I made a single transistor regenerative set next and because the local BBC signal was so powerful I literally didn't even need an antenna at all!
Another single transistor was all that was required to drive a speaker loud and clear.
Why would the resistor help? It just lowers the impedance and what you'd really want would be a high impedance earpiece. Or is having a useful effect by reducing the damping factor of the circuit?
The resistor provides loading for the detector. The crystal earpiece has such a high internal impedance that it is more a capacitive than a resistive load.
The high value resistor in parallel with the earpiece provides a working load for the circuit. Without it the audio signal is largely being shunted by the large internal capacitance of the crystal earpiece.
The resistor is not required if using a high impedance moving coil or moving iron earpiece such as that from an old telephone.
Trust me, if using a crystal earpiece the resistor makes a big difference.
