I do not know if any of you have heard of a crystal radio: (http://en.wikipedia.org/wiki/Crystal_radio), but in a nutshell it uses the very radio waves in the air as its source of power, no outlet or batteries required. From what I understand from the reading, the radio waves received are converted into AC current, which is then converted in DC current. My question is, is the antenna designed only to receive certain frequencies to receive and turn into power, or does it pick up any radio wave it comes into contact to and turn it into power and display the preferred frequency through the earphones? Also am I correct in the assumption that it is AC current that is generated from these waves?
Thanks!!
David

 

Hello,

The power created is only enough to let you hear the sound in a crystal earpeace.
You will need a very large antenna to hear anything.

Here is a page from the EDUCYPEDIA with lots of links on crystal recievers.

http://www.educypedia.be/electronics/circuitsrfcrystal.htm

Bertus

 

Most crystal sets were designed to work on the AM broadcast band from about 510 kHz to 1650 kHz. A long wire antenna worked well enough for receiving signals from 50,000 Watt Clear Channel Stations all over the country especially during a solar max. We are now allegedly coming off a solar min sometime in 2005-2007. Although all kinds of RF energy is present in the antenna only some of it makes it to the detector (crystal or diode) based on the RF tank circuit that connects the antenna to the detector. The rest of the RF energy is simply thrown away. What comes out of the detector is rectified AC consisting of the carrier frequency and the audio envelope.

http://en.wikipedia.org/wiki/Amplitude_modulation
http://en.wikipedia.org/wiki/Crystal_radio

If you do a back of the envelope calculation you will find that there are truly minuscule amounts of power in a crystal set. A few microwatts if you're lucky.

 

Only the radio frequency being received is used to power the audio output of a crystal radio. The actual power levels are extremely small, piezo earphones are extremely efficient.

What Papabravo said, only shorter.

 

I've made a simple crystal receiver and listened to a local AM station from it.

It's amazing to seemingly pull voices out of nothingness!

To answer your questions -

The received power never needs to become DC. If the radio frequency is "detected" with a diode, then the audio signal, which itself is AC, can be heard through earphones.

The answer to the antenna question can be "a little of both". All that is needed is that enough power be received to operate the earphones. If you're close to the transmitting antenna, practically any wire will pick up enough signal to do that.

As you get farther away, you start to need to design the antenna more carefully to pick up enough power. In either case, an antenna will recieve all signals passing by, and unless your receiver rejects them, they will all be detected and heard.

The antenna itself can be designed to perform some unwanted signal rejection, as you said, but that usually isn't enough to completely reject unwanted stations.

From a practical point of view, it can be hard to design an AM radio antenna to just pick up certain AM frequencies. For casual experimenting with crystal radios, it often works perfectly well to just raise as much wire as you can high into the air, like suspending 50 to 100 or so feet of wire from a tree branch. My experience also was that a good Earth ground made a big difference; again, nothing fancy, but be sure to connect one side of the crystal radio to a wire stuck into the ground.

Either use lightning protection techniques on the antenna (see ham radio guides for how to do this) or take the wire down when you're done to prevent a lightning hazard.

 

I do not know if any of you have heard of a crystal radio: (http://en.wikipedia.org/wiki/Crystal_radio), but in a nutshell it uses the very radio waves in the air as its source of power, no outlet or batteries required. From what I understand from the reading, the radio waves received are converted into AC current, which is then converted in DC current. My question is, is the antenna designed only to receive certain frequencies to receive and turn into power, or does it pick up any radio wave it comes into contact to and turn it into power and display the preferred frequency through the earphones? Also am I correct in the assumption that it is AC current that is generated from these waves?
Thanks!!
David

Every ham worthy of the name has built crystal radios in his "yute". I still build them in my decrepit old age.

It's amazing how sensitive the human ear is. You can hear audio signals down in the TENS OF NANOWATTS of acoustic power, which is the only reason crystal radios work so well.

In my radio classes, I give a little talk about this, to put some perspective on relative power levels. A typical STRONG H.F. signal, what we could call an "S9" signal on a modern amateur radio receiver, is 50 microvolts at the antenna terminals. This translates to a measly 50 PICOwatts, assuming an antenna input impedance of 50 ohms. Remember, this is a STRONG R.F. signal!

Now, a standard telephone receiver has a nominal audio level of -13dBm (U.S. standard TELCO level). This is one twentieth of a milliwatt. So, what's it take to bring up a 50 picowatt signal to where it's a comfortable listening level in a set of headphones? Well, we need about 70 dB of gain, that's what! Ten million to one power gain. This can be distributed throughout the receiver: r.f., I.F., and audio chain. Actually, it only takes three stages of transistors, assuming a gain of 23 db per stage, about the theoretical maximum.

Something to think about.

Eric