Simple crystal Radio Question (grounding issue)

[elockpicker]

Simple Radio Question

Hi,

I have two questions about radios.
1)As you know a simple type of radio is one that uses a crystal earphone and a germanium diode.One end is connected to the antenna and the other to the REAL ground i.e soil.
I was wondering : in modern radios we do not need the soil and we just need the antenna.
How is it possible ?

2)what is the circuit model for an antenna ?

Thank you very much.

 

[biff44]

Re: Simple Radio Question

You are talking about a broadcast band AM radio. The AM radio band is very low frequency. As such, an antenna, that wants to be at least one quarter wavelength long, is physically many feet long.

You could use a half wave long antenna (does not need a earth ground connection), that is twice as long as a quarter wave one, but that is even bigger. So you use a quarter wave long antenna, and for the other "side" of the antenna, you use earth ground. The earth ground acts a little like a mirror, and the mirror image of the quarterwave antenna, plus the quarterwave antenna itself, add up electrically to look like a half wave antenna.

Modern radios instead use a bar of ferrite material that has a lot of copper wire wound around it. This makes the antenna "look" electrically like it is a half wave antenna, but it is only 1/10 the size physically.

 

[elockpicker]

Re: Simple Radio Question

Thank you ,but can I assume an antenna as a voltage source (which I have to buffer) ?