# Antenna limits

1. Apr 2, 2005

### mtanti

I have a wierd question I think... I'de like to know how low a frequency and how high a current can a wireless device transmit.

eg, radio frequency is over 1000s of Hz, can it be much lower than that?When you recieve a radio signal to an antenna, a small current is generated which is interpreted as sound. How high can that current be?

If there's no limit to the amperage generated at the recieving end, how can you calculate it?
10x!

Last edited: Apr 3, 2005
2. Apr 4, 2005

### moose

well, just check the electromagnetic spectrum chart, somewhere higher or lower, it wont be a radio signal anymore.

3. Apr 5, 2005

### Antiphon

It's not a wierd question at all.

There are no limits. But there are practical limits. For example, if you start
off by saying that you have a 1 MegaWatt transmitter, then you can determine
how large your antenna has to be if you fix the frequency in order to achieve
a certain efficiency.

You can make a 0.00001 Hz tansmitter and deliver billions of amps through
the antenna. Just not on the earth because the planet is too small to hold the
antenna.

4. Apr 5, 2005

### waht

re

low frequency antenna don't have to be a long piece of wire, you can have large coil of wire that when unwound is just as long.

5. Apr 5, 2005

### SpaceTiger

Staff Emeritus
I'm not an electrical engineer, so this is partly a question. It seems to me that coiling the antenna would give it a higher inductance, and therefore a higher resistance to change in current. Doesn't that limit the amplitude of the signal you can transmit/receive?

Also, I was thinking about the frequency of the receiving circuit. From what I've heard, you "tune" to a specific frequency by creating a circuit that has a natural frequency at that of the desired radiation, creating a resonance. Might there be practical limits to the natural frequency of this circuit? How large can one make a capacitance in practice?

6. Apr 5, 2005

### Antiphon

The area of the coil would have to go up in order for it to be an efficienct radiator.
The number of turns won't make it more effieicient but it would change the way
you would drive the antenna. Look up "Radiation Resistance" to discover how.

The practical limit on capacitance is set by your budget. Bill Gates could