Radio signals for Beginner, Explanation of how radios work Options Track this topic Email this topic Print this topic Download this topic Subscribe to this forum Display Modes Outline Switch to: Standard Switch to: Linear+

[Eagle]

Ok, there seems to be some confusion of how radio waves work, so I'll try to pass on my professional knowledge

A basic radio system needs two basic components: a transmitter and a receiver.

The transmitter determines the range of the radio signal by the amount of power it is pushing out, the more power, the greater the range (for those of you who are interested it work on a distance squared basis, so you need 4 times the power to transmit twice the distance, though again this is simplified)

The receiver takes this signal and converts it in a useable format. The receiver does not change the distance of the signal, this is why receivers can always be small.

In a two way system, on which the RFID system seems to work, require that each RFID to have a transmitter and a receiver. The transmitter has a short range, a few meters depending on model, while the receiver as long as it is within the foot print of transmitter can receive a signal at any distance. This means a RFID tag could receive a signal from a powerful transmitter, but be unable to respond.

This means that the RFID rating purely corresponds to its transmitter.

There are however a subtle means of using this to track an RFID which is out of range its transmitter range. Activate the RFID with powerful transmitter, the RFID logs onto local network and sends a text message, using a pre arranged email/text account (similar to current mobile phones), a hacker then backtracks the signal to nearest fixed node and RFID is located to within a few meters.

The advanced course:

Radio antenna are based on a 1/4 of the wavelength of radio wave, as this is the most efficient transmission mode.

There are two types of antenna omni direction (equally strong transmission in all directions) and directional (stronger signal in certain directions)

Currently the two most common antenna are the pole plus ground plane (which is usually non existent) and the loop, which circular with one or more loops. The pole is more or less omni directional, but is larger, while the loop is 3d figure of 8 (pinch a balloon in the middle, where the more in certain direction, the stronger the signal).

Size matters:
At above 300 Gigahertz, the atmosphere becomes opaque to radiowaves, so is unusable.
At 30 GHz+ rain can cause serious problems with the radio signal.
A 30GHz signal has a wavelength of 10mm, so 1/4 is 2.5mm which means it is probably about the right size to fit into an RFID.
A 3GHz signal has a wavelength of 100mm, so 1/4 is 25mm a bit large for an antenna, but ok for a loop with radius of 8mm.

So at a guess the wifi system has to be using a signal in the range of 3-30GHz. This also allows plenty of bandwidth, because of the signal encoding you generally can't transmit more data than you have bandwith, so 30GHz of bandwidth means that you can transmit about 30Gb of data per second. You just have to careful to limit the radio strength to the minimum possible or else you can reduce your neighbours bandwidth, sort of like radio pollution from shouting.

Funny things about radiowaves is that they bounce off things especial metal, these reflected signals can play merry hell with signal strength, cancelling it out in some locations while boosting it in others, so metal heavy environments may severely reduce signal ratings. The other thing is that they are absorbed by materials, when you buy a cordless phone or doorbell, have you noticed that they have an open air rating (usually ~200m) but if you try this in the home, their range is much less typically ~30m. And that's for a relatively low frequency (400Mhz), it gets much much worse at a higher frequency.

So bear in mind when working at the extreme range of the signal, you may be fine and in fact may get some extra range or you may not have any reception to speak of.