Various radio/wireless reception questions

[AmberClad]

1) Why do radio and TV reception seem to improve if you hold the receiver or antenna?

2) Does the Pringles can trick only work with WiFi? What about radio, Bluetooth, etc.?

3) I have one of those USB thumb drive MP3 players with a built-in FM receiver. Not surprisingly, there is no visible antenna. Why does playing around with the position of the earbud wires alter the quality of the reception?

4) Why does WiFi work fine inside shielded buildings, but cell phones and radios seem to be much more heavily affected?

 

[f95toli]

1) It doesn't ALWAYS improve the reception. In some cases it might and then i presumably because your body works as an electrical shunt, I am guessing the extra capacitance might change the impedance which in turn *might* improve the matching and therefore the reception.

3). Because they are, if I am not misstaken, using the earbud wires as antennas.

4) Shielding only blocks out radiation coming from the outside, cell phones work just fine inside a shielded building; it is just that they can't recieve anything from base stations. However, e.g. DECT phones works just fine. Also, with wifi you can use higher powers and better antennas since you are not limited by power consumption.
Radios operate at much lower frequencies than both cell phones and wifi meaning it is usually much harder to block out the radiation; high-power radio transmitters can cause serious problem when you are trying to do precision measurements.

 

[Paperdoc]

On Item 4, the answer probably lies in where the transmitter and antenna are placed. Cell phone signals from outside a building do not penetrate well through steel girders, etc, so the phones often get weak signals inside modern buildings. In some (like, in retail malls) the owners have mounted repeater transmitters and antennas inside to combat the problem as a convenience to occupants and customers. Now for WiFi, any organization that expects to use WiFi networking in their building will have mounted WiFi transmitters and antennas inside the building and connected to their own internal network. If you want reliable WiFi networking in your building, you don't usually just depend on someone else's network from outside!

By the way, the limits on signal stength are not from trying to use less power in a small battery-operated device. They are imposed by regulation on the maximum radiated power from an antenna in proximity to people. So WiFi vs cell phone signal power is similar at the transmitter. The difference, as I said, is where those are located compared to where you are.

 

[stevf]

2 - do you mean making an antenna out of a pringles can? in that case it might work. What the can "trick" I am thinking of creates a directional antenna. Directional antennas have much higer gain over an omni-directional antenna(the kind that basically looks like a single wire). Simply put, gain means a much stronger signal compared to a plain antenna. The problem with a dirctional antenna is that it must be pointing at the source or receiver. Thats why it works for wifi - you know where the receiver and transmitter are. You could do it for a radio, but the size of your antenna would be much different and you would have to point at the transmitting antenna so you couldnt do it for a car very easily. Hope that makes sense to you

 

[Pulsar]

Because of the different wavelengths, a pringles can is going to have varied functionality with different types of wireless signals.

An antenna works best when it is a multiple or specific fraction (1/2 1/4) of the wavelength of the signal.

Likewise, a direction antennae needs specific dimensions and focal lengths to be more effective. You can google antennae building and they give some specifics.