Is Wifi line of sight?

[Q.E.D]

@mks57

Inversely proportional to the square of r?

Yes, sorry. That’s what I meant, of course. It’s late here!

[Chasing_Dreams]

Dang it, you guys are right, power attenuation of an isotropic point source is inversely proportional to the square of the radius. Same logic about the increasingly rapid loss of the signal power still applies.

[Sage_Rat]

A simple answer is that Wifi is based on line of electricity. It’s just power being beamed through the air. If you have a copper mesh between your computer and the transmitter and it will be blocked off, while as a stack of pillows would be pretty see-through.

Note that humans are largely water, so we aren’t real see-through.

[Rico]

@Corporal_Nobbs

On the topic of Ham (Mmmm HAM)
Yay at long last a thread that I can contribute too. I work in marine communications for the RFA (The Royal Fleet Auxiliary, the Royal Navy’s freight and fuel service) There are three types of Radio waves. HAM radio waves propagate in certain layers of the Ionosphere (High Frequency) so on a basic level they bounce up and down, and travel a greater distance than VHF on say a small pleasure boat. VHF is essentially visual range only. Just reaching the Horizon (depending on hight of the antenna, as the waves do not propagate at all…

I must step in to correct this.

There is one type of radio wave. There are four ways to “modulate” or put information on radio waves. AM, FM, CW, and Digital.

AM is Amplitude Modulation. It’s used not only on the AM broadcast band, but also in the shortwave bands by broadcasters and amateurs on shortwave (the aforementioned Hams). AM (and their cousins Single SideBand or SSB) signals indeed are reflected by certain layers of the ionosphere, some frequencies are reflected better at different times of the day or year.

FM is Frequency Modulation. It’s used not only on the FM broadcast band but by business band two-way radio, older style cell phones, and, again, Hams. FM is a line of sight modulation scheme, but FM signals have been known to “duct,” or bounce off a level of the ionosphere when conditions are absolutely perfect.

CW is not really modulating a wave at all. It’s simply turning a carrier wave on and off in a certain pattern. Hams use this when they send Morse Code.The transmitter is turned on and off with the code key. A common misconception is that the transmitter is sending “beep-beep-beeeep-beep.” It’s not. The beep is made in the receiving radio by mixing an off-frequency signal and creating a “hetrodyne,” or the “beep” tone you hear.

Modulating a signal digitally is the latest technology - and quite honestly, I’m not at all familiar with the many different schemes and formats available. Some are done by sending a digital signal over an FM carrier, some actually send a digital signal with phase shifts.

This is a very simple explanation. For more information see Modulation - Wikipedia or http://telecom.tbi.net/mod1.html

Rico
General Class Ham since 1983

[si_blakely]

@Rico

I must step in to correct this.

There is one type of radio wave. There are four ways to “modulate” or put information on radio waves. AM, FM, CW, and Digital.

This is true. However, Radio covers a large frequency spectrum, from 3 Hz (ExtraLowFrequency) to 300Ghz (ExtraHighFrequency). The various bands have different characteristics, and thus are used in different ways.

ELF 3 - 30 Hz is ground and water penetrating, and can be used to signal submarines (but the bandwidth is low).

VHF/UHF (30MHz-3GHz) is line of sight, and used for voice/television/wifi(2.4GHz).

Longwave (LF 30kHz-300kHz) follows the curvature of the earth and is more reliable for long distance (but low bandwidth), shortwave (HF 3MHz-30MHz) refracts through the ionosphere (better bandwidth, but very dependent on atmospheric conditions).

Si

[Richard_Pearse]

@Raguleader

I recall reading in a book (“Miracle at Midway” by Gordon Prange, I think) that they discovered this effect, or at least unexpectedly saw it in action, during the months leading up to the Battle of Midway. Bomber pilots flying out of Midway Isle were picking up UHF signals from pilots flying around Pearl Harbor, which should have been well beyond the range of their radios. Once they figured out that the radio transmissions were bouncing off the upper atmosphere and coming down much farther away, they started being much more discrete about what they talked about on their radios lest the Japanese be tuning in.

I think it is unlikely the UHF radio was bouncing of the upper atmosphere.

HF radio will reflect off the ionosphere. UHF and VHF won’t. However UHF and VHF will get ducted below an inversion layer. I have experienced very long ranges with VHF radio when there is a strong low level inversion. I think this is more likely to have been the cause of the long ranges the bomber pilots were experiencing.

[mks57]

@1920s Style "Death Ray"

I think it is unlikely the UHF radio was bouncing of the upper atmosphere.

HF radio will reflect off the ionosphere. UHF and VHF won’t. However UHF and VHF will get ducted below an inversion layer. I have experienced very long ranges with VHF radio when there is a strong low level inversion. I think this is more likely to have been the cause of the long ranges the bomber pilots were experiencing.

When I lived in Hawaii, there were almost annual occurrences of ducting that made it possible to have two-way communications with California in the 2-meter Amateur Radio band (144-148 MHz). These openings often extended well into the UHF range.

[N9IWP]

Line-of-siteness has more to do with wavelength/frequency than modulatuon technique. Aircraft radios use AM modulation at VHF frequencie (and so are line of sight)

FM is allowed in the 10 meter amatauer radio band which is at the high end of shortwave.

If you have big enough antennas you can bounce VHF (and above) off the moon.

There is also polarization modulation, but I don’t think anyone actually uses that.

Brian

[drachillix]

@According to Pliny

What weakens the signal? Walls? Trees? Windows? People passing in the path?

Having worked with wireless routers regularly for the last couple years, all of the above. I have even seen connections broken by the presence of several people standing between the router and the recieving machine about 30 feet away.

I have been out to many onsite calls for wireless network problems where moving the router 5-10’ one way or another so that fireplaces, water heaters, or brick walls were not in the direct transmission path. High up on a wall or even ceiling is often an excellent option to avoid loss of signal due to heavy furniture and foot traffic.

Depending on the antenna arrangement even the case of the computer can be an obstacle to the signal.

[butler1850]

I used to install point to point wireless networking, long before there was an 802.1x standardizations (pre A, B, G, N, or anything. In fact, there wasn’t even interoperability between brands) I got out of it partly due to the issues involved in installing equipment in New England (Line of site here is a real problem)

Many of these systems operated in the same ISM (Industrial Scientific & Medical) band that today’s wireless routers operate in. One of the biggest issues we faced was inteference from a variety of objects. The biggest obstacle was any sort of pine needle. The wavelength of the wireless signal is nearly equal to the width of the pine needle, and will block a signal like lead blocks x-rays. Window screen (metal) was also a problem, and is present in many walls as a material to apply plaster to.

Nearly anything that’s the right “size” (or multiple/fraction of “size”) can cause a degredation of signal.

In general in the radio world, (exceptions abound), the higher the frequency, the more directional the signal. VHF/UHF will not arc over a mountain, or around a building. LF, HF signals can. (Ground wave principle) This site discusses propagation of signals far more than I can, and raised a few more interesting points…

Interference. Areas in your house may be the confluence of “bounced signals” that interfere with your reception.

Polarization. The angle at which your antenna is oriented in relation to the other antenna can interfere with your reception.

Other factors can also affect signal strenghts.