Electromagnetic Spectrum: Data Transfer

[TEKcisco]

The most common frequencies in the spectrum that are used in wireless devices are between 30MHz and 30GHz. I was wondering if, data-wise, you have a wireless device that can send data at 30MHz, and one a 30GHz, will the 30GHz device send that data faster?

The 30MHz device should reach that device first, right? (smaller frequency, longer wavelength)

What does frequency mean to data? What does wavelength mean to data? (yes a little redundancy)

Can you give me more info on this and possibly anything else you think might help. I just like to know stuff.

 

[Skogsgurra]

hmm..

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[TEKcisco]

Can you give a a website that resembles English more (a lot more). Or can it be translated via google translator? What is the language?

 

[Skogsgurra]

I may have indicated that you need a lot more information before any answer will be meaningful to you.

There is a lot of physics, transmission theory and plain everyday engineering to be mastered before you can benefit from a simple answer here on Eng-Tips.

First. Wireless transmission uses electromagnetic waves. Their speed is independent of wavelength or frequency. And wavelength has nothing to do with "reaching the target" before a signal with an other wavelength. They all get there after the same delay.

Second. Shannon and other guys have done a great deal of research on transmission theory. There is a fundamental relation between carrier frequency and information density time-wise. But different modulation techniques can make a low carrier frequency carry a lot of information - a lot more than one would expect from the fundamental laws.

In the 30 MHz vs 30 GHz case, it is safe to assume that the information sent in a time unit is higher for 30 GHz than for 30 MHz. But the "travelling time" is equal.

Mr Einstein found out about 100 years ago that the speed of an electromagnetic wave is constant (at least in vacuum and almost constant in air) and Mr Maxwell had done the necessary calculation before Einstein formulated his theories, which have then been verified many times during the years.

As you probably know, this site is for exchange of engineering tips. Your question does not quite fit in that category. Thence my "hmm.."

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Comcokid]

Essentially, the speed of electromagnetic propagation at 30 MHz equals the speed of electromagnetic propagation at 30 GHz. Velocity-of-propagation = C / SQRT(dielectric constant) where C is the speed of light. Vacuum has a dielectric constant of 1. Dry air has a dielectric constant of 1.00001. Don't remember exactly how many zeros are put in front of the 1, but air is just generally taken to be the same as a vacuum.

Also, the propagation distance that can be achieved on the earth is dependent upon frequency. Low frequency signals can travel around the curvature of the earth. High frequency signals are attenuated more by the atmosphere and are line-of-site (fine if you are communicating with a satellite overhead).

Now, the amount of data you put on a signal determines the bandwidth. If you are transmitting a few bits-per-second (i.e. Morse code) your bandwidth will be narrow (a few Hertz wide). Analog voice requires a wider bandwidth (a few Kilo Hz wide).

Data transmission , like WIFI, requires a lot of bandwidth. 802.11 defines 22 MHz wide channels with 5 MHz margin for each channel. Now, if you try to do that at a 30 MHz center frequency, you will find you occupy the spectrum from 19 to 41 MHz, and additionally would require the spectrum from 14 to 46 MHz for any spill-over. You would end up hogging much of the shortwave spectrum. Also since shortwave signals are bounced back by the ionosphere, you might hog much of the shortwave spectrum over a good portion of a continent, and possible portions of the whole earth! A good reason that such data devices are confined to the 2.4GHz and 5 GHz bands - there's room for lots of data channels, the signals are attenuated at a distance, and they're line-of-site.

This is only a few of the issues of what-kind-of-communications is attempted at what-frequency and why.

 

[Higgler]

They go the same speed.

It'd be good to do some reading first and ask questions on the technical stuff you didn't understand after many hours of reading. I believe this website rightly requires you to try to find out on your own, via Google etc, then ask questions.

Sorry for the "go do some reading" instructions, but this website mostly has very experienced technical people and it's most common for beginners to "ask for reference books". Google is great to start with though.


good luck,


kch

 

[TEKcisco]

Sorry guys,

Forgot that the medium that the signal travels through determines the speed, not the wavelength. Remember hearing that in high school a few years ago .

 

[IRstuff]

The propagation delay of a pulse is dependent on the speed of the EM wave in the media. Therefore, to the first order, the time to receive the first edge of a signal is nearly independent of wavelength.

However, the total transmission time of a message is dependent on the wavelength, since the information capacity of the channel is limited by the wavelength. Therefore, a message sent at 30 MHz will take about 1000 time longer to complete transmission than a message sent at 30 GHz, assuming identical coding.

TTFN

FAQ731-376