why do we use frequencies in Ghz for long distance communication?

Question

Like even in the case of Wi-Fi WMAX and even hot spots'

Answer 1

Several of the reasons have already been mentioned in earlier answers, like smaller antenna size and the availability of the frequency (not already used for another purpose). But the primary reason has to do with the concept of modulation.

There are many ways to modulate a radio signal. The original technique developed was amplitude modulation, where the voice or music that you want to transmit varies the amplitude of the radio signal, which is called the carrier. This technique is still in use today for your AM radio stations.

Mathematically, amplitude modulation produces two sidebands: an upper sideband and a lower sideband. If we let the symbol fc represent the carrier frequency (say, 640 KHz), and the symbol f represent the frequency of the sound you want to transmit, the frequencies of the sidebands are (fc + f) and (fc - f). Thus, the total bandwidth of the signal for that radio station is 2f.

You can see that the carrier frequency has to be significantly higher than the highest frequency of the sound that you want to transmit. In fact, the bandwidth places restrictions on how close together you can have radio stations. That's why you don't have stations at 630 KHz and 650 KHz, because their signals would overlap with the station at 640 KHz.

Just to digress a little bit, but many radio amateurs ("hams") use a technique called single sideband (SSB) modulation, which is AM modulation with only one sideband, either the upper or lower. The bandwidth of their radio signal is only f, allowing essentially twice as many hams at one time to use the crowded frequency bands allocated to ham radio.

The next modulation technique put into widespread use is frequency modulation, whereby the sound you are transmitting varies the frequency of the carrier rather than the amplitude. FM has the advantage that external sources of static that affect the signal strength (amplitude) have little effect on the sound quality at the receiver. Thus, FM stations are relatively static free. Also, because the carrier frequency is much higher than those used for AM radio, the bandwidth of the FM station can be much wider, allowing the station to transmit high fidelity stereo.

Now we come to the problem of cell phones. Within any given cell (which you can think of as a honeycomb-like territory overlaid on a geographic region), you've got thousands of individual cell phones competing for the use of a limited number of frequencies. This is accomplished through a technique called time division multiple access (TDMA), in which each cell phone is assigned a time slot by the base station during which it can transmit. Since you've got thousands of time slots, the duration and duty cycle of any one slot is very short. You've got to transmit all your voice data in that narrow time frame. Compression techniques are used (like in .mp3 audio files), but you still have a mind boggling amount of data to be transmitted when all the cell phones are taken into account. Because of the bandwidth required, this can only be accomplished when you have a very high carrier frequency, in the GHz region.

The same considerations apply to Wi-Fi and any other high data rate transmission.

Answer 2

The choice of frequency depends on the data rate and the distance for propagation. Since energy is conserved, a smaller beam will be received at a longer distance for the same power output. Most WiFi antenna are designed to propagate signals horizontally with small antennas. A lower frequency will cause more diffraction of energy through the effective aperture of the antenna, i.e., more energy will be directed downwards and upwards..

Long distance microwave transmissions have similar constraint. To minimize output power for a given range, a smaller beam will allow for longer range transmission.

Answer 3

We use whatever frequencies are still available in our crowded communications spectrum. WiFi is not all that wide without an array of antennas to spread it. The Gigahertz bands require more energy to propagate signals than, say, 900 MHz, but they're available. and when a sales agent says it, it sounds like you're getting an upgrade.

Answer 4

There is a fact that the antenna length is a fraction of the wave length so that assume we will transfer a wave in the human voice frequency about 3KHz the wave length is 100Km so that the antenna length will be in the Kms order but in the case of GHz frequencies; e.g. 3GHz; the wave length is 10cm and the antenna length is in the order of cms. that is not the only limitation but it is a one.

Answer 5

-because the higher frequency is much harder for easdroppers to de-scramble and tune in to your conversations- to keep your privacy!