Why transmit signal in air is so high freq.?

[wccheng]

Dear all,

Why the transmited signal in air is need to use so high freq (such as MHz or GHz)? Could I use low frequency (such as kHz) to transmit in air?

wccheng
thanks

 

[Borber]

Low frequencies like few kHz can be used for transmitting and they are in use today for naval submarine communications for example. It is dificult to build antenna with reasonable good gain at low frequencies and high power transmitters are needed. So higher frequencies are used for radio, television, communications, radio links etc.

 

[v_naren]

1) antenna size = approx wavelength of frequency of transmission
eg. if freq of TX=1KHz => Lamba= wavelength=300000meters!!!!!
this size is impractical..thus we need to use higher frequencies

2) Suppose we want to TX music from A to B...the frequeny range of
music is generally from 20Hz to 20000Hz then the sizr of the antenn
required will range approximately from 15000Kilometers to
15kilometers for TXing the different frequencies. firstly it is quite
obvious that it is impossible to firstly build an antenna of these sizes at
all!!! and even if you build it how can you use one single antenna to TX
the entire range which will have an infinite number of frequencies.
Which is why we use MODULATION btw....if we modulate using a
simple DSB-AM...around carrier at say 50MHz....then the entire
frequency range we need to transmist is only from 49.98MHz to
50.02MHz...so then we only need antenna sizes of the order of
6.0024meters to 5.9976meters...these sizes are much more practical
to achieve and since the range of antennas needed is almost constant
relative to an unmodulated TX we might as well use a single antenna
size with a negligible loss in TXed power.

3) Also...in general the atmosphere or air or whatever allows
transmission of EM waves at a higher frequency with much less
attenuation than with low frequencies...this is just a property of the
atmosphere and since we cannot do anything about it...we must use a
high frequency of TXission.


There are a lot more reasons which are more complicated to explain.

 

[lvjrf]

the noise in low freq is higher than high freq

 

[v_naren]

correct me if I am wrong......but the noise in higher frequencies is much higher than at low frequencies and hence is actually a disadvantage and not an incentive for transmission at high frequency...

 

[Borber]

Noise power or noise figure or equivalent noise temperature is higher at low frequencies.

 

[hrkhari]

One of the resons is the bandwidth allocation for mobile wireless communication highlighted by FCC and etc is set to be at high frequency.

Rgds

 

[djalli]

Low frequencies like few kHz can be used for transmitting and they are in use today for naval submarine communications for example.

Even 300Hz, their antenna being pulled (pulleable antenna) in the sea. Submarines have such antennas. Imagine it. Just a long wire.

The only reason why low frequencies because is impossible to communicate in high frequency deep under water, skin effect, scattering and etc.

 

[djalli]

Noise power or noise figure or equivalent noise temperature is higher at low frequencies.

Even it is not fully studied at low frequencies must exist something as "Windows of noise" occuring in HF.

 

[springf2000]

you can use the basband to transmit the signal,such as the UWB signal,but your UWB basband signal's pulse width should be narrow enough ,such as 1ns

 

[chihhao]

bandwidth allocation is one of the important reason

 

[pyrite]

Here is the common usage of freq spectrum:

 

[claint]

every band has its use,many of them has been allocated for some reason.

 

[flyhigh]

Hi,

As can be seen from the post by pyrine, low freq range (~10kHz) can be used for some important communications. The only advantage of low frequency (Long Waves) is that they have graceful conditions for propagation in the atmosphere, so long range can be achieved using those frequencies. Comercial radio broadcast operating in this band can be heard almost on all of the continents! Of course, there are antenna issuse mentioned earlier.

flyhigh

 

[dd22]

Another main reason is communication capicity, which is realted to channel bandwidth and power. If you use lower frequency, the bandwidth is reduced of course, at 10KHz, you cannot achieve 20 KHz bandwidth. Then to support 1 Mb/s data rate, you have to use very high level modulation scheme and needs a lot of power. I dont think anyone can do this now.

 

[djalli]

I am curious to know becomes channel capacity in 1000m under water?

 

[hoangthanhtung]

Dear all,

Why the transmited signal in air is need to use so high freq (such as MHz or GHz)? Could I use low frequency (such as kHz) to transmit in air?

wccheng
thanks

Simple answer : Because energy(transmitter power) is directly proportional to the fourth power of frequency and is inversely proportional to the distance. So if you want to transmitt the sign to the receiver which is far from the transmitter, with acceptable signal-to-nose ratio (SNR) you have to use high frequency for long distance. You also use low frequency but you have to increase transmitter power and you have accept that distance of signal's propogation is short.

 

[Borber]

Propagation properties of radio waves differs with frequency. As flyhigh said, VLF up to VHF transmitters can be received arround the world while communications on VHF and higher practically needs free line of sight between antennas.
At radio waves propagation we distinguish surface, ionosferic, tropospheric and direct waves.

 

[djalli]

Please use this monograph to figure approx. out how much attentuation is for certain frequency.

There are two sample drawn as lines. Top line is normal sea water.
Bottom line is if water is Adelaide water in Australia. It was nearly become a scandal the water in city of Adelaide in Australia. Presumeably this would be as nearly as worst regular water in our faucets.

For example 40KHz in seawater you would experience nearly 8dB per meter loss. At 1GHz loss is 46dB per meter dashed line. Wow that is something.


AFAIK VLF system is mostly used is maritime underwater explorations. It is 3-20KHz. USA military uses for its submarines the ELF system which is 76Hz, that is right 76Hz communicating deep in ocean.

https://amzn.to/2gRfKqW

Monograph picture is also a good accurate tool.
I found it to be helpful!

djalli

 

[djalli]

Now I just explained with figure the water. Does air contains water vapor, humidity, etc?