Power level on transmission of a signal at certain distance

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bramin19

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can anyone explain how the power will be attenuated in free space?

For eg: if a dipole antenna transmits a power of 10 watts, what could be the power at 1km from the antenna? how can u calculate it?
 

Radiated power is the same measured at any distance, max received level is almost the same at 1 km distance as air attenuation is low and in real "free space" none.
Think about it as a flash light, amount of light that can be received is almost the same at 1 m distance as 1 km. Required size of collector can however be a practical problem at long distance.
Field strength at distance is reduced with a factor corresponding to how much the area of a collector must increase to still pick up same amount of power.
If you know effiency and radiation pattern for both transmitting and receiving antenna, in free space, can Friis transmission equation be used to calculate amount of power that is possible to receive. Friis transmission equation - Wikipedia, the free encyclopedia
In reality is free space hard to find and it can be real complex calculations if ground reflections, moisture or trees and such must be taken in account.
 

you also need to specify the frequency as attenuation increases with freq

So to answer you question specifically you need to tell us what freq that 10W transmitter is on

go to this site and download the FSC program .. its free it will work it out for you
after you put in TX power, antenna gain path distance and freq

FSC Home Page


Dave
 
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An easy calculation about power loss vs distance is: any double of distance will increase the path loss with 6dB.
For example if the received power at 100m is -30dBm, at 200m will be approximately -36dBm.

Also, any double of frequency will increase the path loss with 6dB.
For example if you get -30dBm at 100m using 1GHz signal, you will get approximately -36dBm at 100m using a 2GHz signal.
 

Be careful and keep free space apart from other environments. If the question is attenuation in free space is frequency not a parameter.
 





hello dave

u said that attenuation increases as frequency increases. but i have studied that the carrier frequency which is modulated by a message signal has to be high in order to travel longer distances.


Am I right?

if i am wrong what is the use of the carrier signal?
 

In free space is reachable distance for a given power level not depending on selected carrier frequency.
See my previous post and follow that link to learn that received amount of power at a given distance is not depending on modulation either.

Modulation is about the carrier information content. Of course can you make this modulation more or less demanding for the receiver which can effect amount of required signal/noise ratio to be able to correct decode any information.

Digital broadcast TV modulation is an example were you maybe still can receive a good signal but not good enough to decode any information at all.
Slow Morse type of amplitude modulation can be understood even for real weak received power level. Can any power be detected at all, can also the information in most cases be understood. With just a few Watt TX power can it reliable transfer information around half our solar system as some satelittes in fact do.
A GSM phone can not understand information from a base station 40 km away, even if it is a powerful transmitter, high frequency carrier, optical sight between the antennas. This is due to the GSM modulation method which limits max distance.

"what is the use of the carrier signal?"
A hand lamp can be used to transmit Morse code. The light beam is the carrier. No beam -> no modulation possible -> no information can be received.
Continuous beam do not either transfer any information, besides that in the receiver end do they know that your lamp is still working.
 


I think you are confusing several aspects of RF transmission and signal modulation. Every signal sent on an antenna needs a base carrier frequency. If you turn on the transmitter and run 100 MHz constantly, you will receive a signal, but it will always be on ("1" or high). To send information, you need to modulate, or change, that carrier relative to the information you want to send.

There are many (and complex) methods of modulation. For now, we'll take the Morse-code with a flashlight example. When the light is off (carrier is off), you receive no data. When the light is on for a short time, you get a "dot". When the light is on for a longer time, you get a "dah" or dash. So, the example everyone knows is SOS: ...---... (three short bursts, three long bursts, three short bursts). That would be an example of on-off keying, or OOK modulation. On the old telegraph systems, when the operator tapped the "key", the guy on the other end of the wire would hear a tone. Hold the key down for a short time, you hear a short tone... "dot". Hold the key a long time, get a long tone... "dah" or a dash. Now you could send any letter in the alphabet using that encoding/modulation method. You can do the same thing using an RF carrier. Assume you have a transmitter that runs at 100 MHz. If you key it on, it transmits. When you key it off, it's silent. Using the key, you could now send radio "pulses" through the airwaves, instead of electrical pulses down a wire, to send your Morse-encoded data.

Things get really complex from here, but that is a fundamental basis for sending signals. You need a medium (air or a wire, in our examples), a carrier (light waves, electricity, or a radio frequency), and a way to modulate that carrier signal (a key or switch).

The comment about attenuation increasing with frequency is related to transmission in air, as opposed to freespace (a vacuum). Since air contains water droplets, higher frequencies are more easily absorbed by the suspended water, thus allowing less energy to reach the destination. In freespace there is nothing to absorb the signal (its a vacuum), so it doesn't attenuate more or less versus frequency... it doesn't attenuate at all due to the medium, only because of the energy spreading out as it travels further away from the source.
 


Thanks for replying for me ... been away from the puter for a couple of days so didnt get a chance to respond. You clarified the comments very well Hopefully the OP understands now
The prob is that the OP was referring to freespace but most likely was transmitting within the atmosphere rather than a vacuum so was mixing things up

Dave
 

I had preferred that none even had mention frequency, air or droplets as it rather quick becomes complex with low practical value at this entrance level.
For normal communication frequencies below 5 GHz is attenuation in dry air less then 0.01 dB/km, and in water vapour even less then in air. Yes that is correct, pure water in general is a lower loss medium then air (Nitrogen+Oxygen) both as the gas water vapour and as fluid in droplets.

Why do then frequency be of any interest at all in these propagation prediction models such as Hata-Okumura?

In non free space, such as cities, forest, indoor.. is RF attenuation increased due to a lot of factors, a few:
Non optical sight -> The signal that can reach a receiver antenna must penetrate partly absorbing material or by reflections travels an alternative and longer way.
Interference -> Several direct or reflection signals either cooperate at a given point, or cancels out each other.
Even on a very big field, and maybe optical sight between antennas, must still compensating be done due to ground reflections which interfere with direct wave.

Standard formulas for radio propagation in different types of complex environments exist for which frequency is a parameter, mainly because of that for any size of disturbing object in the environment (house car trees windows people), it is more likely that it is big enough to absorb short wave lengths or effectively reflect them while longer waves can pass through almost unaffected or only gets partly reflected. Rayleigh effect, scattering, depolarization, MIMO, relative speed between antennas (mobile unit) and such can also be needed to be taken in account depending on type of communication and antenna type.
As these formulas assumes a specific environment are they also only accepted as "good enough", for a specific frequency range.
Most famous is the Hata-Okumura model which you must have some knowledge to be able to use correct, as terrain profile, reflections and shadowing not really are considered and TX antenna is assumed to be placed at some height. To compensate for these problem exist other models. This model is also limited to only be used within 150 to 1500 MHz and 1-20 km.
 

I think you should use geometric abstraction level for radiating point in free space. All of your transmitted power (10 Wt) will be spreaded on the area of the sphere with R=1000 meters (S=4*pi*r^2). Receiving antenna will collect power density on it's effective area. So power density is proportional to r^2.
 
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