Electromagnetic Wave Propagation/Antenna - Help Required.

Answered Question
Jun 11th, 2010
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Hi all,

I have some fundemental issues with understanding the how waves are propagated.


Please see the attached file for clarification and some digram examples.

I undertsand the following concepts:

1. The hertz (symbol: Hz) is the SI unit of frequency defined as the number of cycles per second
2. We also know that ll electromagnetic radiation, from radio waves to x-rays, travel at the speed of light. In empty space this speed is approximately 300,000,000 meters per second!
3. Wavelength = speed of light/frequency


So for the examples above we can say that:

300,000,000 / 3hz = 100,000,000 meters
300,000,000 / 4hz = 75,000,000 meters
300,000,000 / 5hz = 60,000,000 meters
300,000,000 / 7hz = 42,857,142.8571....... meters
300,000,000 / 14hz = 21,428,571.4285....... meters

Radio 1 in the UK (99.8 Mhz)
300,000,000 / 99600000 (99.8Mhz) = 3.0120481………… meters
300,000,000 / 2412000000 (2.412 GHz) = 0.1243781……… meters
300,000,000 / 5180000000 (5.180 GHz) = 0.057915…… meters


4. A Wavelength in the 2.4Ghz Frequency is 12.43 centimeters AS IT ENTERS THE ATMOSPHERE FROM THE ANTENNA
Is the above in fact correct?



Question.  Please see statement 5.

So I at Distance 1. Lets say this each distance interval is 1 meter apart, the radio wavelength is 12.5cm
At Distance 2, the the wavelength bigger and then bigger at D3 and D4 (pls see attached)


Also, Please can you explain statement 6 in the document.


I think I am missing a basic radio fundemental point.

Many thx indeed,

Ken

Correct Answer by abersven about 6 years 11 months ago

You statements 1 to 4 are true. When it comes to statement 5 and 6 the answer is that the wavelength remains the same regardless of the distance from the transmitting antenna.


Let’s say we have an access point transmitting on channel 6 which equals to the frequency 2.437GHz. And as you write in your statement 3 the wavelength equals to the speed of light that is 299792458 meters per second divided on the frequency which equals 12.3 centimeters.


When this signal arrives at the receiver that is listening to channel 6 that is on frequency 2.437GHz the same rules applies in this end. The wavelength is the speed of light divided on the frequency. And that is still 12.3 centimeters.


So to summarize the answer: the wavelength remains the same regardless of distance from the antenna. Both laptop 1 and 2 in you statement 6 is receiving the same signal with the same wavelength.

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kfarrington Fri, 06/11/2010 - 07:03
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ps. I have been reading back from last year the post on some of this subject as well.  Thanks to Jeff for all the Input.


https://supportforums.cisco.com/message/1316185#1316185


I guess what I am say is that if a wavelength for a 2.4 Ghz frequency is 12.5cm,  does the wavelength get bigger as it travels.  If so, is there an equation (like the inverse square law) for the redunction of the power in a wavelength as it travels.


It is a very tricky subject so I do apologise in advance if it was covered in the last post and I am just missing the point.



Kind regards,

Ken

abersven Sat, 06/12/2010 - 18:07
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The signal gets weaker when we move away from the transmitting antenna. There are a lot of things you must consider when you want to calculate the loss, and it’s involving huge mathematical models to do this right. So I will not go in to details but give you the model for a free space loss. In real life there is no free space since we must consider antenna gain, height above ground, how the ground is formed and what it consists of. And what is air? There are all kinds of stuff in the air that will result in loss.



Free Space Loss
Free space loss = 32.4 + 20xLog F(MHz) + 20xLog R(Km)
F is the RF frequency expressed in MHz.
R is the distance between the transmitting and receiving antennas.
At 2.4 Ghz, this formula is: 100+20xLog R(Km)

Correct Answer
abersven Sat, 06/12/2010 - 17:53
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You statements 1 to 4 are true. When it comes to statement 5 and 6 the answer is that the wavelength remains the same regardless of the distance from the transmitting antenna.


Let’s say we have an access point transmitting on channel 6 which equals to the frequency 2.437GHz. And as you write in your statement 3 the wavelength equals to the speed of light that is 299792458 meters per second divided on the frequency which equals 12.3 centimeters.


When this signal arrives at the receiver that is listening to channel 6 that is on frequency 2.437GHz the same rules applies in this end. The wavelength is the speed of light divided on the frequency. And that is still 12.3 centimeters.


So to summarize the answer: the wavelength remains the same regardless of distance from the antenna. Both laptop 1 and 2 in you statement 6 is receiving the same signal with the same wavelength.

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