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(Hardeep Singh) #1

Hi,

Today I am going to start a new category here on Member’s Area: Wireless Physics.

Here I’ll be sharing little fact about the wireless from a perspective of physics and will explain why, how and what of that.

You can always see all the posts under the tag #wirelessphysics101 or use this URL: https://members.rootsh3ll.com/tags/wirelessphysics101 to get a list of all the posts under this series.

I’d also need your participation in the same to keep the community active and to keep an active flow of information going.

So here is my first share

Length of an Antenna is relative to the Wave Length

I’d love to see who can explain this for the love of InfoSec and Physics

Let’s start sharing and explaining some facts and help people build a stronger ground in the field of wireless and WiFi security.

Let’s express ourselves in the best way possible, be it a video, audio or text/images/gif whatever. Let’s spread the colours, let’s learn together :slight_smile:


(-) #5

I’m among other things amateur radio hobbyist, with HAREC licence.

Antenna is like a loudspeaker for electromagnetic fields, or like a guitar string that radiates electromagnetic energy.
Like with speakers and strings the size/length affects the frequency response, big speakers work better with lower frequencies, and longer the string lower the frequency and vice versa.

Wavelength is inversely tied to the frequency and the speed of the propagation, for sound in air that is ˜330 m/s, but for electromagnetic radiation the speed is speed of light, namely 300 000 km/s.

L = c/f where L is wavelength, c is speed of light and f is frequency

Now I can finally get to the actual antennas let’s take WiFi as an example frequency is 2.4 GHz so
L = c/2.4*10ˆ9 = 0.125 m
This means that optimal antenna length would be 12.5 cm, but also half and quarter wavelength antennas work, 6.25 cm and 3.125 cm respectively.
That is why 5GHz is the other WiFi frequency, because same antennas can be used for both.

Antennas are better at receiving emissions close to their resonant frequency, just like wineglass can be screamed to cracking.

This piece has less physics and more analogy, but I thought it’s better to explain briefly how the problem can be approached with analogies, rather than going to Maxwell-equations about electromagnetism. If there’s any questions, or wish for elaborations for some parts, I can provide it.

Links for further reading:
https://www.everythingrf.com/rf-calculators/frequency-to-wavelength
https://www.cv.nrao.edu/course/astr534/AntennaTheory.html
https://www.explainthatstuff.com/antennas.html

73 sipa


(Hardeep Singh) #6

Spot on @sipa!

For our readers,
The logic used here doesn’t specifically apply to WiFi (2.4 or 5 GHz ), but the complete Wave Spectrum. From Radio waves to the Gamma Waves, It’ll apply to every single point of intersection.

So, using the same example here’s my analogy on antenna length for radio waves at 100 MHz:

Let’s say you have a base station transmitting at 100 MHz (read, Mega/Million per second) and speed of light is close to 300 Million m/s. Then applying both the values in L = c/f or c/L = f, we get

300,000,000 (m/s) / 100,000,000 (/s) = 3 m

Which means the a single cycle of the sine wave (Wave Length) is almost 10 feet long in the air. See full chart here

So to transmit signals for a 100 MHz of radio frequency, the antenna should be 10 feet long for optimal signal transmission!

Here’s a related one for you @sipa :

Antenna’s are capable of both transmission and reception. and since antenna length is relative to the wave length, then why receiver side antenna (Say mobile phone’s AM/FM antenna) are much smaller considering the 10 feet of wavelength and smaller chassis of a mobile.
Yes, earphone does work as an extender antenna, but still cable is not 10 feet long.

Can you explain this to our readers? :slight_smile:


(-) #7

Mobile phones can use headphone cable as AM/FM antenna, that’s one solution. 1/4 wavelength of 100MHz is 3/4 m, this is easily within length of headphone cabled. There is also other antenna geometries than simple whip-antenna. Also the TXpower of FM broadcast stations is in range of 10-60kW, which is very high.


(Hardeep Singh) #8

It’s fascinating to see EM waves starting with radio waves at very high power tx of upto 60KW
then somewhere in between lies our WiFi at upto 2W then at the very end we see brains of olympic level meditators transmitting gamma rays.
Such a wide variety and still everything is simply an ElectroMagnetic wave.


(-) #9

We also have 1kW wifi transmitters, they’re called microwave ovens. Wifi is limited to 20dBm = 100mW in most places, to decrease interference, because there is only 11 channels on that 2.4GHz band.

Gamma-ray sending meditator sounds very dubious.
do you mean gammawaves?


(Hardeep Singh) #10

Right, it’s gamma waves, I confused that with gamma rays.
Although acc. to article the only similarity between the 2 is their high frequency with low amp.

Here’s the video I was referring to :