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6 comments
Participant

Join Date: Oct 2008
Posts: 3

Frequencies, Penetration and Coverage

11/09/2008 5:16 AM

I am confused as how frequency, penetration and coverage distanace are related. I hear people saying signals with lower frequency can cover a greater distance than higher frequencies and also these signals have higher penetration capacity(pass through objects) becuase they have a greater wavelength.

On the other hand signals with extremly high frequency(in Tera hertz) like X-ray are used in medical world because they can pass through our body(I suppose).

Can anyone explain this to me clearly. I apprecaite if you recommend me a reading for detailed explanation on waves.

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Associate

Join Date: Jan 2008
Location: Bangalore , India
Posts: 48
#1

Re: Relation between frequency, penetration and coverage

11/09/2008 11:20 AM

beautiful question ,,, even i like to have a review on this if anyone can help !

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Active Contributor

Join Date: Jul 2008
Location: Finland
Posts: 22
#2
In reply to #1

Re: Relation between frequency, penetration and coverage

11/09/2008 11:28 AM

Best way to start is google and wikipedia.

http://en.wikipedia.org/wiki/Electromagnetic_spectrum

hmm. linking doesn't work. just copy paste...

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Commentator

Join Date: Apr 2007
Posts: 85
Good Answers: 11
#3

Re: Relation between frequency, penetration and coverage

11/09/2008 12:18 PM

Hello Jammo.

Good question. Mind if I talk about just the RF part?

In conductors the penetration depth decreases with increasing frequency. More specifically, the penetration depth has to do with the rate-of-change of currents induced in the conductor by the RF field. All conductors have inductance which tends to resist a change in current. I'm not speaking here of the inductance as a 'lumped' parameter characterising the surface as a whole, but of the distributed, volumetric inductance of the material. The faster the current's rate-of-change, the more the intrinsic inductance resists the change and the shallower the RF penetration. The phenomenon even has its own name: skin effect. Google Skin Effect to learn more.

As an aside, electric current tends to flow on the outside of a conductor -- even at zero frequency (DC). It doesn't flow exclusively on the outside, of course, but it prefers this configuration because like-charged particles (in this case, electrons) repel one another and migrate to the surface, which is as far as they can go. In the static case, once the system reaches equilibrium there are no electric fields -- and hence no current flow -- in the interior of a conductor. In the non-static case the system never reaches equilibrium and so we have electric fields -- and therefore currents -- on the inside of the conductor. It's a contest between the volumetric inductance and resistivity (volumetric resistance) of the material. Google Faraday Cage to learn more.

For superconductors there are neither electric nor magnetic fields in the material's interior. Absolutely none. Superconductors are a whole new can of tasty, fascinating worms, but if you're interested, google Meisner Effect.

An interesting question you may also wish to explore is this: "Why does RF penetrate poor conductors more readily than good conductors?" For example: graphite versus silver. Understanding why this is so will greatly extend your overall understanding of the subject.

One of my Jr. High science teachers taught me an invaluable lesson that I've never forgotten: "More valuable even than knowing something is knowing where to look."

Kind regards,

TV

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Guru
United Kingdom - Member - Not a new member!

Join Date: Jun 2008
Location: USA/Europe
Posts: 4632
Good Answers: 77
#6
In reply to #3

Re: Relation between frequency, penetration and coverage

11/11/2008 10:47 PM

Hello TheVoices:

That was a pretty good description from the internal radio apparatus to the end of the antenna.

Now explain as expertly, how far a long wave will travel, and how far a very short wave signal will travel. I think this was the gist of the question. No insult intended, OK?

I think it may also help the original questioner, and others if you could explain the importance of 'tuned' antenna's, and the importance of using the correct antenna to receive a particular frequency?

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Guru
Engineering Fields - Control Engineering - New Member China - Member - New Member

Join Date: Sep 2006
Location: CHINA
Posts: 2970
Good Answers: 14
#4

Re: Relation between frequency, penetration and coverage

11/10/2008 6:49 AM

I can anwer your question.

This will concern different field in physics. in acoustic, as lower frequency wave has longer wave length, it can produce diffraction, that is when it meets smalll size object which has smaller size than its and then the wave can go round it and propagonda forward or produce new sub wave to radation out. wheras the higher frequency has smaller wave length, so it can be reflected by loarger size object, this is well known Hygens principle. thats why the lower frqucy signal can cover large area, but higher frequcy has more direction.

the energy of wave has a positive ratio to its frequency, so higher frq has more engery then lower freq.

as x-ray, it has the more short wave it will has more enegy to penetrate the thick object.

we cannt simply say higher freq has hger penetrate capacity around our live space. for example, higher freq, sound wave transmit shorter distance than lower freq.

becasue air hsa more strong absorb capability.

you maybe a still a middle school student, try to learn more physics in the class. you will this.

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Guru
Engineering Fields - Control Engineering - New Member China - Member - New Member

Join Date: Sep 2006
Location: CHINA
Posts: 2970
Good Answers: 14
#5
In reply to #4

Re: Relation between frequency, penetration and coverage

11/10/2008 6:54 AM

#3 give you another simple that sometimes higher freq signal cannt penetrate more depth than lower freq. current.

not bad.

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