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> Antenna Gain and Directivity., What is the difference between the two?
Lord Loh
  Posted: January 06, 2006 12:25 pm
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What is the difference between the two antenna parameters - Gain and Directivity

Which is practical (not considering the ideal case of isotropic radiators)?


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Geek
Posted: January 06, 2006 01:39 pm
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Gain and directivity are related - you cannot have directivity without gain and vice versa.

The main difference lies in polarization. A dipole has gain, only because it's directive off its sides. I has a toroidal pattern in free space. If it's vertical, it'll be omnidirectional, therefore having no directivity in the plane of use. If it's horizontal, it'll fire off the back and sides in a figure-8 patter, having the same gain and it'll have directivity.


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Nothing40
Posted: January 06, 2006 04:43 pm
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Imagine maybe a flashlight,with a focusable beam.
If you adjust the beam so that it's wide,and covers a larger area,it's not as bright,but if you focus the beam into a tight,narrow path,it's brighter along that path,but it doesn't throw light off to the sides anymore,it's more directional..

It's like concentrating your RF signal.
If gain goes up,directivity goes up also,and vice-versa..


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joejester
Posted: January 06, 2006 07:56 pm
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The antenna gain is the comparison of the antenna with the reference antenna -- a dipole.

The directivity is the focusing of the radiated energy in a pattern other than omni-directional. Naturally focusing all the power in a direction other than omni-directional will increase the received energy at the focus points ... and the distance at which you can receive the energy.

You can have increased energy [compared to a simple dipole] in other omni-directional antennae. 5/8th wavelength is a higher gain than a simple dipole.



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Lord Loh
Posted: January 07, 2006 05:48 pm
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So I make my antenna directive by adding a parabolic reflector... I am now concentrating all my power in one direction.

Could I not compare this with a dipole and call it gain?

As Nothing40 said:
If gain goes up,directivity goes up also,and vice-versa..

So what is the difference?

In my text book, the gain is calculated by taking the radiation pattern of a test antenna against a dipole. (I think it is normalized against the dipole)

Directivity equations add one extra term... eta. the efficiency.

Then, a few paragraphs later, they call the gain a more practical quantity than the directivity as accounts for the efficiency.... I am confused...


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joejester
Posted: January 08, 2006 12:43 am
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QUOTE
So I make my antenna directive by adding a parabolic reflector... I am now concentrating all my power in one direction.

Could I not compare this with a dipole and call it gain?


Yes you could call it gain.

QUOTE
As Nothing40 said:
If gain goes up,directivity goes up also,and vice-versa..

So what is the difference?


For Nothing40's statement to be absolutely true, it has to be the only way to get more gain from antenna is to have a directional antenna. There are omni-directional antenna's that have gain when compaired to the standard antenna.

A 5/8ths wavelength is +3 dB compared to the standard antenna. It is an omni-directional antenna, as is the standard antenna. We did not add directivity to realize the gain.


Visit http://www.phy.davidson.edu/instrumentation/NEETS.htm and download module 10 - - it discusses antenna's and should provide you with the answers you seek.
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Nothing40
Posted: January 08, 2006 07:18 am
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Sorry,I was half asleep when I posted....(kinda sleepy now,too)

A 5/8wave antenna would have more gain than a 1/4wave..but even that is "focused",it's just in a different plane. It "squishes" the signal down vertically,so the radiation pattern would look kinda like a donut around the antenna,instead of more like a sphere,for the 1/4wave. (sorta generalised.)
It trades some vertical signal (where it's just getting sent off straight into the sky) for more horizontal signal.

The 5/8wave antenna creates "gain" by having a different radiation pattern,still omni directional,but it looks different in the vertical plane.

Hrm,can't seem to find any examples on Google with a quick search..


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Posted: January 08, 2006 10:42 am
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^^--- this is actually what I wanted to say when my train of thought derailed ---^^

laugh.gif


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Nothing40
Posted: January 08, 2006 07:07 pm
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Heheheh laugh.gif


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joejester
Posted: January 08, 2006 09:50 pm
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If my memory serves me correctly, and if it don't then I'm suffering from CRS ... Cant Remember $hit ... there are illustrations in the book I posted in the link.


There are methods to squish the signal from the typical donut ... from adding top loading elements, kind of like that 1/8th wavelength in the 5/8ths wavelength antenna. Typically TLE's are for very short antennaes. I've seen them on 625 and 700 foot monopoles in the VLF band.'

For the life of me, I don't know why I typed dipole, when I meant the 1/4 wavelength monopole as the standard antenna. It must be an attack of the CRS disease.

This post has been edited by joejester on January 08, 2006 09:53 pm
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