Unity Gain antenna

[Icom 706 King Cobra]

hello all..



i am always looking at different antennas..

i have likly bought too many of them..lol..



anyway i am seeing many antennas that are listed as a Unity Gain..

Could someone explain to me the benifits or lack of benifits of such an antenna..

i see these antennas for anywhere from 30 mhz to 900 mhz..



i do talk on many different frequencys..

yet i am unsure as to what exactly unity gain is..



seems to me these are zero gain antennas..

so is the benifit of these antennas broad banded swr range..

or is there some other benifits to be had verses a say high gain antenna..



Thanks




</p>

 

[freecell]

"A 'unity gain' antenna is one that has a gain of '1'. Meaning that the power radiated by the antenna is the equivalent of '1' times whatever the input power is. It's symantics more than anything, gotta know how to 'read' it."....



an antenna that is said to have a gain of unity over a particular reference antenna has a gain of exactly 0. Unity Gain = 0db., not 1db.. . let's not drag transmitter power into this and muddy it up. the expression of that value is unnecessary here in determining what's taking place in the antennas (gain or no gain) and is not relevant. it is usually assumed that tx power into both antennas under test is identical when comparisons are being made. the same antenna can be said to have gain if the comparative reference is changed, let's say to the isotropic source.



if the reference antenna above is a halfwave dipole and we change it to the isotropic source instead, now our antenna can be said to have approximately 2.14 dbi., or 2.14 db . of gain when compared to the isotropic source.



one of the ways that a particular design is able to provide gain is by compression of the wavefront envelope. keep in mind here that the isotropic source is a sphere which radiates a uniform signal in ALL directions all at the same time. naturally when we move to the earthly aspect here it's clear to see that when compared to the isotropic source the "bottom" of the generated wavefront is compressed upward by the presence of the earth underneath, in stark contrast to our isotropic model. the next place one would want to compress the pattern further is on the top. acting in the same manner as the upward compression, eliminating radiated energy at high angles and redirecting it at angles closer to the horizon yields still more overall gain. compressing the top of the generated wavefront downward and eliminating large amounts of skyward wasted radiated energy while re-directing it into lower radiation angles is one definition of antenna gain. this is but one way antenna gain is achieved. the law of the conservation of energy is not violated here, which says that energy is neither created or destroyed. it is merely re-distributed.



here is an image which details this wavefront compression by comparing a 1/4 wave monopole against a vertically polarized 1/2 dipole both over earth ground.



www.firecommunications.co...ntenna.gif



notice that the volume of the two wavefront envelopes are identical and notice how the downward compression present in the 1/2 wave extends the length of the lobe at the horizon while eliminating wasted power at the upward angles.


</p>Edited by: <A HREF=http://p067.ezboard.com/bworldwidecbradioclub.showUserPublicProfile?gid=freecell>freecell</A> at: 5/29/04 2:39 pm

 

[freecell]

"but what professional antenna builder would advertise his antenna with the term "Unity" in the vain in which I hear this discussion going. Who would say my antnnena has 0 Gain, ZERO Gain?"



i'm not talking about antenna advertising claims or using the term Unity in them. that's not what i'm advocating here.



"Who would advertise their antenna has isotropic qualities."



there is no commercially built antenna with the characteristics of the isotropic source....it can't be built. so the answer would be no one. has nothing to do with what i'm talking about.



you can go with whoever you want, it doesn't matter. the definition given above in my previous post is the one used by the industry. (not the advertising industry) that you disagree with it doesn't matter. the definition is given to quantify certain relationships between antenna systems.



*An antenna exhibiting 0db. of gain when compared to a standard reference such as the vertical 1/2 wave dipole (*sometimes called a "unity" gain antenna), is an antenna with no gain. the only antenna in practice that fits this description is the isotropic source. it exists only in theory because the feedline cannot be connected to the center of the isotropic sphere without interrupting the surface, causing an imperfection in the radiating surface. this is the only thing confining it to theory and keeping it from the practical. nevertheless, its radiation characteristics represent a known value.



since the 1/2 wave vertical dipole is the first practical and basic design that exhibits any gain when compared to the isotropic source it is commonly used as a reference for determining the gain of larger antennas, especially in terms of wavelengths, if only because it can actually be constructed and put into operation.



another example would be 2 different directional designs that exhibit the same amount of forward gain, whatever the figure. when the 2 antennas are compared against each other it could be said that either of the antennas has 0db. of gain or gain = unity when compared amongst themselves. it doesn't say unity ratio, it says unity gain. gain is not expressed as a ratio. gain is expressed usually as a whole number and/or a whole number and some fraction in decimal form, not as a ratio.



understanding this is simply a starting place so that we are able to quantify gain figures of differing designs and establish a basis for comparison.



with every one arguing about the method of measurement it's no wonder the industry could never settle on one of the two standards.



the isotropic antenna is the only real 0 db. gain antenna. it's the starting point for measuring the performance of every antenna ever built. these are merely established references.


</p>Edited by: <A HREF=http://p067.ezboard.com/bworldwidecbradioclub.showUserPublicProfile?gid=freecell>freecell</A> at: 5/29/04 8:17 pm

 

[freecell]

"yet i am unsure as to what exactly unity gain is."



you're right marconi, he made a statement. here's the answer assuming we re-phrase it as a question.



what is unity gain? (a unity gain antenna)



unity gain = 0db.. an antenna that produces no gain when compared to the reference antenna as specified by the manufacturer. the two most widely used reference antennas when quoting antenna gain is dbi. (db of gain referenced to the isotropic source) or dbd. (db of gain referenced to the 1/2 wave dipole)



the use of the term "unity" in reference to establishing gain figures for antenna advertising will only serve to make things more confusing, as is already evidenced. for the sake of simplicity i would interpret it to be the isotropic as it allows the manufacturers/advertisers to inflate the figure by another 2.14 db., making the figure appear that much more impressive. even though the isotropic antenna is unable to actually be constructed, it is the only true unity reference, if only because of its ability to radiate all energy in all directions. since it is a sphere and can only exhibit true isotropic properties in free space, unimpeded by the influence of pattern distortion that would effect it when operating over a ground mass, it literally represents a total absence of any wavefront compression whatsoever, the logical starting place for the design of any antenna capable of producing measurable gain whether it be by wavefront compression, multiple elements excited in-phase and/or parasitically energized elements as those used in the typical yagi or quad designs or complex phased multi-element arrays.



here is an image/radiation pattern of an antenna having unity gain.



www.firecommunications.com/unity.gif



here is the same antenna with various degrees of wavefront compression superimposed to demonstrate one method of increasing the gain of a monopole antenna.



www.firecommunications.com/antenna.gif




</p>Edited by: <A HREF=http://p067.ezboard.com/bworldwidecbradioclub.showUserPublicProfile?gid=freecell>freecell</A> at: 5/29/04 11:38 pm

 

[freecell]

"if Unity applies to gain, and it is as you state, why would anybody advertise this quality for their antenna."



because it's the truth. refreshing, isn't it?



If you work in the city with a lot of high rise buildings, a unity gain antenna with its round pattern may help. Lots of times your repeaters are on the tops of buildings, or you can reflect power off tall buildings to get better signal coverage. But if you're in a rural, mostly flat, area and need long haul, I can't think of many reasons why you wouldn't want to consider a high gain antenna with its flat pattern out toward the horizon.



the gain resulting from wavefront compression that i mentioned in previous posts may not always provide the necessary coverage in all types of terrain, which i alluded to earlier. a unity gain antenna and the almost spherical pattern generated comes in quite handy in sprawling urban areas because of the large amount of signal radiated at the higher angles.



remember the two image links in my last post?



so the reason why someone would want to use the term in advertising is because for those in the know a unity gain antenna has some distinct advantages over the gain types in urban settings. it's a selling point.


</p>Edited by: <A HREF=http://p067.ezboard.com/bworldwidecbradioclub.showUserPublicProfile?gid=freecell>freecell</A> at: 5/30/04 10:42 am