why can't we have an isotropic radiator?

[armanio]

Hi All,

I need to know :why can't we have an isotropic radiator? I have a logical answer for it (at least for me) which is :
we can't have a spherical radiator and if we do , then we will not be able to feed it.

But my professor, told me that it is related to what is called "Fuzzy Orange" and I don't know what is fuzzy orrange.

Can any one help me with it?

Thanx in Advance
ARMANIO

 

[flatulent]

What is really meant is that there cannot be a linearly polarized field that is isotropic. The turnstile antenna field is linear at the edges and circular at the apex and elliptical in between.


As a thought experiment, think of an antenna that is vertically polarized at the equator. As you go to the pole the vertical field from all of the edges meet and are out of phase making a null. Any type of linear polarization will end up with a null somewhere.

 

[sergio mariotti]

It's well know that is impossible to build an isotropic radiator.
The reason mainly due:
1) to the fact that the feed line broke the simmetry
2) antennas usually are sensitite to only 1/2 of the radiation (i.e. H or V poarizzation or RH or LH polarizzation...)

But try to think to an Heated (or cooled) Eccosorb sphere. It'is an almost perfect blackbody isotropic radiator.
Of course you must choose properly the dimension (mutch times greather than lamda) and the type of Eccosorb (i.e. for millimetric waves use CR110).
Of course the nature of the signal emerging from that type of radiator is "white gaussian noise" so don't hope to see a CW signal !

Do you want an example in nature? Think the Sun.

But if you ask me what is the pratical use insteaded the use of a standard reference antenna, my answer is that i still prefer the standard reference antenna.

 

[dowjones]

hello,

It is a quite interesting question and I think that, with some remarks, there is no reason why an isotropic antenna cannot be made.

remark 1: The IEEE says that an isotropic radiator must be lossless. this is impossible

remark 2: One polarisation must be selected (it is not possible to make an antenna sensitive to all polarisations -linear, horizontal, circular Rh +Lh-.

remark 3: don't think of a single antenna (it should lead to a point, which is not possible), think of a 3D array

therefore the design could be a 3D array, sensitive to one polarization and with some loss, but with a nearly-spherical radiation pattern.

if we consider a cube with one antenna on each facet (I think on patch antennas with directivity of about 6dB), and with a power divider 1:6 feeding all the units, the pattern should be nearly-spherical, sensitive to a field component (theta for instance). The pattern will have ripples, due to the interaction between closer elements patterns, but even with that the pattern should resemble a sphere.

regards

 

[Ricky761]

My idea for an isotropic radiator is feeding at least three radiator in an equal distance and in co-phase and in the height corresponding to its lambda ( wavelenght ).