Adding noise given by Noise Factor

<makolber@yahoo.com> wrote:

> spp wrote,

>> When I visualize this, it seems it violates the antenna 
>> recipricocity theorem as it would suggest driving the antenna
>> with a given power would create twice that power in the EM
>> field.  Something it not right with the idea, and I'd like
>> to see more of a description, perhaps a literature reference.

>that was my statement. (not Peters)

Oops.  Sorry, Peter.

>I spent a little time doing a literature search but didn't find anything
>worth quoting..

>but I'll give you these thoughts....

>every conductor with accelerated charges (which I will now refer to as current) 
>radiates a field.

>If you place an antenna in an existing field, there will be a current
>induced into the conductors.

>The current distribution will be changed depending upon how the antenna
>terminals are terminated.  The three corner cases are open circuit,
>short circuit and matched.  Since the current distribution changes, the
>re-radiated field changes.  You can think of it in terms of conservation
>of energy as well, the power delivered to the LNA must be taken from the
>EM field and hence the field is changed by the presence of the loaded
>antenna.  And the EM field is  changed differently if the antenna is
>shorted or open instead or matched.

>Another thought is the operation of a Yagi antenna.  The reflector and
>directors obviously re-radiate a field in that case.  The spacing and
>length of these are adjusted such that the rre-radiated field
>re-enforces the field at the "driven element".

>Due to conservation of energy (ignoring losses) a shorted or open
>antenna must re-radiate ALL the energy it absorbs.    A matched antenna
>delivers some power to the load, therefore the amount re-radiated must
>be less. 

Hmm.

Last I checked, light is EM radiation.

So let's say you have a 200" telesope mirror pointed in the direction
of a light source and an optically back object in its focal plane.  I 
cannot see how any significant fraction of the power incident on the 
mirror is "re-radiated".

When you get down to something like a dipole, there is significant
re-radiation going on by the mechanisms you describe, but these
factors are already taken into account when stating the antenna gain in 
dBi.  Thus a 0 dBi antenna has a cross-sectional area of (IIRC) 
0.06 * (lambda)^2 .  The receiver designer (in the far-field case)
can correctly assume that the RF flux incident on a region of this area 
is 100% coupled into the receiver front end, assuming no mismatch,
resistive losses, or similar inefficiencies.  

So in the typical case the receiver designer does not need to worry
about these re-radiation effects, and they are not part of what
needs to be modeled in this thread.  

Steve