"rickman" wrote in message ...

On 10/13/2014 1:36 PM, wrote:
gareth wrote:
Quoting from Electromagnetism
By F.N.H.Robinson
in the Oxford Physics Series
1973 edition
ISBN 0 19 8518913
Chapter 11, Radiation,
page 102
Formula 11.11

Has in the equation for radiated power the term

(2*PI*L/LAMBDA)**2

where L is the antenna length and LAMBDA the wavelength,
thereby showing that the radiated power decreases when the
antenna length decreases.

I will read up further and report further...

You do that and while you are at it take note of the fact that the
expression you give is unitless and can not be power.

You will also find that the total power radiated by an antenna is the
surface integral of the average Poynting vector over a surface enclosing
the antenna. The surface usually chosen is a sphere in the far field to
keep the equations "simple".

# He is taking a portion of the equation and presenting it out of context
# assuming that this is a valid way to consider what he wishes to show. I
# would like to see the full equation. The devil is in the details.

# I remember once when I was looking at a link budget and an equation I
# was presented with contained a relationship with the distance which was
# not a square. I questioned the source of the equation meaning how it
# was derived. The person who gave it to me brought me the book and said
# it was by one of the authorities in the field. lol I'm sure the guy
# was an expert, but I wanted to know why the power didn't drop off with
# the distance. I expect this was an equation that was empirical as the
# context was over ground distance including likely obstructions and many
# factors changed the formula from the free space model.

Yes, that can be a tricky subject. If an isotropic radiator is assumed,
then the surface area of the surrounding sphere will vary with the square of
the radius. That produces less power per square unit as distance increases.
So, path loss from the squared term depends on the radiation spreading out
over distance.

There is no loss due to distance itself, but to the radiation spreading.

Example: for a lossless microwave dish, if all the radiation transmitted
arrives at the receiving dish, then there is no path loss.

"Wayne" wrote in :

There is no loss due to distance itself, but to the radiation spreading.


I'm wondering now if what Gareth is concerned with is the same as divergence
in an asperic lens output with a laser diode. Assuming the diode has an
emitter width of a very few microns (is already usually only one micron in
one axis even in a multimode diode with a single 'stripe' emitter pattern)
then a large enough single asperic lens will make a finely directed but broad
beam, but if you want it very narrow as well, it diverges more widely and
various optic methods will tame it a bit, but there's no real substitute for
a single mode diode if possible to use one for the wanted power.

Assuming it is NOT possible, the multimode diode needed will demand a bigger
lens to match its power efficiently into a well directed, 'collimated' beam.
It seems to me that this is more than just an analogy, but maybe fundmentally
similar to the difficulties with energy density, accuracy of form, low loss
of materials used, aperture size for emission, and maybe several other things
I've seen mentioned recently about this subject of small antennas. Including
the fact that eben if the laser beam IS highly divergent, the small aspheric
lens is just as efficnt at prjecting the power as the larger one, so long as
all light from the diode gets coupled through it without spill or reflection.

I hope that's not too off-topic, but it seems to be that I might get some
learning from responses to this one...

"Lostgallifreyan" wrote in message
. ..

"Wayne" wrote in :

There is no loss due to distance itself, but to the radiation spreading.


I'm wondering now if what Gareth is concerned with is the same as divergence
in an asperic lens output with a laser diode. Assuming the diode has an
emitter width of a very few microns (is already usually only one micron in
one axis even in a multimode diode with a single 'stripe' emitter pattern)
then a large enough single asperic lens will make a finely directed but
broad
beam, but if you want it very narrow as well, it diverges more widely and
various optic methods will tame it a bit, but there's no real substitute for
a single mode diode if possible to use one for the wanted power.

Assuming it is NOT possible, the multimode diode needed will demand a bigger
lens to match its power efficiently into a well directed, 'collimated' beam.
It seems to me that this is more than just an analogy, but maybe
fundmentally
similar to the difficulties with energy density, accuracy of form, low loss
of materials used, aperture size for emission, and maybe several other
things
I've seen mentioned recently about this subject of small antennas. Including
the fact that eben if the laser beam IS highly divergent, the small aspheric
lens is just as efficnt at prjecting the power as the larger one, so long as
all light from the diode gets coupled through it without spill or
reflection.

I hope that's not too off-topic, but it seems to be that I might get some
learning from responses to this one...
############

Well, it may be slightly off topic and certainly out of my field of
experience, but I find it more interesting than Gareth's misused equations

"Wayne" wrote in :

Well, it may be slightly off topic and certainly out of my field of
experience, but I find it more interesting than Gareth's misused equations



Thanks. I do try. I figure even if I am imprecise, I can either try to
entertain or at least come at it from an angle that might be useful, perhaps
not just to me.