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Wayne · October 14th 14, 05:28 PM posted to rec.radio.amateur.antenna

"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.

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