On 9/11/2015 1:10 PM, gareth wrote:
"Wayne" wrote in message
...
"gareth" wrote in message ... "Wayne"
wrote in message
...
Well, I don't know what you are calling "retarded potential".
It is the term for the electric potential that must have existed on an
antenna when
analysing the radiated signal at some distant point.
It is termed, "retarded" because the radiation originated at some time in
the past.
OK now I know. I hadn't heard the term before.
But, how about my question?
" if the radiated power is equal for two different antennas, why would the
field strength be different, except as related to pattern differences?"
The radiated power just is spread out differently.

Quoting from Electromagnetism by F.N.H.Robinson in the Oxfors Physic Series,
1973 edition ISBN 0 19 851806 4, Chapter 11, Radiation and page 100 ..

The radiated power is proportional to the current times the antenna length
divided
by the wavelenght, and all squared.

Therefore, to achieve the same radiated power from a short antenna, the
current
in the antenna has to be higher.

There is no contradiction there. Current is not power. Power is
voltage times current. Since the impedance of a short antenna is not
the same as the impedance of a larger antenna, it makes perfect sense
that the current for a given power level will not be the same.

--

Rick