"christofire" wrote in message
...

"Richard Harrison" wrote in message
...
Chris wrote:
"The formulae for all the field strengths can be found in reliable books
such as Kraus "Antennas"."

I agree.

On page 40 of Kraus` 3rd edition of "Antennas" is found:
"For a 1/2-wave dipole antenna, the energy is stored at one instant of
time in the electric field, mainly near the ends of the antenna or
maximum charge regions, while a 1/2-period later the energy is stored in
the magnetic field mainly near the center of the antenna or maximum
current region."

My preceding statement was before reading Kraus:
"A standing wave antenna stores energy in the magnetic field near its
center during one half of the cycle and in the electric fields near its
ends during the other half cycle."

My statement lacks clarity and precision. I am a poor engineer who has
never worked as an educator. Chris` point? Close but no cigar? OK, I
deserve the critism.

Best regards, Richard Harrison, KB5WZI


I hardly dare to say it but, actually that's incorrect for the radiation
field (which is what I wrote about). The radiation resistance of an
antenna accounts for its ability to radiate power into the surrounding
space and, like all other resistances, the peak of current co-insides with
the peak of applied voltage - so one doesn't occur '1/2-period later' at
all. What's described in the passage above is the situation in respect of
the temporary storage of energy in the 'reactive near fields'
corresponding to a reactive component of the terminal impedance, not the
radiation resistance. I would expect the latter to be of greater
importance to those interested in communication.

I wouldn't disagree with the statement that stored energy is concentrated
in the regions near the 'maximum charge regions' but if you plot the
equipotent lines around a dipole and equate the amount of energy stored to
the electric field strength it illustrates that the spatial distribution
of energy in the electric field is similar to that in the magnetic field
... as one might expect.

Chris

Of course, I meant to write 'equipotential' lines, but the doorbell rang at
the moment I was typing that. 'Equipotent' sounds a bit like 'omnipotent',
but in a shared manner (e.g. Greek gods)!

Reading the quotation again, even the '1/2-period later' seems incorrect.
For the reactive part of the terminal impedance, the peaks or zero-crossings
of current and voltage are separated in time by 1/4 of the period.

Chris