"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

christofire wrote:

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

That's a good explanation. It might help some people to visualize the
process by comparing it to a series RLC circuit, which its feedpoint
impedance resembles over a moderate bandwidth. In both an RLC circuit
and an antenna, the current and voltage aren't in phase, but they're not
exactly in quadrature (90 degrees out of phase) either. This means that
during each cycle, some of the energy entering the RLC circuit or
antenna is stored and some is consumed. In the RLC circuit, the stored
energy is stored in fields in the capacitor and inductor; in the
antenna, it's stored in fields near the antenna -- the near field. And
the consumed power is dissipated in the resistor in the RLC circuit; in
the antenna, it's radiated. The antenna's equivalent to the RLC circuit
resistance is, of course, the radiation resistance, which "consumes" --
radiates -- some of the applied energy each cycle.

Roy Lewallen, W7EL

Chris wrote:
"I hardly dare say it but, actually that`s incorrect for the radiation
field (which is what I wrote about)."

That`s chris` prerogative. Note the near field is also called the
"induction field". One reason, its energy returns to the source each
cycle. The far field emergy has escaped or radiated. Its energy appears
as a resistive load on the source.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Chris wrote:
"I hardly dare say it but, actually that`s incorrect for the radiation
field (which is what I wrote about)."

That`s chris` prerogative. Note the near field is also called the
"induction field". One reason, its energy returns to the source each
cycle. The far field emergy has escaped or radiated. Its energy appears
as a resistive load on the source.

Best regards, Richard Harrison, KB5WZI


You mean, those "antenna/rf-magnetic-fields" are NOT leaving the
radiator at the speed of light, but being "stored in the ether?", to
then collapse and induce an electric field back into the element which
first generated-such? sly-grin

Sorry, I know, this will be perceived as "troll-territory." :-(

Regards,
JS

"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