On 5/26/2011 10:37 PM, K7ITM wrote:
On May 26, 4:53 pm, John wrote:
Assume a non-reactive antenna. Start with a half-wave dipole.
What is the radiation resistance and what is the terminal resistance?
Thanks,
John
Radiation resistance can be, and is, defined differently by different
writers. Often the term is used without clarifying definition, and of
course that leads to trouble. Beware any time an author throws around
"radiation resistance" without carefully defining how he means it.
There's a nice section (section 17; page 118) on it in King's
"Antennas" chapter of King, Mimno and Wing's "Transmission Lines,
Antennas and Wave Guides." I highly recommend reading that passage,
though I know that not everyone likes King's writing as much as I do.
(I may be able to supply a PDF of it, if you can't find the book.)
Generally, radiation resistance is associated with power actually
radiated by the antenna: i^2 * R(radiation). One possible definition
is "that portion of the resistive component of the feedpoint terminal
impedance that represents radiation." But when the feedpoint is not
at a current maximum, "radiation resistance" is sometimes (often?)
taken instead to be the resistance which, when multiplied by the
square of the current at the current maximum, would result in the
value of the radiated power.
Orfanidis has an explanation in Chapter 15 of his online
electromagnetics textbook
http://www.ece.rutgers.edu/~orfanidi/ewa/ch15.pdf
page 612
Not a very complete discussion, as might be found in Kraus, but at least
it's online.
There's also Prof. David Jeffreries's website
http://personal.ee.surrey.ac.uk/Pers...es/radimp.html
"The classical way to calculate the radiation resistance is to surround
the antenna with a hypothetical closed surface in the far field,
calculate the values of electric field and Poynting vector on this
surface in terms of the antenna terminal current I, integrate the power
flow per unit area, represented by the Poynting vector, all over this
surface, to determine the total outward travelling power in watts, and
equate this power to the quantity II*Rrad/2 as discussed above. "