On 5/27/2011 12:37 AM, 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.


Of course. It all makes sense now. Feeding a 1/2W dipole in the center
causes one to measure approximately the Rr. However the dipole can be
fed at some other point. Even fed at one end where the feed point
resistance can be in the hundreds to thousands of ohms.

So, radiation resistance is a convenient device (along with current at
that point) that we use to represent power "lost" from the antenna
through radiation. Good. I can understand that.

Gotta be very careful of the definitions. It was in Kraus' Antennas For
All Applications that raised my question.


Presumably the "terminal resistance" is the resistive component of the
impedance seen at the antenna's feedpoint terminals. It's generally a
good idea to specify that the current is the same magnitude and
opposite direction in the two feedpoint terminals, and that they are
closely spaced--a tiny fraction of a wavelength apart.


I understand.


The difference between the two represents power dissipated in the
antenna itself. Of course that's generally a pretty low percentage of
the total in self-resonant antennas, but in short antennas (loaded
dipoles and monopoles, and tuned loops that are very small compared
with the wavelength) the power lost in heating the antenna and
associated loading reactances can be significant.


That, too, is understandable.


Cheers,
Tom

Thanks again, Tom, for this.

Also, I understand Jim's reference to integrating the volume to get the
radiated power and, hence, Rr. Thanks, Jim.

Cheers & 73,
John