has a relative permittivity that's often significantly different than 1.
(Vacuum *is* 1, and air is very close to 1.)
Think of it this way: The speed of light in a substance is 3*10^8/sqrt(k) m/s,
where k is the relative permittivity. And of course, a wavelength (lambda)=c
(the speed of light in the material)/f (the frequency of operation). Hence,
to calculate dimensions to build something like "a quarter-wavelength dipole,"
you need to know the relative permittivity of the material surrounding that
dipole so that you can correctly compute c. When you have a plastic housing,
you have a mixed environment that's mostly air and somewhat plastic, so the
*effective* dielectric constant is "something" greater than 1 and your
dimensions need to be changed accordingly or the antenna's resonance will
occur at a lower frequency.
--
This is all first-quarter undergraduate electromagnetics material, so you
might want to take a look at the books targeted towards such students.
Popular tomes include Cheng's "Field and Wave Electromagnetics," which assumes
you have a solid grasp on undergraduate calculus. If you're a little skay
there, I'd suggest something like "Introduction to Electrodynamics" by
Griffiths or "Electromagnetics" by Kraus (which is a truly excellent book -- I
think it's a fair statement that many people who received A's in traditional
EM courses couldn't solve many of Kraus's problems, even though he uses far
less fancy math than most... Kraus was all about real world applications...
and searching for E.T. in his spare time
... his death a few years ago was
a great loss). If you want to avoid the textbook approach, something like
"Electromagnetics Explained" by Schmitt is good for the working engineer.
In many cases, the more "colorful" the cover, the less mathematically involved
the text will be.
Something like "Classical Electrodynamics" is good if
you ever need to let a little wind out of your sails...
---Joel
