Antenna matching to housing ??

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martin

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What I didn't understand is that why plastic housing will effect the antenna characteristic?

My co-worker told me that is possible to effect about 200MHz in a

2=2E4GHz system.

Please comment.

Thanks. Best regards, Boki.

Boki,

The plastic housing can affect the antenna's characteristics because plastic 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

Great introduction, I can recall the formula now, but I am not good on this course.

Cheng's book is our textbook, but too many courses, only study enough to pass... sad.

However, it is a very good a new start point by your introducion!

Thanks a lot! Great!

Best regards, Boki.

Hi Boki,

Well, you're certainly not alone... electromagnetics is often one of the least favorite courses in EE curriculums, given the heavy math involved and the (initial) difficulty in seeing the real world applicability.

I was always intrigued with antennas and had one of those rare professors who had done Real Commercial Development in the area (he now works for TDK, still designing antennas); it certainly helped.

---Joel

Sounds great, cheers !

Best regards, Boki.