antenna-theory.com

[splendor]

hi guys,
when I read through the website,I found this statement "larger volumn "usually leads to larger band width.
I wonder why,can someone explain it please? Thanks

[bigSteve]

If you imagine an infinitely thin dipole, you can understand that the electric current really has no degree of freedom there. It has one mode, it must terminate at zero current at the end and for a given frequency, will be sinusoidally distributed along the wire.

Now, if you fatten the dipole up, there becomes more freedom, more modes, that can "fit" on the wire. That is, there is more than one current distributions that satisfies all the universal rules of maxwell's equations. When you have more freedom or more modes allowed on a wire (as in the case of a fatter dipole), the bandwidth increases. More modes means more frequencies around the center frequency will radiate on the structure.

[splendor]

Hi bigSteve,

Very good explanation. Thanks.
Also, in the case of microstrip antenna. Can we say thicker have more fringing field modes? I heard some explanation from Q out of the volume of an antenna. Does thicker substrate mean higher Q ?

Thanks.

Splendor

[AntennaStudent]

Hi,

Can you please explain what modes are? Thank you

[admin]

A mode is basically a current or voltage distribution.

So if you have a very thin wire antenna, then there is only one way the current can flow on the wire and produce radiation. If the wire is fatter, there are more current distributions that satisfy all the boundary conditions and can produce radiation. This extra freedom allow additional frequencies to radiate, increasing the bandwidth.

[R. Fry]

The reactance of larger diameter antenna conductors changes less with change in frequency, other things equal.

Therefore the amount of the r-f spectrum meeting the limit desired for an acceptable antenna input impedance is greater for large OD conductors than small OD conductors.

R. Fry

[AntennaStudent]

Can you please recommend a textbook that I can read as an undergraduate? I need to know the why's of antenna theory. The website antenna-theory is very clear about explaining the basics of antenna. However I need to know the why's.

Thanks

[AntennaStudent]

Thank you R. Fry and admin for answering my question.

[R. Fry]

[AntennaStudent]

One more question.

I need to be able to explain how changing the radius and length of a dipole antenna affects the realized gain.

I also designed a Yagi using HFSS and now I have to explain the different values I have for the Front-to-Back ratio and resonant frequency.

The problem is that I feel completely lost and don't where to look for an explanation or even what kind of information to look for. I've tried reading Antenna Theory by Balanis but keep getting lost in the math. Fundamentals of Applied Electromagnetics by Ulaby is too simple.

[bigSteve]

The radius of a dipole antenna shouldn't affect the gain much. The length should - the longer it gets the more directive it is and hence the gain is larger.

It's not clear what you are asking for on the Yagi question. Front to back ratio isn't very clear at all for a simple explanation. Resonant frequency should be the same for the dipole antenna that is driving it - i.e. odd multiples of a wavelength.

[AntennaStudent]

Hi bigSteve,

I need to explain the why the Front to back Ratio, resonant frequency and relative gain changes at the reflector-feed distance is changed. For the first design I only have a reflector and feed. The director was added at a later stage.

What can I do to gain an intuitive sense of antennas because I just can't see things the way my internship supervisor does. I'm unable to analyze results and give an explanation for them.

Thanks

[AntennaStudent]

Lol...thanks Botham but I think having a PhD also has something to do with it.

[arka]