A basic doubt on microstrip p@tch antenna

Hi all,
I have this niggling doubt for some time. May appear foolish to some but I want to ask it. In many microstrip antenna design ( Considering only Rectangular for the moment ) , it turns out that the width may well exceed the length ( including the fringing fields) W may be greater than λd/2 (λd being the wavelength in the effective medium). Then why doesn't the antenna resonate at a lower frequency ? It always resonates around the frequency corresponding to the length only ( might vary slightly though ). WHy does this happen. Then what are all the parameters controlled by W ? ( impedance ? Pattern ? Efficiency ? Thanks for your time.

-svarun

Length of the microstrip antenna determines the operating frequency dominantly.
Width affects the radiation efficiency.

Fringing fields are mainly present at the radiating edges. In the case of an ordinary rectangular patch there are 2 radiating edges λd/2 apart and with a lenght equal to W, hence the influence on radiation pattern. The fringing fields at the non-radiating edges, W apart, are negligable.

Best regards.

the feed is a major factor in determining the dominant mode and corresponding resonant frequency.

Agree with Loucy.

The W (length) is the route of the feeding input wave.
So it determines the resonant freq, and so on.
And, the polarization is also determinedby the feeding direction.

loucy said:

the feed is a major factor in determining the dominant mode and corresponding resonant frequency.

Click to expand...

Hi Loucy,

I am still a beginner in this field and so can you elaborate a bit more on that or can you tell me where can I read more on this. Thanks for your time.

-svarun

hi svarun

here some expln on microstrip antenna.

PL

In some books published in the 90s they sometimes refer to the two dimensions with a distinctive name.

The length L is called the Radiation Length.

The width W is called the Resonant width.

Sometimes the L and W appreviation can swarp around in some texts.

Question aries: If the p@tch is square how would one determines which is the length and which is the width???

The feed is the answer. Normally the feed is aligned center along one of the edge. This edge and the opposite one is therefore the radiation lengths L. Leaving you the two edges in the orthogonal direction, namely the resonant width W.

As mentioned in one of the above post, the resonant width is equal to Guided_Wavelength / 2. In practice, the radiation length L is usually need to be chosen and rarely calculated.

I suggest you try Ansoft Designer. There you set up your materials and layers, then Designer can calculate/estimate for in seconds the dimensions for the linear or circular polarised p@tch antenna including the locations of the 50 ohm feed point.

Hope this help!

Dear ,
I think it could be described better this way:
considering the patch shape, it could support some specific EM modes, forexample, for a rectangular one, there are TE01,TE10,... . well, if we solve the bondry condition diffrential equation of patch with cavity method, we'll see that modes are like,say : sin(x/lamda/2);sin(y/lambda/2);sin(x/lamda/2)*sin(x/3*lamda/2); and so on.
well, this means that the p4tch shape itself can support just few specific shapes of EM fields. if you feed the rectangular patch from the middle of one edge, the modes of the other edge is not excited becouse of their shape. if you feed it from the diagonal, both of them would excite, and this is used for a long time to have CP polarization.
i hope you can undrestand my means, with my weared language! ), please let me know if more comments you need.
you can find all of these in the great book: "microstrip antenna handbook" of bahl and bhartia.
rgds,
Marti

Microstrip antennas resemble dielectric loaded cavity [two PEC walls (top and bottom) and four PMC sidewalls] and they exhibit higher order resonances. Through cavity model analysis, we can get field distribution and cutoff frequency of different mode.
For all patch antennas, we have h«L and h«W,
If L>W>h, the dominant mode is TM 010 mode
If L>W>L/2>h, the second order mode is TM 001 mode
If L>L/2>W>h, the second order mode is TM 020 mode

Use the Field Equivalent Principle (Huygens' Principle), microstrip patch is represented by an equivalent electric current density Jt at the top(there is also current density Jb at the bottom of the patch which is not need). The four side slots are represented by the equivalent electric density Js and magnetic current density Ms:
Js=n x Ha and Ms=n x Ea
where n is the unity vector normal to each side wall, Ha and Ea represent magnetic field and electric field at the sidewalls.
Because microstrip antenna has very small height-to-width ratio, the current density Jt at the top is much smaller than the current density Jb at the bottom, it is assumed it is negligible here and it will be set to zero. Also it was argued that the tangential magnetic fields along the edge of the patch are very small, ideally zero. Therefore the corresponding equivalent electric current density Js will be very small (ideally zero), and it was set to zero here.
Thus the only nonzero current density is the equivalent magnetic current density Ms of four sidewalls. Through my attached figure of field distribution, you can get the magnetic current density at each sidewall, and you will understand why only L contribute to radiating, that is why we call these two radiating slots. while other two will contribute to the imaginary part of the imput impedance.

Best Regards,

Thanks so much , sassyboy, mamali and asdfaaa. Things are clearer. I will go after the book by Bahl and Bhartia.

-svarun