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multiband vs wideband

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Jul 22, 2007
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Need your help about this matter...

For microstrip antenna there is some techniques/methods to generate multi-frequency or enhance the bandwidth of the antenna...
I see some of those techniques are relatively the same, for instance slot could be used to generate multifreq and also widen the bandwidth...

What kind of factor (for example: slot) can distinguished the results (whether is caused multifreq or wideband)?
Or is just based on trial and error (luck)? But I doubt about it, there must be at least some theory behind of it. Could anyone in this forum explain it to me?

Thanks in advance

Hi, Micro_tech:

For multi-band, you are trying to create a few distinct resonant frequency bands (high Q). For wide band, you try to make a very wide bandwidth (low Q). I think I have posted my comments on wide band antennas before. For all those wide band antennas, one thing in common is that they do have enough space for the energy to be stored. When energy is concentrated in a small space, the resonance is normally high Q. A good example is a typical microstrip antenna. Most of the energy is concentrated between the patch and the ground plane. For a slot antenna, it allows energy to stored in a much bigger space, it is higher Q. Another example is a patch with some EBG ground. Basically, the EBG ground allow the energy to be spreaded horizontally. It makes the antenna wider bandwidth. Regards.
Hi jian,

Thank you soo much for the explanation...

so the red line that distinguish is the Q factor...
we can use the same method (e.g. adding slot to basic patch), and it could generate multi-band or widen the bandwidth, depends how the slot change the energy (or can I say the current) distribution in the patch...
please give me comment about my summary above (because I'm not too sure about it)

I'll look for your comments on wide band antennas...

I'm sorry I have more question about multi-band, if you don't mind....
Is there any specific criteria (beside resonant freq. must be more than one), antenna can be said a multi-band?
Or is just simply all of the frequencies should have VSWR<2 ?

because what I know is, basic patch antenna besides its TM10 freq, also have another resonant frequencies (e.g. square patch antenna, TM10 freq=950MHz its also resonant for freq 2.1 GHz although not as good as in the 950 Mhz).
So can we call it multi-band antenna also?

Once again thank you for your time in helping me...

Hi, Micro_tech:

If you look at a microstrip antenna, it involves the patch and the ground plane. The current on the ground plane is kind of opposite to the patch. Their far fields are basically cancelling each other. The main radiation source is the fringing effects of the patch. The energy is basically bouncing back and forth between the two ends of the patch. There is little energy radiated from every round trip. Such a situation makes it very high Q. A slot on a patch normally can create much more radiation. The E-field on the slot is equivalent to magnetic current. For most slot antennas, you can find other equivalent magnetic current source opposite to the one on the slot. The resonance is normally much lower Q.

VSWR < 2 means that the return loss is less than 10%. It is one criteria for a good antennas. However, VSWR < 2 does not necessary mean it is a good antenna. For some structures, the VSWR < 2 while the radiation is low. In such a case, the radiation is lost as material loss and surface wave loss. In such a case, the antenna is not a good antenna even VSWR < 2. The fundamental requirement is that the radiation efficiency is high. When the radiation efficiency is high, it means that most of its energy is radiated, instead of returned or lost as omic loss.

Best regards,

Hi jian,

Thanks again for your time helping me...

so in other word, it is common that a simple patch antenna could have more than one resonant frequencies (with RL < -10dB), because basically microstrip antenna has high Q factor. Is this correct?

About good antenna criteria, thanks for your explaination, your right.
But can we know the "radiation efficiency" is high or not just by the simulation software like IE3D? just as simple as to find out the VSWR<2 or RL<-10dB?

Thanks and regards

Hi, Micro_tech:

No matter whether an antenna is high-Q or low-Q, you can always tune it to get VSWR < 2.

On IE3D, you can find the radiation patterns. From the radiation patterns, you can find the radiation efficiency (ratio between radiated power and net input power). It is the most important factor for determinng how good the antenna is radiating. For VSWR or RL, you check the S(1,1). Low RL means that there is little reflected power back from the antenna. It does not mean the radiation efficiency is high. However, for most antennas, when the basic shapes are fixed, changes of dimensions normally may not affect the radiation efficiency much. However, it will affect the VSWR or RL much. In such a situation, you just check VSWR and RL when you tune the antenna. When you get VSWR < 2 or RL < 10%, it may mean you are getting kind of the best from such a configuration. However, where it is good or not good, you still need to check the radiation efficiency. Regards.
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Hi jian,

Yes you're right, we can tune the antenna to get VSWR < 2, for example by moving the probe feed postion.

But, what I want to make sure is, it is normal for a simple microstrip patch antenna (without any additional methods) having more than one resonant frequencies?

Please give me your comment, thanks

Hi, micro_tech:

A patch antenna can certainly can have more than 1 resonance. Normally, when the length is about integrals of 1/2 lambda, it is a resonance. There are many more others. Regards.

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