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I suggest multiple-ring antennas. I was simulating them a while ago and got very good results. Try to look them up in IEEE, or I could upload an article for you.
Hi: I think the rule for big volume for wideband antennas still applies. No matter what kind of antennas, we do need to have enough volume for the stored energy. Microstrip antnenas are of narrow bandwidth because the energy is mainly limited to the small region between the patch and the ground. A slot antenna can have much wider bandwidth. The reason is that the stored energy can be spreaded into more space on both sides of the slot. In some sense, the space around the slot can not be used for other purposes, if you put some components there, it will change the stored energy there and it will change the antenna's performance significantly. From what I see, some wide band antennas look take less space. However, they use the air around it for the stored energy. You have to allocate the space for it in order to make it wide band.
Jian, if you can correct me in the following discussion, it would be helpful.
We look at the definition of Q factor from two viewpoints.
\[ Q= \frac{Energy stored in the deivce per cycle}{Energy dissipated per cycle}\]
Also
\[Q=\frac{f_0}{\Delta f}\]
where \[\Delta f\] is the 3-dB bandwidth.
We need as low a Q as possible to attain greater bandwidth. If you consider the radiated energy as a loss mechanism, then it means that lesser stored energy and greater dissipated (radiated) energy must mean higher bandwidth.
Please point out what is wrong with this argument.
Hi, svarun: I think your formulas are correct. Yes. In order to be wide band, we need to have less stored energy from the formula for Q. However, it does not mean we need to have smaller volume for it. From my experience, when the volume for the energy (not the total stored energy) is small, the antenna is always narrow band. When a small antenna has a wide bandwidth, its field normally spreaded into the surrounding area. As I mentioned, a slot antenna is like that. Even a slot antenna has low profile, however, its field and energy is not confined near the slot. It is spreaded into the surrounding much more than a microstrip antenna. Another example is EBG structure. It can make an antenna low profile while it can be much wider bandwidth than a microstrip antenna. However, please remember, its field and energy is spreaded horizontally. Much of the energy and field is spreaded into the region surrounding the patch in the horizontal location. If you change the structure not very far away from the patch in the horizontal location, you will see change in an EBG antenna's performance. It is a clear indication that much field and energy is there. The above are just from my observations. You may try to check whether it is true. Regards.
I agree with you. I am reading Wheeler's paper on "Small Antennas " and that is also giving me some idea on the "stored energy" concept that you mentioned.
If u r looking at microstrip antennas then u slot and e shaped patches have been reported which have considerably high bandwidth. Also how abt using planar monopoles which have significantly high bandwidth.
I just located a copy online - I've been reading over the same paper myself. I'm also looking for other related papers myself on small antennas in the paper requests forum. Anyway, here's a link to Wheeler's seminal "Fundamental Limitation of Small Antennas" paper of 1947 along with several others (including Wheeler's 1975 paper "Small Antennas"):
However, some modifications have been made to Wheeler's formula relating radiation Q/bandwidth to volume, so although the Wheeler paper is a must read, I would then refer you to McLean's paper "A Re-examination of the Fundamental Limits on the Radiation Q of Electrically Small Antennas" of 1996, which is attached below.
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