antenna-theory.com

[threecat2]

Hi,

I would like design a low power antenna to transmit signals in the 3-5GHz range, and there is a size limit of 500um for the antenna. That would give me L = lambda/20 in the case of a short dipole. I'm hoping to place the receiver tens of centimeters away from the transmitter. Finally, I'd prefer a structure that would allow wide bandwidth applications.

So I'm looking for the right antenna shape that will maximize transmission efficiency, and ease impedance matching at the same time. Any suggestions?

Thank you.

-threecat2

[bigSteve]

One of the fundamental rules of antennas: If you want bandwidth, you need volume. And 1/20th of a wavelength is not it.

A short dipole won't work for what you want. The bandwidth will be extremely narrow, so there's no way to impedance match it over such a wide frequency range. You're transmission efficiency will be less than 10% even if you tune it and it is in band.

You do mention you'll have a cable 10s of centimeters to the receiver. If you can (i.e. this cable is not shielded behind other metal) you should try to use this as part of your antenna. See for instance:

The Infinite Balun: http://www.antenna-theory.com/definitions/infinite.php
as used in the spiral antenna page: http://www.antenna-theory.com/antennas/travelling/spiral.php

You'll need to get more creative with your available space, don't think of it as an antenna isolated in a structure, but an integrated antenna within the structure so you can radiate whatever it is you're sitting on.

[threecat2]

Hi bigSteve,

Thanks for your input. I'd like to correct your posting by saying that I do not have tens of centimeters of wire to the receiver. The transmitter will be isolated, and I meant that the desired wireless transmission distance is tens of centimeters long.

If a dipole has such low efficiency, would a patch or spiral antenna (tens of microns wide) be a solution?

Thanks again.

[bigSteve]

With such a small volume all antennas will be fairly inefficient. However, because you are transmitting over such a short distance you may be able to get away with a pretty inefficient antenna. Try the short dipole and see if it allows communication.