antenna-theory.com

[dxdp<h/2p]

Hi,

I am looking into stroke detection application using microwave technology. Any sugestions on which antenna to use? Antenna should be low profile, size 25x25x10mm, working frequency 0.5-3GHz.

Thanks inadvance!
dxdp<h/2p

[Schubert]

You're looking for a small antenna with wide bandwidth. Note that a half wavelength at 500 MHz is about 833 mm, so you don't have anywhere near that. So you'll end up with something that is pretty low efficiency.

This may be fine as you probably aren't trying to communicate over long distances anyway. So to figure out what you need you should provide more details. Is the antenna implanted in the body or on the outside? What is it communicated to?

[dxdp<h/2p]

You are right regarding efficency. My best simulated result so far shows an efficiency of 2%. Antennas are placed outside the body and are communicating with a network analyzer. S-parameters measurements are perfomed. Distance between antennas is 25cm. Permittivity of the human head is around 40.

According to my calculations lambda/2=0.3m@500MHz in free space and 0.047m in human head having permittivity 40. How do you get 833mm?

What could theoretical efficiency be for this type of application? Regarding dimensions of the antenna I specify 25x25x10mm, which should corresponds to lambda/4@500MHz in permittivity 40 media.

Since antennas are to be placed in an array, it is required that they are shielded from the backside in order to avoid coupling between antennas.

[Schubert]

Ok yeah I screwed up the 500MHz half wavelength calculation, sorry about that.

I still am not quite understanding your test setup though. Antennas are on the outside of the body and you are measuring s12 between two antennas? And the antennas are on either side of the body?

The permittivity of human flesh is probably around 40. However it has some conductivity too and this is gong to kill your efficiency. So any improvement in efficiency due to increased epsilon making your antenna electrically larger will be offset by energy absorption within the body.

As for "theoretical efficiency" I think you are most interested in maximum coupling (s12) between the two antennas. My rough estimate is you are looking at 20 dB of pathless and antenna gains on the order of -17 dB, so something like -60 dB of isolation between your ports. However, I'm not 100% sure what's going on so that is a rough estimate.

[dxdp<h/2p]

setup is as following:
- antennas are placed 18cm apart on each side of the human head phantom
- s12 is measured between antennas and the radiation should go through the head
- conductivity of the head varies with frequency (0.5S/m@0.5GHz to 2.5S/m@3GHz). Therefore it is of interest to work with low frequencies
- s12 should be maximized since its change is the measuring signal of the system. During bleeding electrical properties inside human head change therefore one expect to measure a change in s12 (as well as in s11).

Therefore the question is - which antenna type has best potential to deliver high s12 between 2 antennas, covering 0.5-3GHz band (RL=-10dB) with high enough efficiency. And another question is which efficiency one should count with?

[Schubert]

I feel like I'm giving you some free consulting so this will be brief. My notes:

- you have no real reason to maximize s12 if you are only looking for delta.
- looks like it will be difficult for you to perform stroke detection in this manner unless you have a reliable "baseline" measurement for the patient in question. That is, if you measure the heads of a few people you will likely see radically different s12 measurements
- make sure your setup is repeatable - slight differences in head position will destroy your setup
- I would recommend an experiment of some sugar-salt water solution (to simulate conductivity and permittivity of body), and replace some of this with a balloon of blood in random spots. See if you can see the delta.

Sounds like a good idea, I hope it works. Certainly stranger things have happened

[dxdp<h/2p]

the reason I want to maximize the s12 is to keep the system simple. By this I mean that the measurements could be performed with a network analyzer and a switch without additional amplifiers or other hardware required for high sensitivity measurements.

You are right regarding the repeatability. Antennas should be fixed in a robust setup. Measurements will look different for different patients. The system is to be calibrated on a phantom before measurements on patients.

However question regarding which antenna type to use remains.

Thanks for taking your time and considering this topic. Hope you can find a balance between giving and receiving feedback. Please feel free to not reply if you feel that this balance is broken.

[bigSteve]

I don't think you'll need additional amplifiers even if you're antennas aren't too good. Imagine a good antenna has efficiency of -1 dB, a horrible antenna has efficiency -10 dB, but you network analyzer can already measure s12 of -70 dB. So you won't need additional amplifiers. The trick will be finding a trade off between what antennas couple together well through the body or head material.

[sonia600]

According to my calculations lambda/2=0.3m@500MHz in free space and 0.047m in human head having permittivity 40. How do you get 833mm?