Antenna Near Field

[Microfielder]

Is there any way to minimise the length of the near field for a microstrip radiating at 403MHz. The strip is 1/4 lamda, and the substrate is Zirconia (Er=35). Thank you.
Best regards.

 

[Higgler]

Can you re phase your question.

Is it a microstrip antenna 1/2 wavelength?
Are you trying to make a small antenna because you are limited in space?
Are you using it as a near field sensor and the near field energy is changing your polarization?

kch

 

[MacGyverS2000]

Can you re phase your question.

<chuckle>

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Microfielder]

It's a 1/4 wavelength, spiral microstrip antenna. It is side fed and the substrate has an Er of 35.
The antenna is to work in the human body. I assumed that if the near field could be made as short as possible, loading effects of the human tissue would be minimal. Hence a large Er for miniatursiation.
I was wondering if a cover of high Er would help?? Or is there an alternative?
Size is critical as the antenna must be as small as possible.

Thank you.

 

[biff44]

I have worried about antennas and body handling before. The biggest problem is that the body will capacitively detune the resonant frequency of the antenna. This will change the apparent antenna gain. So you want to pick an antenna type with as broad a bandwidth as possible, so that you can move the resonant frequency of it but still respond efficiently to what you want to transmit. So a simple quarterwave length antenna is not so good, but an antenna made of a length less than quarterwave, with lumped capacitance loading at the end, it better.

Also, I would try to separate the metal line of the antenna as far as possible from touching the human flesh or liquids. Some sort of coating would help. It does not have to be the same dielectric constant as the substrate. You do not state your operating frequency, but if the coating becomes thick in respect to a quarterwavelength, you will want to actually pick the coatings dielectric constant such that it will act like a radome.

 

[Higgler]

Macgyvers2000, I actually forgot the r (r is also known as a Pirate's favorite letter, my kids told me that one).

Microfielder, you are into a very challenging antenna area. Alot of people are trying to make a good system for inner body sensing.

A few tips,
don't let the metal of the antenna touch the body. i.e. insulate it. Hence, a cover to keep it away from the body will help.

research RFID antennas, They couple efficiently at lower frequencies.

Don't let your bosses dictate 403 Mhz, that may be the wrong frequency.

The most common MRI's in the hospital use 63 MHz, GE is one of the biggest makers of MRI's.

http://en.wikipedia.org/wiki/Magnetic_resonance_imaging

MRI's use 63 Mhz mostly, also 41 Mhz, and 300 Mhz.

If you pick a frequency that's used in an MRI, you could put the patient in the MRI and leverage the MRI electronics receivers. Put your transmitter on test, let the MRI machine measure signal strength as it moves through the body and have strength vs. location plotted with the image generation. If you have too much metal in your antenna, that's not good in an MRI.

What's your required efficiency? If you haven't done a link budget to communicate with whatever you plan on communicating with, then you don't have any antenna gain spec's. Gain will probably be in the -40 to -80 dBi range.

good luck,
sounds like a fun project.
kch

 

[Microfielder]

Thank you Higgler and Biff44. You are right, the gain is about -40 dBi when implanted.
If antenna diameter constrains the NF, would spiraling even further through tighter curling effect the spatial distribution of the NF? I guess I am asking if there is a way to calculate the near field distribution as a function of the shape of the antenna?
Thank you for your responses. Much obliged.

 

[biff44]

It would help if we knew what frequency!

 

[Microfielder]

Unfortunatly its fixed to 402-405 MHz

 

[biff44]

A circular coil (a trace with a series tuning capacitor) might be less sensitive to body proximity than a quarterwave antenna whip!

I once experimented with non-tuned inductive coils. These are just low resistive loss inductive coils (in your case a 1-turn coil) that are not tuned to resonate. They are driven by a current source, rather than by a 50 ohm matched amplifier.

The benefit is that since they are non-resonant, the proximity of liquids or flesh can not mistune the antenna, so its phase characteristics are not wildly dependent on keeping everyting away from the antenna.

The drawback is that you have to drive the thing with some current, and that means having a sufficient battery.

I am not that up on antenna propagation theory, but I think this type of antenna might only work well in the near field. Only sends out a varying magnetic, not electromagnetic, field. Any good field theory guys out there to confirm?