How to choose Insulator (or Substrate) for Implanted Antenna?

[nvt088]

I want to choose an insulator that give the highest radiated frequency for implanted antenna.
My antenna works at 403 MHz, inside the body with dielectric constant =42; conductivity = 0.64 S/m.
I tried many ways to choose the insulator but haven't succeeded.
How to choose the insulator? Is there any method,mathematical platform for this problem? We can discuss here.
Many thanks.

 

[jiripolivka]

I think Teflon is the only suitable implantable dielectric. Anything else has pores that support inflammation.As the wavelength in a tissue will make your antenna small, maybe even a glass substrate might be usable. Consult doctors who implant pacemakers or pacemaker makers, they have a long-time experience with suitable materials.

 

[nvt088]

I think Teflon is the only suitable implantable dielectric. Anything else has pores that support inflammation.As the wavelength in a tissue will make your antenna small, maybe even a glass substrate might be usable. Consult doctors who implant pacemakers or pacemaker makers, they have a long-time experience with suitable materials.

Many thanks for your answer.
Do you know any method or mathematical basic (Equation, etc) for this issue? In my case, I want to choose a substrate to give the highest radiated efficiency?
Many thanks.

 

[FvM]

The general method is to solve the maxwell equations for the choosen antenna geometry and optimize the parameters.

Most people will use a numerical EM solver like HFSS for this purpose. You didn't yet mention antenna geometry, if it's fixed or supposed to be varied for optimal antenna performance. I guess, you are targetting to an electrical small antenna.

 

[jiripolivka]

Many thanks for your answer.
Do you know any method or mathematical basic (Equation, etc) for this issue? In my case, I want to choose a substrate to give the highest radiated efficiency?
Many thanks.

In my opinion the Teflon or glass substrate will have almost no effect on the antenna parameters, it will make only the mechanical support of the conductive structure of your antenna. This should possibly be gold-plated to prevent interactions with the surrounding tissue. The tissue permittivity will define the electrical lengths of the conductive structure.
Instead of equations I think experiments will show much more in the real world. I would start with a good phantom modeling the tissue, and test antenna prototype for matching and radiation pattern. In a real tissue I will expect much worse problems to occur.

For tissue permittivity data, I like Mr. Alabaster's Ph.D.thesis, I found it on Google .

 

[biff44]

I have not done implanted antennas, but did do underwater antennas. I might suggest an antenna with some sort of thicker dielectric surrounding it. That way, as the antenna is moved around in the body, and it encounters different surrounding dielectric constants, it would stay relatively tuned to the correct center frequency.

Also, I would suggest one of the higher dielectric constants, so that it mimics the flesh closer, and there will be less of a reflection of the antenna to flesh interface.

 

[nvt088]

The general method is to solve the maxwell equations for the choosen antenna geometry and optimize the parameters.

Most people will use a numerical EM solver like HFSS for this purpose. You didn't yet mention antenna geometry, if it's fixed or supposed to be varied for optimal antenna performance. I guess, you are targetting to an electrical small antenna.

Thank FvM,jiripolivka and biff44 very much for your help.
I also used HFSS for my simulation. My antenna is a rectangular loop antenna with length=Lambda at 403.5MHz.

As jiripolivka said, I know experiments are very good ways to approach problems. But my present stage is at simulation. I want to choose a insulator with suitable dielectric constant for implanted antenna to get the highest radiated frequency.

As biff44 and some papers mentioned, higher dielectric constant will make less reflection. But Whether or not radiated power increase with higher dielectric constant?
In the beginning, I also think that less reflection results in higher radiated power. For example, at 403.5MHz, tissue have dielectric constant=42, and conductivity =0.64 S/m. And choosing insulator with dielectric constant=42 will have higher radiated power than insulator with dielectric constant =1 (Vaccuum).

But when I use HFSS to simulate and calculate radiation frequency, the efficiency of vacuum insulator is higher that the efficiency of insulator with dielectric constant =42. I tried many times but the result is the same. Is this possible that my method for simulation and calculation of efficiency is wrong?

Does this issue relate to distribution of E and H field? When simulating, I see that with vacuum insulator, I have higher H field magnitude than others while E field magnitude is almost the same in free space (Outside of body phantom). I measure E and H field on a same line when changing the dielectric constant of insulator. And this result lead to a higher efficiency in the case of vacuum insulator?

By the way, can I ask how to increase the magnitude of H field of an antenna (changing type of antenna, dielectric...)? Because H-field is more stable than E field in body enviroment?
Many thanks!
Thuan

 

[jiripolivka]

The antenna alone may be perfect but the tissue in which it will be embedded has so variable parameters that calculations have almost no sense. I saw many papers with various data (and tried myself without any success) but most antennas simply do not behave as intended. Tissue loss is the most important problem.

As the other antenna out of the tissue will be located close enough to the embedded antenna, the power transmission is not a problem. I would expect say 30-60 dB loss between the transmitter and receiver due to propagation in a tissue, and no antenna gain. Then design your system to operate with such loss.
Antenna radiation pattern will also be badly affected by the tissue, so do not expect any positive outcome from it. Get ready to solve system problems. You can try modeling by phantoms but I have found the real tissue so different and disturbing that technically any theory failed to work.

 

[FvM]

A general remark in advance. An RF engineer or anyone involved in the development of RF links should gain an intuitive understanding of electromagnetic phenomena. I'm under the impression that you are still far off.

Secondly, the problem should be specified more clearly. You are talking about radiation effciency. In general terms, this is power transmitted to the far field relative to transmitter output power. In detail it's important to distinguish different loss components:

- reflected power would be only counted as losses if the transmitter impedance is fixed respectively you are unable to match the antenna impedance correctly
- internal antenna losses. Can be varied by antenna design to a large extent.
- losses in the surrounding media. Probably the most interesting part in your application. How can the antenna geometry optimized for lowest "external" losses.

Finally the role of receiver antenna. The above efficiency scheme assumes transmission to far field, which may not describe your application correctly. If the receiver is still in the near field, the receiver antenna will be paired to the transmitter, e.g. a magnetic antenna with respecctive orientation to receive from a magnetic transmitter antenna (loop).

My guess related to the original question is that a thick, low Er substrate will be best for a magnetical antenna.