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IWR6843AOP: Radome Design, dielectric constant of glass?

Mark Dresser
Prodigy 20 points
Part Number: IWR6843AOP

I have an application that requires a glass radome due to environmental conditions that would melt any of the likely plastics.  I have been studying the SWRA705 design guide, in which table 3-1 lists the relative permittivity (Er) of glass as 5.75.

If I look at the parameters for N-BK7, which is a well documented optical glass, the index of refraction (IOR) at visible wavelengths is approximately 1.51, trending downward as the wavelength increases.  Using the relation IOR = sqrt(Er), this would suggest Er somewhere around 2.3.  Unless the index of refraction is non-monotonic between visible wavelengths and mmWave wavelengths, there is something that I don't understand...

Searching online, I've found references to Er of glass between 2.3 and 6.3 but some are for esoteric types of glass and most of them are from sources that leave me with questions.

Can anyone suggest where the figure of 5.75, used in SWRA705, originated and/or what I am missing? 

Thanks

Mark

  • 0
    TI__Expert 6930 points

    Hello Mark,

    Thanks for reaching out to us, please allow me 24 hours to look into this.

    Regards

    Ankit

  • 0 in reply to Ankit Mohanty
    TI__Expert 6930 points

    Hello Mark,

    Please check the section 10. References of the Application Note SWRA705 for the references. You can find the source of DK/DF of various material types in point 7. Dielectric Constant, Strength, & Loss Tangent

    Regards

    Ankit

  • 0 in reply to Ankit Mohanty
    Prodigy 20 points

    Hello Ankit,

    Thank you for the information.

    That reference explains where the number 5.75 (@ 10 GHz?) came from.  Unfortunately the referenced table appears to be quite old and doesn't have any any data for performance above 10 GHz (most of it <=3 GHz).

    Interestingly enough, the data sheets that I was able find for Corning 7059 quote a different number for dielectric constant: 5.84 @ 1MHz and index of refraction 1.531 @ 643 nm.  If both are correct then the dielectric constant varies widely with frequency and neither number is of much value at 62 GHz.

    Are you aware of any sources that characterize radome material properties at 62 GHz? or any test facility that could measure the dielectric constant of samples at 62 GHz?

    It seems like this would be a useful resource for anyone using your mmWave devices.

    Mark

  • 0 in reply to Mark Dresser
    TI__Expert 6930 points

    Hello Mark,

    The dielectric constant of Glass varies between 3.7-10. Materials with lower Dk (dielectric constant) and Df (loss tangent) are recommended for radome designs. Typical materials used in radomes are PBT (Polybutylene terephthalate), Plexiglas, Polycarbonate, Teflon® (PTFE), Polystyrene, and ABS.

    I am not aware of any sources that characterize radome material properties at 62 GHz. Usually, the material Dk value listed are within 10GHz. The Dk value for Glass material at mmWave frequencies needs to be requested from the third-party manufacturer. TI has partnered with companies to offer a wide range of solutions using TI mmWave radar sensors and related services. These companies can accelerate your path to production using mmWave radar. Within this partner network, you will find companies with capabilities of designing both radomes and lens's. To quickly browse our third-party solutions and find the right third-party to meet your needs, use this search tool MMWAVE-3P-SEARCH Design tool | TI.com.

    Dielectric loss or dissipation factor will start playing role in thicker material. i.e. lossy material or higher loss tangent material as n increases losses also increases hence reduction of link budget margin. This makes it harder for detecting weak object like plastic tubes. Hence it's desirable to select low loss dielectric materials for the Radome. 

    Regards

    Ankit

  • 0 in reply to Ankit Mohanty
    Prodigy 20 points

    Thanks Ankit,

    I will look into third-party partners that may be able to help with my questions.

    As noted in the original post, plastics are not an option due to environmental conditions.

    Mark