Microwave Materials, Spectrum then EMI effects on SNR of WiFi, BT.

There are a lot of measurable parameters in microwave dielectrics which affect;

• Dielectric Constant , Dk between conductors and thus Impedance , Zo
• scattering parameters in a transmission path from mismatch and losses
• skin depth: compare wavelength of IR vs uWave
  • Longer λ is better for skin effects but worse for loss tangent
  • Microwave Ovens:
    • Consumer: f= 2.45GHz +/-100~120Hz sidebands λ=12.2cm, P=1.8kW max
    • Commercial: 915 MHz +/-100~120Hz sidebands λ=32.8 cm, P=3kW max+3kW IR(opt.) The modulation sidebands depends on power line frequency (doubled with many harmonics) Then there are SIGNIFICANT 1/4 λ sidebands typ 300MHz due to the geometry of standing waves in the chamber but again modulated with power line harmonics and not high bit rates. The spectrum will show quasi-peaks and not modulation and the modulation is dispersed at low f by the rotating turntable and contents reflecting off the oven walls.
• compare this with WiFi: including but not limited to 2.4 GHz, 5 GHz, 6 GHz, and 60 GHz frequency bands with frequency hopping within a BW of 10 MHz, 20, 40 , 80MHz etc which is rather immune to 2.45GHz in a narrow band unless the front end is compressing with WiFi signal <= -78 dB and Rician Fading loss of 10dB

Test the SNR on a Laptop with a strong signal -70dBm and a weak signal-80dBm which is borderline on 54Mbps but OK on 11Mbps. Then put near a microwave oven and look for LOS (loss of signal). There are many windows tools to convert RSSI (received signal Indicator to dBm) for any WiFi signal on a Laptop or Mobile.

Contamination is more critical for interference, e.g. acid rain, snow on Sat dish, body reflecting/blocking antenna.

• Loss Tangent is the critical factor in a dielectric that absorbs power (heat) and the high dielectric constant (Dk=80) lowers the impedance by a factor of 80 but does not mean it is more lossy but can conduct more current in series with the cycling waves.

Oil, plastic and fat have a Dk around 4. The presence of contaminants and additives in FR4 makes it a poor dielectric for microwave with the exception of special blends of epoxy/fiberglass such as GETEK and others use more polyamide which affects cost a bit but reduces the loss tangent enough for use in the ISM (928MHz) and 2.4GHz bands at a 10% cost increase of the PCB.

Experiment DIY

This means de-ionized, sodium-free pure water does not heat up quickly in a microwave oven.

The presence of salt in any dielectric is an ion that makes it significantly more conductive. Test a glass of clean water and one with a tbsp of salt after 60 secs for temp rise. BIG difference in temp rise ('C only not 'F no exceptions for Engineers even in the Excited States).

That is Loss Tangent loss due to ionic impurities.

Disclaimer

I have measured RSSI accurately to 0.1dB for loss of signal and BER on WiFI extensively in the past, but not now. When 54Mbps fails, 11Mbps works and near threshold 1dB rise in SNR is huge ~2 orders of magnitude difference in BER vs 1dB rise SNR. The Laptop antenna motion of <1mm in rotational angle aiming for a reflection, can easily change 1 to 10 dB from multipath when the range is far.

I do have an E/H Field Strength meter but it is not frequency selective up to 5GHz so I could not test this and my 10yr old laptop power connector is dead (solder joint) so I could not make measurements, but my own experience is Rician Fading is alot more serious (>-10dB) on weak signals from body / wall reflections out of phase than the effect of a microwave oven affecting the Wifi/BT radio if well designed.

The Physics of water like any dielectric don't have a microwave resonant frequency The absorptive effects have been characterized from 10cm to 10um rise with lower wavelength but this is enhanced by salt ions.

@Olin's assertion to resonant frequency is totally bogus. Water has absorptive properties over many decades of f without this resonance.

Conclusion

http://www1.lsbu.ac.uk/water/microwave_water.html#loss This is a good reference but fails to discuss the ionic behaviors of contaminants but clearly agrees with my assertion that there is no interference.

Post Mortem

I take everything back. I pulled out an old Panasonic 1800 W microwave oven and it interfered with our Wifi. Unlike the Samsung 1kW.

posted about 4 years ago

CC BY-SA 4.0

3y ago

TonyStewart‭

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