TL:DR The bathroom is high in humidity which absorbs and reflects microwave energy. Ceramic does not. Just try to heat up your empty coffee/tea mug with another cup of water in the microwave oven.But avoid painted cups with gold rims. you'll get arcs where it burns out. The bathroom also has more reflective surfaces from fixtures, tubs water etc.There are several well-defines principles that cause signal fading in the home from low power routers to higher carrier (5GHz vs 2.5GHz) to reflections in weaker areas called Ricean Fading. This is compensated by Router choice, router setup in WiFi using browser, location near reflected surfaces including walls, floor, mobile WiFi auto equalization which attempts to correct errors, retry or re-train channel, lower baud rate, switch channel in order to avoid some major reflections. But most often just moving a couple mm or more is all you need to get out of a deadspot. Deadspots are the reception of carrier signals delayed by half-wave length from any and every surface ( humidity and metals more than others) near the same amplitude. So in the fringe zones deadpots will drop 5 to 15 dB just by moving 1 or 2 mm away from the deadspot.
https://www.speedtest.net.
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In addition, you can try adjusting the retry or re-training channel, lowering the baud rate, or switching channels to avoid major reflections or rely on the automatic setup. However, sometimes simply moving a few millimeters can be enough to escape a deadspot, as these are many tiny zones where carrier signals are delayed by half-wavelength from surfaces and experience significant signal degradation. The one wavelength that cancels out is a relative term and can be a broad range within the 10M, 20M, 40MHz etc band at 2400 MHz or 5GHz. Where the signals is strong reflections are not a problem, it is where the signal is weaker ,then reflections are every half wave from the damp wall in the bathroom.
Anecdotal
In a previous home when we moved in, we could not get a modem hookup for 5 days. So I found a home several doors down that I could connect with by resting a laptop on a table near the rear window and aiming a large tree of moisture-filled leaves to reflect the signal and get internet when the signals were weaker 20 yrs ago less likely to all be locked up. If I moved the laptop moe than 1 degree or 1mm , I would lose the signal. So it can be used additively but usually is just a nuyciance with droputs.
In terms of Wi-Fi bandwidth, there are different combinations of 20 MHz bands that can be grouped to achieve higher bitrates. However, to take advantage of these higher bitrates (>54 Mbps), you generally need to be in close proximity to the router. If you prefer a more reliable connection, you can choose the lowest common rate, such as 11 Mbps on the 2.4 GHz frequency. Factors such as Friis Loss, which is the loss in signal strength over distance, and Shannon's Law, which defines the relationship between bandwidth and signal speed, play a significant role in Wi-Fi performance. Furthermore, Ricean Fading loss becomes more dominant with greater distance and more reflections, causing signal degradation.
If your Wi-Fi speed drops below 54 Mbps or 11 Mbps, it indicates that there may be issues with power levels, echoes, reflections, location, or antenna orientation. Achieving speeds of 150 Mbps or using 5GHz frequency can be challenging unless you have a clear line of sight to the router, and it may not perform as well as the 2.4 GHz frequency in certain environments.
In the past, a Windows application could be used to plot signal levels in dBm while slowly moving objects or walking in the signal path. This method allowed for visualizing signal strengths and potential deadspots.
Factors such as Ricean Fading (reflections), co-channel collisions, and the higher threshold of the 5GHz frequency contribute to Rx noise sources. When the 2.4GHz carrier signal drops below -75 dBm, it becomes easier for the signal to drop below the demodulator noise threshold. The SNR (Signal-to-Noise Ratio) threshold depends on various factors, including router power, mode, channel, bandwidth, selection criteria in setup, antenna orientation, location near walls, mobile device orientation, motion near deadspots, etc. To determine if packet errors are occurring, you can conduct tests such as transferring videos on the LAN or using websites like speedtest.net.
When excessive retries, dropouts, or lower efficiency are observed, the mobile device automatically adapts its algorithm to correct the issues by changing the channel bandwidth (20, 40, 60, 80 MHz) through manual or automated methods selected in the router setup. Experienced users can also improve the router's power, central location, and Wi-Fi driver setup to enhance performance.
Router setup is typically done through a web page accessed via the local gateway. Mobile devices can be configured to change from the default automated setup, which can be useful in weak signal areas.
The theory for line-of-sight transmission without ground reflections can be computed using Friis Loss calculators, which show that the loss is proportional to frequency and inversely squared with distance. However, higher data rates with compressed bandwidth require stronger signals due to the Shannon-Hartley law. Indoors, the effects of multipath fading, known as Ricean Loss, dominate Wi-Fi communication since signals reflect to some extent for all frequencies and rates, albeit at different positions. The ratio between the Wi-Fi signal (S) and Ricean Noise (N), known as S/N or SNR, is crucial. Technically, it is the Carrier-to-Noise ratio (CNR) that gets demodulated to the baseband signal SNR. Outdoors, the range between transmitter (Tx) and receiver (Rx) is limited by the receiver's white noise level threshold. The signal is calibrated when not moving to equalize the losses and phase shifts caused by echoes or reflections.
When high error rates occur, the modem in the mobile device should step down the data rates to tolerate a slightly lower SNR. For example, good performance can be achieved with a 2.4GHz 54 Mbps signal when the signal strength is above -75 dBm. If it falls below -80 dBm, you can try adjusting antenna locations or stabilizing a laptop, as even a 1 mm change can make a 6 dB difference in signal strength (e.g., from -82 to -76 dBm), potentially improving the data connection from little or no data to almost error-free.
If the aforementioned methods do not improve the situation, you can consider adding an old router to connect to the new router/modem. This setup can act as an extender or provide dedicated coverage to another area, such as basement corners or the third floor, by utilizing both 2.4GHz and 5GHz options and selecting the best configuration.
Reflections are present everywhere and can be particularly problematic when using a low-power modem/router or when the router is not centrally located due to other computer connections.
