porthole wrote:My Fell MOB fob will not shutdown the engines with either spray or heavy rain,
That behavior probably occurs because the criteria for engine shut-down probably requires more signals attenuation than the amount that occurs if the fob gets a bit wet.
porthole wrote:...nor will it shut down dropped into the wash bucket.
As for dropping in a wash bucket, I assume you mean a bucket with water in it. I would expect more radio signal attenuation to occur from total immersion of the fob in water. But the bucket is also anticipated to be still on the boat.
The effect of water molecules on propagation of radio waves varies significantly with the actual frequency. The relationship is the HIGHER the radio frequency, the HIGHER the chances of absorption of the energy in the radio frequency wave. This is due to the relationship between the wavelength of the radio wave and the size of the molecules in the intervening substance.
For more background, read this summary of an Institute of Electrical and Electronic Engineers (IEEE) paper"
Review of Temperature and Humidity Impacts on RF Signalshttps://ieeexplore.ieee.org/document/9299327Excerpt from IEEE Paper wrote:Signal strength of radio waves propagating from transmitter to receiver can be affected by atmospheric conditions even in line-of-sight situation. These atmospheric conditions cause the propagated waves to vary from the anticipated range. Higher frequencies can result in weak signals as they have shorter wavelengths. High frequency increases the chances of hitting obstacles such as water molecules in the atmosphere when radio waves propagate. This will cause a weakening effect as water absorbs some of the energy from the waves and scatters the rest of it.
Most boaters are already acquainted with this behavior of radio waves and water if they have ever heated any food in a microwave oven. The heating that occurs in a microwave oven is the result of (water) molecules in the food absorbing the energy of the radio waves being directed onto them. The frequency of radiation used in a microwave oven is generally optimized to cause water molecules to absorb the radio wave energy. The basis for this is examined in another paper,
Physics of the microwave ovenhttps://www.sfu.ca/phys/346/121/resourc ... _ovens.pdfIn the section under the heading
Heating of food in a microwave ovenAbsorption of microwaves by waterthe following explanation is provided:
from source cited above wrote:Electromagnetic waves may be absorbed by matter in many different ways depending on their
wavelength and the state of the matter (gas, liquid, solid)...solids and liquids may absorb microwaves due to the polarization induced by the external oscillating electric field. In a microwave oven, the electrically dipolar water molecules absorb most of the microwave energy.
The inference to be made from these observations: the frequency for the remote transmitter devices in a wireless engine shut-off safety switch system was most likely selected on the basis of not being particularly sensitive to being absorbed by water.
The actual frequencies in used by either manufacturer are not known to me, but I would expect that the frequencies used would need to be in a radio band that permitted such unlicensed use. Further investigation in to the particular devices should be able to discover what FCC-allocated radio band is being used.