I see mention on the GAIA web site [G] that @avi is involved with, that the tides can affect the Internet service. This is something that we see in our remote wireless networks in Scotland too. This interesting phenomenon is worth a thread.
Here is is such a thread... (1/n)
It turns out that even obstructions *near* the straight line between both ends can cause problems. A signal from one end can reflect off of the obstruction and arrive at the other end. This reflected path is longer than the straight line.
It matters how much longer it is. If it is exactly one wavelength longer, or two, or three, the reflected signal will reinforce the direct signal and it will appear stronger. However, if it is half a wavelength, it will interfere and cancel the signal.
In between, the signal will be affected more or less. If the obstructions are complicated, the reflections will cause the received signal to get all jumbled up.
There is a sequence of ellipses corresponding to the phase difference (even multiples of 180 degrees means whole wavelengths, odd multiples mean half wavelengths) called Fresnel zones where this happens.
But a very interesting thing happens in mountainous coastal areas and islands with significant tides...
Because radio waves do not travel through the hills, we tend to make wireless links between them. Sometimes over water.
There is a nice echo of the traditional patterns of communication here. Before we built roads through the mountains, it was easier to visit your neighbours by boat. The social structure was essentially tied to the water. Some places on the mainland only got connected to the road network in the 1970s and this radically changed their character. A few are still not connected.
But that's a digression.
Radio waves reflect off of the water. If the water is in the zone where reflections can adversely affect the signal (which depends more than anything on the height of the antennas and the distance between them) what we see is the length of the reflected path changing as the tide rises and falls. Sometimes perhaps it is outwith the Fresnel zone. Sometimes it is within it.
And so, signal strength, or even existence of a link, will change with the tides.
It is even possible, because the wavelength depends on the frequency, to change to a different frequency in anticipation of the water level adversely affecting the one in use. This "slow frequency hopping" is hard to get right but in principle it could work.
Or, we can consider the problem in reverse and use the signal strength of the radio links to measure the level of the water to within a few cm! So long as the sea state isn't too exciting.
@rattlesden yes, it would do. tide wouldn't matter because both ships are (ideally) floating on the water, but unless the ships are both extremely tall (with antennas 50-70m up) there will definitely be reflections off the water.
something in the depths of my memory is telling me that the water is more or less mirror-like for signals at different frequencies and it may be that the 150Mhz or so of marine VHF is less affected for this reason, but i'm not sure.
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