High frequency directional antennas are set up 14 km apart, 1.5m from water level [I recall them saying 1m on the video, but in the experiment note,
location and height, I've added below, it says 1.5m] . Both are on points along the coast of a large body of water, with sufficient coastal curvature for there to be only water between the two points. Thus, there are no objects or land masses obstructing them.
According to the experimenters, the curvature of the Earth over that distance, for an antenna 1 m above ground, would present an obstacle of 3.84 m. This should be sufficient to prevent the antennas from remaining in radio contact.
The above image illustrates what is described above. (The house is in the image as an example of an object 3.84 m high.)
I've read online that radio waves, especially small ones, would be almost entirely unaffected by gravity. Really large radio waves can bend ever so slightly around the Earth's curvature, beyond line of sight, but it's nominal. This would seem to be in contradiction to what this experiment has demonstrated.
Coordinate Info [Added Later]
On their web site (which I've looked up since posting this question, to get more exacting details), they state the locations of the two antenna as:
Team A: São Lourenço do Sul, RS 31 ° 22'42.37 "S 51 ° 57'40.79" W Team B: São Lourenço do Sul, RS 31 ° 30'0.91 "S 52 ° 0'26.88" W
I've checked, and the distance between those points is 14.24 km. I used this tool to check. Here is a screen shot of the result.
Radio equipment [added June 9]
The equipment used in the experiment was two sets of the following :
1 Radio Ubiquiti Bullet M5 HP Approved by Anatel to 400mW operating in the band 5800 Mhz
1 satellite dish Aquarius Approved by the FCC with 24 dB gain and 4 degrees of openness
1 UHF Radio HT
HT VHF Radio 1 with 2 dB omni directional antenna
1 radio HT VHF / UHF dual band
All other info on the experiment is detailed here.
Location and height
In the video cited, I got the impression one antenna was on land, and the other antenna was on a boat out on water. But looking at the locations given in experiment notes, it looks to me that both are at points along the coastline. I was not sure why that was. I've since seen this note on the experiment notes:
Note: Team B coordinate the currently appears on the water, but when in 2011 the experiment was conducted had a cove where the equipment was installed. The equipment of both teams were positioned at 1.5 meters the water level height.
What is the scientific explanation, using accepted laws of physics, to explain how these high frequency radio waves can make contact with the opposite antenna over a distance of 14 km?
Additional related info
A similar experiment was conducted by the same researchers, using a laser beam. It was transmitted across a distance of 33.78 km, at 1.5 m above water level. It too was successfully transmitted between the two points with that distance between them.