About two-and-a-half years ago I wrote:
Recently I came across a published dissertation that studied the propagation of VHF signals over water in a very scientific and systematic way, looking at signals coming across the English Channel. I tucked it away for further study on some cold winter evening...
A bit later than I expected, I revisit the article I mentioned above, a research paper presented by Chow Yen Desmond Sim, a Ph.D. candidate at the University of Leicester in 2002 and presently a professor in the Department of Electrical Engineering at Feng China University in Taiwan. (You can find the entire 202-page article on-line as a PDF
for your own reading pleasure.)
The paper presents research conducted on the propagation of VHF and UHF signals over seawater. (The then) Mr. Sim collected a great deal of careful measurement of signal levels that propagated over two paths of open and unobstructed ocean between three locations in the Channel Islands of the United Kingdom. As might be expected, his paper contains a great deal of mathematical descriptions and details of his experiments, but otherwise it is quite readable, particularly the introductory portions and the narratives describing the behaviors observed.
The method of testing employed two transmitting locations which directed signals at a third receiving location, with the distances involved as 33-km and 48-km (20.5 and 29.8-miles). The antenna heights were fixed at approximately 15-meters above the mean height of the sea. The tide variation in the area typically averaged about 5-meters, but the lowest tide was only 0.5-meters and the highest tide was 10-meters. As a result, the data collected includes a variation in the antenna heights at both the receive and transmit sites, from about 10-meters (33-feet) above the sea at highest tide to about 20-meters (66-feet) above the sea at lowest tide. This data provides a method to investigate the influence of antenna height on the path loss. Dr. Sim found there was a linear correlation between signal strength and tide height (and thus antenna height). He noted:
5.2.2 Calm sea (Jersey to Guernsey)
The first major effect observed from the sea parameter...[is]...the signal amplitude (in dB) displays approximately linear relationship with the tide height (in metres).... Lower signal amplitude was observed at high tide with a reduction of approximately 1.1 dB per metre increase in tide height...
To reword slightly, when the effective antenna height above the sea was decreased (because the tide height increased) the signal level decreased in a linear manner, with the relationship of a reduction in signal of 1.1 dB for each meter decrease in antenna height above the sea.
The Summary section of the paper repeats these observations:
During times of calm sea state for both, Jersey-Guernsey and Jersey-Alderney showed a linear relationship between the signal amplitude and the tide height although the Jersey-Alderney data showed some fluctuation in the signal across the linear region. The signal amplitude decreases as the tide height increases and this accounts for approximately 1.1dB in signal amplitude reduction for every metre of tide height increase.
In the Conclusion section of the paper these observations are again mentioned:
Chapter 7: Conclusions / recommendations and suggestions for further work
The objective of this thesis was to investigate the current understanding of over-sea VHF/UHF propagation at different path lengths, antenna heights, season and weather conditions...
Measurements from both receiving sites showed a linear relationship between the signal amplitude and the tide height (approximately 1.1 dB in signal amplitude reduction for every metre of tide height increase). This is consistent with the smooth earth diffraction theory obtained from ITU-R P.526-7, which suggested that smooth earth diffraction (especially over sea) is the dominant propagating effect for distance below 40 km [25-miles] between the transmitter and the receiver.
My interpretation of these observations and conclusions: increasing antenna height above the sea for VHF Marine Band communication is an effective method to improve the strength of your transmit signal as received by other ships when the distances are in the range of 20 to 30-miles. Even with antennas that were already mounted substantially above the sea at 66-feet, decreasing antenna height by 3-feet (at both ends of the path) caused a noticeable and consistent 1.1 dB loss in signal.
Note that for the VHF signal Sim used a frequency of 248.375-Mhz. I suspect this frequency was chosen because it would be free from other signals and was available or authorized for this experiment. With a bit of thought one can see that conducting the experiment using a frequency in the VHF Marine Band would not have been particularly wise, as transmissions from other ships could have interfered with the tests, and authorization to transmit would have been likely very difficult to acquire. I believe the ITU allocation in Region 1 for 248.375-MHz is to Fixed-Mobile services. Perhaps a special temporary authority was obtained to use that frequency from the U.K. for the experiment's duration. The methodology was quite extensive, including testing of both vertical and horizontal polarization, with paths involving co-polarization and cross-polarization, and with signals with both amplitude and phase modulation.