Join Date: Aug 2006
Location: Arlington, VA
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The difference between, e.g., 4mHz, 6mHz, and 8mHz waveforms is primarily their LENGTH. Also, as Steve pointed out, their BEHAVIOR is different because they penetrate and are reflected by different layers in the ionosphere.
A 6.227 mHz wave is 936/6.227 = 150 feet long. A 8.152 mHz wave is 936/8.152 = 114.8 feet long.
Shorter waves tend to be more appropriate for communication over longer distances, particularly in daytime. If, for example, you were in the Bahamas and wished to contact a station in Europe during daytime, you'd be better off trying on 12mHz or 16mHz frequencies than lower ones. At night, however, it could very well be that 6mHz or even 4mHz would do the trick.
That said, the reason you are seeing differences in reception in a marina may have VERY LITTLE to do with wavelength and propagation. Rather, it's likely to be associated strongly with RFI...that generated on your own boat, on nearby boats, and by equipment belonging to the marina itself.