Re: [Swprograms] (ot) Fw: [roadgeek] Distant radio stations

[Joe Buch <joseph.buch@xxxxxxx>]

At 01:13 PM 7/20/04 -0400, Richard Cuff wrote:
>   We were yakking over in a "road geek" e-list regarding FM tropo / 
>E-skip.   John Mayson (who had a scanner article in MT a few months back)
>was  wondering about salt water's affect on FM.   You can see my quote
>below...I know salt water (and fresh water) greatly  aid groundwave
>propagation for mediumwave and longwave signals...do they do  anything for
>VHF and UHF frequencies?     

It is not the salt water that enhances the probability of extended VHF and UHF propagation over the ocean.  It is the temperature differential between the air immediately above the water and the air higher up which has been heated over adjacent shore land.  The normal thermal model has the air cooling as one goes higher in altitude.  When the upper air is warmer than the lower air, conditions exist to refract the signal back to earth.  Theoretically tropospheric refraction can happen at any frequency between DC and light.  That's why we see mirages of distant objects over the sea at times.  You can get the same effect over big fresh-water lakes.

I live at sea level in coastal Delaware about a mile inland from the Atlantic.  We often have thermal inversions here caused by warm air blowing from the west over New Jersey and then passing out to sea.  During the summer, I regularly watch channel 55 from Riverhead Long Island, channel 28 from Providence RI, occasionally see channel 21 from Nassau County Long Island, but never stations from New York City.  The path to New York City from here is over the New Jersey land mass but the path to eastern Long Island, Providence, and Boston is over the ocean with a layer of warm air above the cold air.

We also see on occasion see the same kind of inversions over Delaware Bay and Chesapeake Bay which often bring in stations from Philadelphia, Baltimore, Washington and Norfolk.  I have seen stations from as far west as Hagerstown, MD.  All of this is on a rabbit ears antenna on VHF and an indoor bow-tie on UHF.  I can remember times where every channel on the UHF dial had a signal from someplace

When I was a kid I worked at a beach just east of Jones Beach Lot 9.  I remember seeing hazy pictures of the Empire State Building sticking up above the horizon on hot muggy afternoons.  It was my first mirage.  I could see the building down to abot the 30th floor which was well below the horizon at that distance.  In this case there was cool air blowing from the south under the warm air above Queens and Nassau Counties.  The same inverted temperature profile was causing tropo bending at optical frequencies.  I worked in the deserts of southern California and New Mexico for about 15 years.  I saw many a mirage while driving to work as the rising sun heated the upper air while the lower air remained in shadow of a mountain ridge.

I often wonder if HF propagation is modified by such tropospheric bending effects.  Theory says yes it will be if the height of the warm air is high enough in terms of HF wavelengths.


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Joe Buch
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