Greg wrote:
"Sea gain"? Sounds interesting - please explain. Once while
vacationing on the shore of the Gulf of Mexico, Anna Maria Island,
Florida, I was pulling in all kinds of DX at night on the 20 meter ham
band on my Radio Shack DX-440 portable - much more than I ever did at
home (inland).

The effect is far more dramatic on the MW band!

MW propagation is greatly affected by ground conductivity. FCC
regulation 73.190 (figure R3) is a chart of ground conductivity for the
United States. In New England and the NYC area where MW propagation is
notoriously poor, it ranges from 0.5 to 2mmho/m. Here in the South
where it isn't much better, figures run between 2 and 8. (that includes
Florida) In North Dakota where a single 5kw station can cover the
whole state, figures between 15 and 30 are common.

The chart considers the "ground" conductivity of seawater to be
5000mmho/m. (yes, that's 200 times better than the best land)

Two examples of direct observation:

- Wilmington, NC, at night. Car radio in the beach parking lot: Jamaica
blasting in at entertainment quality on 760 and several other
frequencies. I start driving inland; within a block the signals are
noticably weaker. Within a mile, they're barely audible. When I get
back to my motel about five miles inland, Jamaica is totally gone.

- Massachusetts, high noon, car radio about five miles inland west of
Newburyport. CHTN-720, Prince Edward Island, local quality. Drive
about 20 miles inland to Andover; CHTN gradually fades into nothingness.

Interesting that you observed this on 20m. Maybe less local
interference? The ground conductivity shouldn't have much effect on
shortwave -- greater *distances* (i.e., being within maybe 200 miles of
the Atlantic in Florida, rather than being within 900 miles here in
Tennessee) should make a big difference but I wouldn't think the
difference between Tampa and a Gulf island would be that dramatic.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com