Sea Gain: The Numbers Don't Lie
 
Mark Connelly, WA1ION - 13 JUL 2005
 
My article "Web Site for U. S. Station Signal Strengths - A Valuable Resource", dated 23 JUN 2005, described groundwave signal strength charts available at "http://www.v-soft.com/ZipSignal/zip_answer.asp". In that article I evaluated a few stations to see how much distance was required to reduce the strength of a given non-directional station to a particular level.  This analysis gave some results that seemed quite in step with what I've noticed when I've done my own signal strength observations from a number of inland and coastal sites in the New England states.

The distance required for a given amount of path loss turns out to be about 5 to 10 times greater over a salt-water route as compared to across flat to slightly hilly land of fairly low conductivity.

WJDA-1300 (Quincy, Massachusetts: 1 kW) has approximately the same signal strength at Bar Harbor, Maine (distance 203 miles / 327 km over water) as it has in the Pinehurst section of Billerica, Massachusetts (distance 23 miles / 37 km over land).  This represents a ratio of sea distance to land distance of 8.8.  This is fairly typical for that part of the dial.  Ratios of over 10 are possible, especially if you're dealing with the 1300-1700 kHz range and making the over-water strength measurements right at the shoreline instead of using the average value for a given coastal town (which may include inland areas).  On the lower end of the dial, ratios are more like 5 to 7, but are still significant.  If a shoreline is backed by mountains (as in the Pacific Northwest) instead of relatively flat land, the over water versus over land ratio could be much more extreme, perhaps something like 20 above 1300 kHz.

The next phase of research was to see what the signal strength difference (in dB) is for two nearly-identical length paths, one over seawater and one over land.  From the formula Pmult = 10^(dB/10), one can compute the amount of power that would be needed to produce the same signal strength on the land route as can be produced with the station's normal power on the sea route.  The "Pmult" power multiplier, e.g. 100 for a 20 dB strength difference, times the power, e.g. 1 kW, shows (in this case) that the station would have to run 100 kW to have the same signal strength over land that it has on the same distance over water with just 1 kW.  A 30 dB difference gives Pmult=1000, meaning that a megawatt would be required to get the same signal over a given amount of land that a kilowatt gets across the same expanse of sea!  This may seem beyond belief, but the tabulations below (from ZipSignal data) show that 20 dB is a rather ordinary difference to find here in the New England area.  30 dB is certainly not out of reach on the upper end of the dial, especially if signal strength data for the coastal site is based on beach, rather than town-center, results.  As stated earlier, a West Coast or Hawaiian location involving a mountainous rocky coastline could give even more extreme values.  The over-water signal strength of a 1 kW station may not be replicable over the same land distance at ANY power level of which present transmitter technology is capable.

Cases below involve stations having non-directional, or nearly non-directional, daytime antenna patterns.

Case #1
Freq = 730
Call = WJTO
TX QTH = Bath-Brunswick, ME
Power, kW = 1
Approx. TX-RX Distance, mi. = 29
Approx. TX-RX Distance, km = 46.7
Coastal RX site	= Cape Elizabeth, ME
Inland RX site = Augusta, ME
Coastal RX level (dBu) = 73.9
Inland RX level (dBu) = 55.8
delta dB = 18.1
power multiplier = 64.57
comment: A 15 to 20 dB delta is common on the lower end of the dial.

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Case #2
Freq = 950
Call = WROL
TX QTH = Saugus, MA
Power, kW = 5
Approx TX-RX Distance, mi. = 14
Approx. TX-RX Distance, km = 22.5
Coastal RX site	= Hull, MA
Inland RX site = Billerica, MA
Coastal RX level (dBu) = 90.4
Inland RX level (dBu) = 69.9
delta dB = 20.5
power multiplier = 112.2
comment: 561 kW to get the over land signal comparable to 5 kW over water !

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Case #3
Freq = 1230
Call = WESX
TX QTH = Marblehead, MA
Power, kW = 1
Approx. TX-RX Distance, mi. = 18
Approx. TX-RX Distance, km = 29
Coastal RX site	= Cohasset, MA
Inland RX site = Billerica (Pinehurst), MA
Coastal RX level (dBu) = 77.0
Inland RX level (dBu) = 54.3
delta dB = 22.7
power multiplier = 186.21

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Case #4
Freq = 1240
Call = WBUR
TX QTH = West Yarmouth, MA
Power, kW = 1
Approx. TX-RX Distance, mi. = 22
Approx. TX-RX Distance, km = 35.4
Coastal RX site	= Edgartown, MA
Inland RX site = Pocasset, MA
Coastal RX level (dBu) = 78.2
Inland RX level (dBu) = 52.8
delta dB = 25.4
power multiplier = 346.74
comment: A whopping difference!  The land route is over exceptionally dry sandy soil (scrub oak / pitch pine forest) that, although flat, eats up signal strength to a greater degree than the "loamier" soil in the Boston area.  Over 346 kW would be needed to put the same signal into Pocasset (north of Falmouth) as is produced a bit farther away in Edgartown (Martha's Vineyard) with just 1 kW!

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Case #5
Freq = 1300
Call = WJDA
TX QTH = Quincy, MA
Power, kW = 1
Approx. TX-RX Distance, mi. = 19
Approx. TX-RX Distance, km = 30.6
Coastal RX site	= Marblehead, MA
Inland RX site = Bridgewater, MA
Coastal RX level (dBu) = 75.3
Inland RX level (dBu) = 52.4
delta dB = 22.9
power multiplier = 194.98
comment: A typical delta dB for this part of the band.

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Case #6
Freq = 1360
Call = WLYN
TX QTH = Lynn, MA
Power, kW = 1
Approx. TX-RX Distance, mi. = 13
Approx. TX-RX Distance, km = 20.92
Coastal RX site	= Hull, MA
Inland RX site = Burlington, MA
Coastal RX level (dBu) = 82.2
Inland RX level (dBu) = 60.8
delta dB = 21.4
power multiplier = 138.04
comment: A small amount of land (Revere Beach / Point of Pines barrier) along the start of the coastal TX-RX route may reduce the coastal RX level (and resultant delta dB) slightly.

===

Some conclusions: The kind of differences seen in these groundwave measurements correlate quite well with differences seen in some kinds of foreign medium wave DX at night.  Some of the low-angle skip is as severely reduced by an overland path as groundwave is.  Long-term measurements of European, African, Middle Eastern, and South American DX signals taken at Rockport, MA and Rowley, MA on the coast north of Boston show differences of anywhere from 10 to 40 dB better than the same stations heard in inland suburbs when using the same set-up: Drake R8A receiver, active whip on car roof.  High-band longer-haul Africans and Brazilians heard during auroral activity, especially at sunset or earlier, show the greatest difference: often S-9 at the shore and usually less than S-3 (if even noticeable at all!) inland.  Six S-units is conventionally 36 dB.  That's a mind-boggling power multiplier of 3981.  This tells me that a 250 watt Brazilian "graveyarder" heard at the shore would have to be packing close to a megawatt to hit the same S-meter reading somewhere inland maybe only an hour's drive away.  Surprising though this might sound to some, I find this entirely believable based on experience.  When the signal path is nearly parallel to the shore, you can jump from a lengthy mostly over water route to a mostly over land one in a fairly short distance driving inland at a right angle to the coast.  This is the situation where the biggest signal changes occur.  One classic example of this is the exceptionally quick drop-off in signal strengths of the Atlantic City, NJ area stations (1020, 1160, 1340, 1400, 1450, 1490) when driving a very short distance up Route 134 in Dennis, Cape Cod, MA from Lower County Road (near southward-facing beaches) northward to Route 6.  These signals "drop like a rock", maybe 30 dB, over the course of a 10 minute drive, even though the change in actual geographic distance is infinitesimal.

It is hoped that those in other areas such as Oregon/Washington will try using ZipSignal in conjunction with their own measurements to see how these findings relate to their areas, both in terms of groundwaves and low-angle long-haul foreign DX.