Web Site for U. S. Station Signal Strengths - A Valuable Resource
 
Mark Connelly, WA1ION - 23 JUN 2005
 
In a recent IRCA DX Monitor, a Web site "http://www.v-soft.com/ZipSignal/zip_answer.asp" was stated to be a way to get a strength listing of all AM & FM groundwave signals over 50 dBu level for a given US postal zip code.  This is an excellent research tool, right on par with "dxtuners.com" in saving the DXer a lot of time as well as the cost of gasoline and airline tickets.  The site only lists US stations (no Canada, Cuba, Mexico, etc.)  The strength listings are of particular interest to DXers who wish to study propagation over seawater versus over land, to see how effective stations' directional patterns are, to evaluate the directivity and efficiency of their various receiving antennas, and to determine the sensitivity of portable and base-station receivers.
 
Signal levels are given in dBu and mV/m.  The lowest level given, 50 dBu or 0.32 mV/m, is still a good strength signal by DXer standards, likely well over S-9 on a Beverage or amplified loop, but not necessarily suitable to an average listener possibly driving through areas with noisy power lines.  Signals over 75 dBu or 5.6 mV/m would generally be considered local quality.  Anything over 100 dBu or 31.6 mV/m is superlocal, able to penetrate into most steel-frame buildings.  At Hull, MA (zip code 02045) the local fire-breathing dragon WBZ-1030 registers a hefty 131.4 dBu or 3735.3 mV/m, probably enough to peel the paint off the boats at the nearby marina.
 
By obtaining a listing of signal strengths that is reasonably accurate for a given location, the values can be used as a "transfer standard" to calibrate a receiver's S-meter readings with a given antenna.  If the antenna is something well suited to field work, such as an active whip, S-meter readings taken on roadtrips can, to a fair degree of accuracy, be converted back to approximate dBu and mV/m values.  A good calibration Excel table would be done at a number of frequencies since the efficiency of a receiving antenna can vary across the dial.  Active whips are typically somewhat less sensitive at the low end (530 kHz) than at the high end (1700 kHz). 
 
The closest post office here in the Pinehurst section of Billerica, MA is zip code 01866, located 0.6 miles / 1 km from my house.  The readings for that zip are, therefore, quite accurate here.  Here are the top ten AM's based upon daytime level.  The number not in parentheses is dBu, the number in parentheses is mV/m.
 
1. WRKO-680 day: 115.1 (569.62); night: 115.0 (564.53)
2. WEEI-850 day: 84.5 (16.83); night: 83.8 (15.54)
3. WWZN-1510 day: 83.1 (14.27); night: 73.9 (4.97)
4. WEZE-590 day & night: 80.0 (10.01)
5. WBZ-1030 day & night: 78.3 (8.24)
6. WBIX-1060 day: 77.3 (7.33); night 58.4 (0.83)
7. WTTT-1150 day: 77.0 (7.07); night: 63.7 (1.54)
8. WRCA-1330 day & night: 73.1 (4.54)
9. WROL-950 day: 72.1 (4.01)
10. WILD-1090 day: 72.0 (4.00)
 
Another station worth mentioning is WCAP-980.  At night it turns on the directional-array soup and goes from a 68.4 dBu day value up to 77.1 dBu after dark.
 
From the above list you can see that WWZN, WBIX, and WTTT take quite a hit going from day to night configurations.  Of these, WBIX is the only one which actually reduces power.  In the Truro, MA listing WWZN goes up by 3.3 dB at night.  It's not surprising that WWZN is one of the strongest stateside signals into North Africa.  But go west past Framingham, MA and it's totally useless at night.
 
There are a couple of other Web sites that I use in conjunction with the signal-strength finder site.  One finds the distance between two zip codes.  In the following example, I checked the distance from Atlantic City, NJ 08401 to Chilmark (Martha's Vineyard), MA 02535.
"http://www.zipfind.net/Results.aspx?TYPE=DISTANCE&ZIP1=08401&ZIP2=02535"
Result was 238.68 miles, equal to 384 km.
 
The US Postal Service "http://zip4.usps.com/zip4/welcome.jsp" page can find a zip code based on city + state, or address + city + state.
 
The site shows what DXers already know in terms of signals propagating much farther over salt water than land for a given amount of signal strength reduction.  Examples below clearly illustrate this.  All stations selected are non-directional for fair evaluation purposes.
 
*** Case 1: WJDA-1300 Quincy, MA (1 kW)
 
At Billerica (Pinehurst), MA 01866
Distance 23 miles / 37.0 km (land)
52.7 dBu, 0.43 mV/m
 
At Bar Harbor, ME 04609
Distance 203 miles / 326.6 km (water)
52.8 dBu, 0.44 mV/m
 
Distance ratio 8.8
About the same level at each site.
 
*** Case 2: WMID-1340 Atlantic City, NJ (1 kW)
 
At Port Elizabeth, NJ 08348
Distance 29 miles / 46.7 km (land)
50.6 dBu, 0.34 mV/m
 
At Chilmark, Martha's Vineyard, MA 02535
Distance 239 miles / 384.6 km (water)
50.5 dBu, 0.33 mV/m
 
Distance ratio 8.24
About the same level at each site.
 
*** Case 3: WCBS-880 New York (City Is.), NY (50 kW)
 
At Deer Park, LI, NY 11729
Distance 25 miles / 40.2 km (land)
71.2 dBu, 3.64 mV/m
 
At Falmouth, Cape Cod, MA 02540
Distance 172 miles / 276.7 km (mostly water)
74.5 dBu, 5.33 mV/m
 
Distance ratio 6.88
WCBS is 3.3 dB stronger at the more distant site.
 
*** Case 4: WLYN-1360 Lynn, MA (1 kW)
 
At Billerica (Pinehurst), MA 01866
Distance 14 miles / 22.5 km (land)
58.0 dBu, 0.80 mV/m
 
At Harwich, Cape Cod, MA 02645
Distance 70 miles / 112.6 km (partial water)
58.7 dBu, 0.87 mV/m
 
Distance ratio 5.0
WLYN is slightly stronger at the more distant site.
 
The above cases show dramatic differences in paths over land as compared to over salt water.  It must be stressed that the differences can be significantly greater than shown because the web page just gives strengths over the average extent of a postal zip code.  These values more often typify the central business district of a given town rather than an optimum beachfront site that would show a much more exaggerated signal enhancement.  Much of what applies to groundwave also relates to low-angle skip such as would be experienced with Transatlantic signals arriving before, during, and up to an hour after local receiving-site sunset.  The effect is very obvious when stations such as Morocco-612 can "bulldoze" WGIR-610 NH on an ordinary car radio at sunset at Granite Pier in Rockport, MA.  If you add seriously directional receiving antennas such as a Kaz, Flag, Beverage, or phased verticals to an already-directional location, then the DX really gets interesting as many of us know.