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When an AM radio station transmits its signal through the transmitter and out to the tower(s), there are two signals that are created.  One is called the 'groundwave' signal as it is literally the signal that travels across the ground, radiating out from the antenna(s).  The second kind of signal leaves from the transmitting antenna(s) and travels into the atmosphere.  This is known as the 'skywave;' signal.  This skywave signal can either be absorbed by the atmosphere, or it can refract/reflect back to the surface of the Earth and allow it to be heard in receivers far from the original transmitting location.  Which result we experience depends entirely on the time of the day!

During the daytime, the sun heats the Earth's atmosphere and creates layers within the atmosphere that are only present during the daylight hours.  For the purposes of AM daytime propagation, the main layer we are interested in is the "D-layer".   The D-layer forms and retracts on a daily cycle starting two hours before local sunrise and finally disappears entirely roughly two hours after local sunset.  It is much stronger during the summer months, due to longer periods of sunlight creating a stronger layer.  This D-layer absorbs the skywave signal of AM radio stations and does not let them refract or reflect back to the surface. 


As such, this leaves only groundwave as a means to propagate a radio station's signal to their audience during the daytime.  Because of this, signals do not travel as far during the daytime as they do at night, meaning radio reception during the day is usually limited to those stations in your local area and those within a 100-300 mile radius.

Many factors will influence how far a groundwave signal travels;  the transmitting power of the radio station, the antenna pattern they use, the elevation and surrounding topography and even the conductivity of the ground itself!

Ground conductivity is an immensely important factor in determining how far a groundwave signal will propagate from the  transmitter. As a grounwave signal travels across the ground after leaving the transmitter, it immediately experiences loss of signal strength due to ground absorption.  However, if the ground over which the signal travels is highly conductive, meaning it is a good conductor of electricity and electromagnetic waves, then there is less loss and more of the signal is able to travel further.

The absolute best ground that a signal can travel over with the least amount of loss is saltwater.  This is why coastal locations are able to receive groundwave signals from much further away than normal. 


As an example, once on the South Carolina coast, I was able to receive AM stations from Virginia, Washington D.C., New York and deep into Florida while sitting on the beach in the middle of the day. Had I traveled a few miles inland, those signals would have disappeared, it was only because I was sitting along ocean saltwater that these signals were receivable at this time of the day.  For this reason, many DXers refer to beach locations and the ocean in general as a 'saltwater amplifier'. 

Even freshwater is a great conductor of radio signals.  So DXers that live along large lakes or even rivers can notice a signal boost from the water.

Further inland, DXers that live in areas that have ground that is highly conductive such as in the fertile Midwest area of the United States will see groundwave signals travel much further than DXers in less conductive areas (the vast majority of the East Coast states for instance have poor ground conductivity).

The map below, from the FCC, demonstrates the varying ground conductivity of the various areas of the United States.  The higher the number, the more conductive the ground is and therefore an expectation of better groundwave propagation.

You can even download a .zip file of the ground conductivity maps for the entire United States, in small sections, by visiting the FCC page here.  As you can see from the map above, parts of Kansas, Oklahoma and up into the Dakotas have the highest amounts of ground conductivity in the country.  As a result, any groundwave signal that originates in these areas will travel further than they would over a less conductive terrain.  Also, any groundwave signals that originate from outside of this area but make it to this higher ground conductivity region will continue to travel with less signal loss due to the higher ground conductivity.  


The signal strength of a groundwave signal is usually displayed in what are called 'contours'.  The contours reflect the predicted signal strength within a geographic area based on the transmitting power, antenna pattern, ground conductivity, elevation and topography, etc.  Most stations use three contours for determining their listening audience: the 2.0 mV/m signal makes up what is known as the 'local' contour and is the area where the station can be heard the loudest.  This is the main service area of the station.  The 0.5mV/m signal or the 'distant' signal, represents the secondary market/audience of the sation.  Finally,  the 0.1mV/m signal or 'fringe' signal, constitutes areas that are largely considered 'out-of-market' for the station but are able to receive them on a regular basis any way. 


The coverage map, below, from 750 WSB in Atlanta, Georgia shows the predicted coverage of all of these contours with their daytime, groundwave signal.  The innermost area with the numbered counties represents the 'local' contour.  




In the audio samples, below, we hear radio stations received during the daytime from New Orleans, Louisiana in each of the contours.  

The first station, in the 'local' 2.0mV/m contour, is WYLD, Hallelujah 940, located in New Orleans.Notice how loud the station is.  There is no fading, no static.

Our next station, in the 'distant' 0.5mV/m contour, is 1320 WRJW, located in Picayune, Mississippi. The station is much weaker now, but still audible.  There is noticeable static in the signal now.

Our final contour, the 'fringe' 0.1mV/m contour, is  exemplified here by 1330 WEBY, located in Milton, Florida just outside of Pensacola. The station is a bit weaker than WRJW, above.  However, one thing that helps is that WEBY is located on the Florida coast.  As such, there is enough saltwater between WEBY and the receiver in New Orleans to keep much of the signal intact.  That is not the case with our final example.

In the final audio sample, below, 810 WSJC in Magee, Mississippi ​from which New Orleans is located well within their "fringe' reception contour and is located on a more inland location than WEBY, above.  As such, while still audible, it is not as consistent.  Some of the wording is a bit more difficult to make out and we have a considerable amount of static in this signal compared to the others.

WYLD - New Orleans, LA - 2/24/20 - 16:00 CST - Daytime Local Reception
00:00 / 00:25
WRJW - Picayune, MS - 2/24/20 - 16:00 CST - Daytime Distant Reception
00:00 / 00:17
WEBY - Milton, FL 2/24/20 - 16:00 CST - Coastal Daytime Fringe Reception
00:00 / 00:34
WSJC - Magee, MS - 2/24/20 - 17:00 CST - Daytime Fringe Reception
00:00 / 00:19


As the sun begins to set each day, the D-layer of the atmosphere begins to weaken and disappear.  As such, skywave signals that were previously being absorbed by this layer of the atmosphere are now traveling to the much higher F-layer where they are able to be refracted/reflected back to the surface of the earth.  Additionally, during the daytime, the F-layer actually splits into two laters, an F1 and F2 layer, with the F2 being the higher of the two.  As the sun sets, these two layers merge into one and create the F-layer that AM radio signals find with their skywave signal.

This cycle occurs 365 days of the year.  The creation of the D-layer in the morning, the splitting of the F-layer into two layers and then in the evening when the D-layer disappears and the F-layer combines into one. These atmospheric mechanics are driven buy our sun and as long as it continues to burn in the sky each day, this process will continue.

At nighttime, since signals are now free to travel much further with their skywave signal, a couple of things come into play.  One, your radio dial will be filled with stations on practically every frequency.  Where during the day you may have had frequencies with no signals at all, the dial is now filled with one or more signals on each frequency!  This is because during the daytime, there may be no stations located in your area on a frequency, but at night, the signals of stations much further away are able to make it to your receiver.

A great example of this is 750 kHz.  In my area, we have no stations on this frequency.  Being located in South Carolina, the fringe signal of 750 WSB as shown in the coverage map above extends into a large portion of South Carolina, just not all of the way to my location.  Since we are located that close to the fringe signal, no stations can be in my area on 750 or they will cause interference for some listeners trying to listen to WSB.  So, during the daytime, 750 is dead with no signals (once during the winter, I thought I had a faint copy of WSB, but nothing strong enough to positively identify).  At night, though, WSB dominates the frequency as heard in the audio sample, below:

Thanks to SDR technology, we can actually now SEE the difference between the AM band during daytime compared to nighttime.  The larger number of signals is actually visible on the SDR 'waterfall'.  The time-lapse video below, provided by SDR Musings, shows very clearly the effect D-layer absorption has on the mediumwave band, by limiting the number of signals of available stations.  Then, around 6pm (which is his local sunset), you see the number of available stations explode as the D-layer disappears and skywave propagation takes over.

You can also see the impact of skywave signals by reviewing the coverage maps of radio stations for their nighttime signal (those that are not daytime-only, of course).  Below, is a coverage map from 870 WWL in New Orleans, Louisiana.  The map on top shows their daytime coverage which basically hugs the Gulf Coast ifrom parts of Texas into Florida.  Again, the saltwater of the Gulf of Mexico helps propagate the signal over such a wide area here, in addition to the southern Louisiana area having relatively high ground conductivity (they are a 15 on the FCC maps, linked above).  

However, look at the second map, this is the nighttime skywave coverage map.  Further, that is just showing what is considered the 0.5mV/m skywave signal, their .1mV/m fringe skywave signal stretches out much further than that.  Indeed, WWL can be heard over much of the nation and is often received by stations well into South America, Europe, down into Australia, etc.  

There is a Web site (link on left of this section, or by clicking here) called Radio Locator that can provide daytime and nighttime coverage maps of radio stations in the US.  You can also get them from the FCC using their single -frequency maps, although I have noticed these can be hit-or-miss in their completeness and accuracy.

Skywave signals can travel hundreds, even thousands of miles depending on conditions, your receiving setup, the frequency you are listening on, among other factors.  If you want a great indicator of just how far a skywave signal can travel, try tuning in to one of the "clear" channel frequencies shown below.  These are your best bet to hear far away stations as they are often limited to 1-3 stations at night on that frequency of any considerable power or non-directional antenna pattern.  All other stations on that frequency (and the adjacent frequencies) must 'protect' the designated clear channel stations so as not to cause any interference with them.

The frequencies below are considered both a 'clear channel' frequency and have stations with 'class A' protection from interference​.  They usually contain 1-2 50,000-watt non-directional stations (usually one East Coast, one West Coast).  As such, they should yield stations from significant distance from most DXers:

US Class-A clear channel frequencies:

  • 640

  • 650

  • 660

  • 670

  • 700

  • 720

  • 750

  • 760

  • 770

  • 780

  • 820

  • 830

  • 840

  • 870

  • 880

  • 890

  • 1020

  • 1030

  • 1040

  • 1100

  • 1120

  • 1160

  • 1170

  • 1180

  • 1200

  • 1210


Canadian Class-A clear channel frequencies:

  • 690

  • 740

  • 860

  • 990

  • 1010

  • 1580


Mexican Class-A clear channel frequencies:

  • 730

  • 800

  • 900

  • 1050

  • 1220

  • 1570


Bahamian Class-A clear channel frequencies:

  • 1540

The frequencies below contain stations that receive class 1-B protection.  That means they have multiple stations that are 50,000 watts, but will carry directional antenna patterns to avoid intefering with one another.  1110 kHz is a great example with WBT in Charlotte, North Carolina and KFAB in Omaha, Nebraska.  Both are large 50,000 stations but at local sunset, will change their antenna patterns to keep from causing interference with each other

US Class 1-B clear channel frequencies

  • 680

  • 710

  • 850

  • 1080

  • 1110

  • 1170

  • 1500

  • 1510

  • 1520

  • 1530

  • 1560


Shared US/Canadian/Mexican Class-1-B clear channel frequencies:

  • 540 (Canada and Mexico)

  • 940 (Canada and Mexico)

  • 1000 (US and Mexico)

  • 1060 (US and Mexico)

  • 1070 (IUS and Canada)

  • 1090 (US and Mexico)

  • 1130 (US and Canada)

  • 1140 (US and Mexico)

  • 1190 (US and Mexico) and 1550 (US and Mexico)


While skywave can bring stations from far distances, the period around local sunrise and local sunset, called greyline, can produce very interesting propagation as it can contain both groundwave (which is able to travel further due to enhancement from the setting/rising sun) and skywave propagation.




WSB - Atlanta, GA - 12/2/19 - 06:00 EST
00:00 / 00:29

Starting two hours before local sunrise/sunset and running until roughly two hours after local sunrise/sunset, we enter a period of time known as 'greyline enhancement.'  During this special time of the day, the D-layer begins its regression (afternoon/evening) or build-up (morning) while the two separate F-layers begin the process of merging into one (or separating back out again in the morning). As such, there are parts of the atmosphere where groundwave is the dominant propagation method and others where skywave is occurring.

But the atmospherics are not the only magic happening on the bands during this time of the day.  Greyline also holds a few other special tricks up its sleeve.

This period is called 'greyline enhancement' because signals caught within the dawn/dusk window actually see an enhancement in their signal levels all along that daylight terminator.  This occurs all around the world as the line between day and night races around the globe.  Along that line, signals are boosted by a sudden enhanced propagation path.  

A great example of this occurred for me recently on 1080 kHz.  I have been chasing 1080 WTIC in Hartford, Connecticut for nearly 20 years since I first heard them in Brasstown in the early 90s.  Since then, I just haven't been able to pick them out of a mess that includes signals from KRLD in Dallas, Texas, WHOO in Kissimee, FL, WKJK in Louisville, KY and WFTD in Marietta, GA.  I have tried listening at sunset before they change their antenna pattern to protect KRLD, I had tried in the morning when they should be changing back to non-directional.  Always, nothing but the usual stations.

Then, once December early afternoon, my SDR recording captured a weak but unmistakeable ID from WTIC.  I went back and reviewed the data and the greyline angle just happened to have Hartford in it at that time, so I was hearing an enhancement of the WTIC signal because of the greylinev that allowed me to hear them!  The audio samples, below, are what I caught.  Do you hear the ID?

The greyline itself is not the only enhancement.  Sunset and sunrise are also the times that the FCC uses to dictate when a radio station must either begin/end their broadcast day (if they are daytime-only) or change their power/antenna pattern (because they have differences between day and night power/pattern). 


In the old days, a US DXer could park on a frequency such as 1580 and listen to daytime-only stations rise up with greyline enhancement, catch their sign-off complete with national anthem, and then disappear.  This would continue for hours after your local sunset as the greyline terminator traveled across the country.  

These days, there aren't as many daytime-only stations so a signoff isn't as prevalent.  However, you can still notice stations that rise to the top of the frequency with their greyline enhancement, then disappear as they change power/pattern, leaving the frequency open to other stations left behind.  While this can happen on most regional or local frequencies, I have continued to have particular luck on 1580 and 1570.  I have more stations logged on 1580 than on any other frequency!

WTIC - Hartford, CT - 12/3/19 ID after traffic
00:00 / 00:27
WTIC - Hartford, CT - 12/3/2019 - TOH ID
00:00 / 00:40
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