2500 Miles With Only 1/2 MW on AM BCB

That got your attention didn't it? Now notice the power is 1/2 MW and not 1/2 mW. MW is megawatt and the station is the former WLW 500,000 Watt "Nation's Station".

There are many on line historical stories about this station but here is one which I had not seen before and it is a fairly interesting read, especially since it was published while the station was still operating at 500,000 Watts. It details some of the technical aspects of the station as well as other information about studios, etc. I found this at the Nostalgia Air website which is nice to visit if you are interested in old time radio equipment.

I used to drive by the transmitter site every work day going to and from work. The main tower is still there and is still in service as is the transmitter building and the "engineer's home". The cooling pond is also there but it is not used.

There were reports of the station's audio coming from fences and teeth fillings so perhaps these and other factors resulted in the station's power being reduced to 50 kW which is it's current power.

Hope you enjoy the piece of history.



kc8gpd's picture

there is currently a Tennessee fm running 100kW vertical 300kW horizontal polarization.

Carl Blare's picture

Wow I just woke up from more than an hour touring WLW on the amazing link provided by Radio8Z. It read like a Ray Bradbury other worldly story, except that it was all real and happened decades before the space race!

Seeing the five lovely WLW station models posing with man-sized power tubes has renewed my interest in starting the KDX Cheerleaders. I can hear them already: "Go Carl!"

Next was the trip to Nostalgia Air where I printed out the old Zenith Phono Oscillator schematic, but couldn't find the diagram for my RD7000Y Transoceanic, despite the hundreds of Zenith models listed.

Now here we are back in the future, with 1/10th Watt and no cooling pool.

Great links!

Carl Blare

Rich's picture

According to FCC groundwave propagation charts for 700 kHz, WLW at 50 kW produces a groundwave field of 0.1 mV/m at a distance of 275 miles, over a path conductivity of 8 mS/m. This is about the field intensity lower limit for useful, if somewhat noisy daytime groundwave reception on a good AM receive system in an area with low r-f noise levels.

With 500 kW and other things equal, the 0.1 mV/m contour moves out to 356 miles.

So an increase of 10X in radiated power produces a coverage contour increase of only 1.29X -- probably less than most would expect.

But, using 500 kW increased their coverage area by about 68%, which is significant.

radio8z's picture

Rich, I wondered about their range claim also. During daylight hours I have heard them in Bay City Mich., weak but readable, at a distance of approx. 320 miles.

Perhaps their 5000 mile diameter claim was based on nighttime range and they used to be a true clear channel station so there should have been no co-channel interference. Table top and console radios of that era had external antenna connections and probably functioned much better with wire antennas at pulling in distant stations than today's radios. There was also much less electrical noise in homes then. Another probability is that the pamphlet was written by the sales/P.R. folks who sometimes stretch things a bit.

Growing up 40 miles from the transmitter site I had heard personal accounts of the "singing fences" from some old timers when I was a kid.

Carl, this site has available lots of free manuals for download.


PhilB's picture

I agree that a 5000 mi. diameter is extravagant and is likely a marketing distortion reporting very unusual skip conditions.

On the other hand, the ground wave range doesn't tell the whole story either. Back in the '30s, the largest listening audience was in the evening hours. After "twilight" the MW frequencies experience fairly reliable skip when the ionosphere D layer stops absorbing and the E and F layers reflect the waves. You can pretty much say that skip is normal at night at 700 kHz.

I'm absolutely convinced that the night time skip would regularly exceed the ground wave range by a very large amount. 500 kW would be especially likely to produce a very long skip range due to the signal surviving multiple ground/ionosphere reflections. Trouble is, the absolute limit of skip range isn't predictable due to solar variations.

Rich's picture

To develop the earlier comments about nighttime skywave coverage from WLW...

WLW is one of the AM broadcast stations defined as Class A, which in the lower 48 US states are given FCC protection from interference to their 0.5 mV/m skywave signals at the contour where they exist at least 50% of the time.

No other stations on that frequency are permitted to have a groundwave or skywave signal whose field exceeds 0.025 mV/m at that contour for more than 10% of the time. This is a protection ratio of 20:1. All of this is determined by the rather complex process described in 47 CFR §73.182.

For 50 kW Class A stations this provides a nighttime, secondary, protected coverage area having a radius of about 750 miles (regardless of their assigned frequency).

However there are many areas where Class A stations can be received well at night beyond 750 miles from the transmitter, depending on the number and location of other co-channel stations on that frequency. For example, most nights I can receive the Class A signals from the New York City area, which is a great circle path length of about 910 miles from my location in west central Illinois.

Many 50 kW Class A stations can be heard fairly regularly at night over about 2/3 of the lower 48 states. This was an intended result from the allocations policy of the FCC to provide at least nighttime AM broadcast signals to areas of the country that received no, or few useful daytime signals.

So WLW's claim of a 2500-mile nighttime coverage radius for their 500 kW operation probably was true, considering the relative lack of other signals on the frequency and the lower r-f noise levels back then.

Nowadays operating at night in the lower 48 states there are two stations with omnidirectional radiation patterns, and four stations with directional patterns on WLW's frequency. There are many more in Cuba and Mexico.

PerryNH's picture

"Now notice the power is 1/2 MW and not 1/2 mW"

Thanks, I was wondering why the 100 MW AM transmitter I just ordered on ebay had a shipping weight of 2400 pounds!

wdcx's picture

WLS could be heard in the daytime on a table radio in Kalamazoo.

Druid Hills Radio AM-1610- Dade City, FL. Unlicensed operation authorized by the Part 15 Department of the FCC. We do not censor free speech and hide public information.

Rich's picture

WLS could be heard in the daytime on a table radio in Kalamazoo.

Kalamazoo, MI is within the 0.5 mV/m daytime coverage area of WLS (see http://user.pa.net/~ejjeff/wls72map.html).

As for nighttime coverage possible (occasionally) by 50 kW Class A stations, see this post: http://radiodiscussions.com/smf/index.php?topic=228685.0.

KSL is in Salt Lake City, and KNX is in Los Angeles. Reportedly both were heard in Newfoundland early this morning, 4 Feb 2013.

The great circle path length from LA to the geographic center of Newfoundland is about 3,000 miles.

MICRO1700's picture


Bruce, Past Part 15 operations - MICRO1700 and the Dog Radio Stations