WLW 500 kW Directional vs. 50 kW Nondirectional 5 mV/m and 1 mV/m 50% Skywave Map

[Kelly A]

Class As were and are protected to the 0.1 mV/m ground wave. There are at least several of those coverage map images available online. Even smaller stations had coverage maps that show the 0.5 and 0.1 mV/m contours, especially those prepared by Kear and Kennedy.

0.5 mV/m signals are still listenable away from electrical wires, especially remote store parking areas away from stores, and in rural areas.

In the 1960 NAB Engineering Handbook, it even shows a recommendation that city factory areas receive 25 mV/m or more. I can tell you for sure that inside heavy manufacturing plants and deep inside malls, you need 100 mV/m outside those buildings to be heard well inside. And you always did.

A couple years ago the ITU changed the minimum mVm for the average person to hear a receivable MW signal from 5mVm, to 10mVm. .5 might be doable for some DX'er in the hinterlands with a long wire, but for your average listener? No way.

 

[davideduardo]

Those original coverage maps showing field strength borders of .5mVm are a lost classic, because it's wholly impractical that given the noise floor today, anyone could receive a signal attenuated that much.

The ITU even more recently "determined" that 15 mV/m was needed in "populated areas" for an AM to be listenable. The change in the noise floor has moved the listenability level from as low as 1 to 2 mV/m in the 30's to the high signal level needed today.

Of course, most "good signal" AMs were created in the 30's, and since then their useful coverage has deteriorated by as much as 50% to 60%, while cities have sprawled outwards into areas where those 1930 facilities never saw a need to serve.

 

[Schroedingers Cat]

Enter a New York Zip Code in Manhattan, to see how the 1560 might stack up next to a 5 kW or 10 kW high dial position AM station near the Meadowlands. Look at all the 50 kW stations with several hundred mV/m signals.

Zip Code Signal | V-Soft Communications Propagation & FCC Allocation Software Broadcast/RF

zipsignal.v-soft.com

 

[radioman148]

Interesting stuff. Thanks SC.

 

[rfry]

... I don't know if you could have only 50 kW equivalent toward Toronto with just two towers ... . So I think we both agreed a while back that the pattern on that [Proposed 500 kW WJR Coverage] map is pretty much fictional.

I'm not so sure of that belief actually, SC.

To check it out I looked at the daytime field intensity that would be present from 500 kW WJR a little east of Traverse City, maybe around Kalkaska. That is a path length of ~193 miles from the WJR transmitter site, at about N340°E azimuth.

Using the WJR DA pattern below and the M3 conductivity map value for that path, that 500 kW DA field near Kalkaska would be about 0.47 mV/m — which is consistent with the 0.5 mV/m contour shown for the Kalkaska area on the WJR 500 kW contour map posted earlier in this thread.

Also note that the array below consists of only two towers.

 

[Schroedingers Cat]

Notice that the 500 kW 0.5 mV/m contour and the 50 kW 0.5 mV/m contour diverge where the signal hits Canadian land at the base of the Bruce Peninsula, which would conform to the Treaty. WJR 760 would not have interfered with CBL 740 (now CFZM) like WLW 700 with 500 kW nondirectional did with CFRB, then a nondirectional Class A on 690 kHz in Toronto. CFRB was forced to move to 1010 with a DA, and with WINS 1010 being just somewhat more than 300 miles away. It took a lot of fancy footwork to consider CFRB to be protected like a Class A on 1010, which it still is internationally. I'd like to see the Canadian government's technical argument that it was some kind of equivalency. At the very least, they should have given CFRX 6070 a substantial power increase.