I-A and I-B stations (Was Re: WKBR-AM on 1240)

["Martin J. Waters" <mwaters@wesleyan.edu>]

>Joe Ross wrote:
>What was the difference between I-A and I-B?

	Initially, the I-A stations had their channels to themselves at
night in all of North America. As with all these things, there were a few
exceptions and the exceptions grew a little over the years, but that
basically was the plan. These stations included WBZ, WFAN, WABC, WCBS,
KDKA, WCAU  (whatever it is now), WSM, WLW, etc. If my memory and a quick
count are correct, there were 25 of these channels for the U.S. and others
for the other nations (690, 740, 800, 860, 900, 940, etc.) For a long time,
the U.S. actually kept its I-A clear channels really clear, compared to the
other nations. Canada always seemed to be messing around with secondary
stations on some of its channels and Mexico allowed the U.S. and Canada to
put some secondary stations on some of its channels (800 and 1220 come to
mind).
	The other group of clear channels ( for the U.S., 680, 710, 810,
1060, 1070, 1080, 1090, 1110, 1130, 1140, 1170, 1190, 1500, 1510, 1520,
1530, 1560) was assigned to I-B stations. They were designed to share the
channel with one or more other I-B stations, either in the United States or
in one of the other countries. Nearly all the U.S. stations used
directional antennas at night, while the I-A stations did not need to
unless they wanted to aim their signals (as with WBZ). Some of the I-B
channels were assigned only to the U.S., so there were usually two U.S.
stations assigned to them. 1080, WTIC and KRLD, is an example. 1190, WOWO
and KEX is another. Some channels are shared with either Mexico (1090,
1140, maybe others) or Canada (1070, maybe others) and may have one or two
U.S. stations and one or more foreign stations.
	The other difference from the I-A channels was that secondary
stations could be assigned to the I-B channels at night. The I-B stations
received protection of their skywave signal to its 0.5mv/meter 50% contour.
That's why you could have assignments made on 680 and 850 in Boston, for
example, but not on 640 or 1160, although those Class I-A stations were
just as far away as the I-B stations on 680 and 850. The U.S. assigned only
one station rather than two on a few of its I-B clear channels, including
KNBR / 680 and KOA / 850. The history of AM radio in Boston was affected a
lot by the fact that the only U.S. Class I assignments on 680 and 850 were
2,000-3,000 miles away. This allowed the 50 kW assignments in Boston to be
made eventually, as well as other substantial assignments in Raleigh (50
kW), Cleveland, Baltimore, etc.
	One of the results of the reclassification of the I-A and I-B
stations all together as Class A was that the former I-A stations lost a
great deal of their nighttime signal protection. The lumping together
formally reduced their protection to the old I-B standard of 0.5 mv/meter
50% skywave. That's why you now have full-time stations on 1030 as close as
Memphis and Florida, places where WBZ used to be perfectly listenable at
night, because a signal somewhat less than 0.5 mv is OK if the channel is
really clear. The rule change also is why you have the Massachusetts
assignments on 640, 830 and 890, and other assignments in New England on
840, 870 and the C.P. for 720.
	In practice, the FCC says its policy is to protect the Class A
stations at least to a radius of about 750 miles, although it doesn't
actually do that in all cases. IMO, the I-A stations used to put out
reliable every-night signals to at least 1,000 miles. For example, the
non-DA Chicago signals are not protected in New England, because it's too
far away, but 670, 720, 780 and 890 all put good skywave signals into this
area. At a minimum the standard should have been set at 1,000 miles and the
signal level at whatever that 1,000-mile signal is -- somewhat less than
0.5 mv, perhaps.
	Yours truly, in excruciating detail.
	(Just call me the guy who's still p***** off because I could not
get WCCO on election night or the next night to hear first-hand about the
wrestler-elect in Minnesota.)

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Have you patronized the skywave signal of an AM Class A station today?

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