[BC] Major market FM Short Spacing

[Mark Humphrey]

Dave, you are correct.

I just checked my copy of the FCC's pamphlet "Standards of Good
Engineering Practice Concerning FM Broadcast Stations" dated January
6, 1946, which was apparently based on the FM rules (sections 3.2xx in
those days).

The only interference showing requirements at that time were
co-channel (a 10:1 D/U ratio) and first-adjacent ( 2:1 D/U ratio) but
the following paragraph goes on to say "Objectionable interference is
not considered to exist when the channel separation is 400 kc or
greater.  Accordingly, FM broadcast station in the same city or same
area may be assigned channels 400 kc apart.  In the assignment of FM
broadcast facilities the Commission will endeavor to provide the
optimum use of the channels of the band, and accordingly may assign a
channel different from that requested in an application."

I also found some interesting language in the section on transmitter
location: "As a means of minimizing interference problems, it is
expected that stations adjacent in location will generally be assigned
frequencies that are generally adjacent.  Insofar as possible,
frequency assignments for stations at separated locations will also be
separated."    Of course it makes sense to do this, but at some point
the FCC stopped encouraging "co-location" of stations closely spaced
in frequency and that's probably when the second- and third-adjacent
requirements were added.

There were three classes of FM stations in 1946:  Community,
Metropolitan, and Rural.   Community stations (similar to today's
Class A ) weren't even required to determine contours, while
Metropolitan stations had to determine distance to the 1 mV/m and 50
uV/m, and Rural stations also had to show 20 uV/m.  The predicted
coverage map had to show all interference areas out to the 50 uV/m
contour.  Nothing is mentioned in this pamphlet about ERP limits, but
there's a chart of standard FM transmitter power ratings which goes
all the way up to 100 kW!

And the section pertaining to antenna systems says "It shall be
standard to employ horizontal polarization.  If the use of vertical
polarization appears desirable in special circumstances, its use may
be authorized upon a showing of need."

Mark

On Tue, Feb 2, 2010 at 11:28 PM, Dave Hultsman <DHultsman5 at aol.com> wrote:
>
> In the East and on the West Coast,  I seem to recall reading that after WWII there was 500 kHz. spacing in some areas or the spacing was changed to 800 kHz.   Some of these stations did not change frequencies.  Also the east coast from Atlanta to Richmond has many short spaced allocations that have been grandfatered for many years.  I don't know if was changes in rules for frequency spacing or milage seperations. I believe that many of these go back to the pre 1950's.  Also during that time there were many more super powered FM's.
>
> Some of you FM historians can probably better chime in on this.
>
> Dave Hultsman