[BC] AM Allocation research

[Mark Humphrey]

There are many closely spaced Class C AMs in the northeast --  for
instance, WHAT 1340 in Philadelphia, which is only 70.3 km from
co-channel WRAW in Reading, PA and 95.1 km from WMID in Atlantic City.
 The M3 map assumes a conductivity of 4 in this region, but in most
cases it's actually much lower, so these old allocations were probably
based on measured conductivities.

Many of the more recent Class C "drop-ins" were allowed under
now-deleted language of 73.37(b) which allowed relaxed protection,
such that the 1.0 mV/m contour of the new station couldn't overlap the
0.05 mV/m interference contours of other co-channel stations (assuming
250 watt power) in locations where the new station would provide the
first local AM service.   One example is WCDO 1490 in Sidney, NY which
began operation in the early '80s.  WCDO's site is only 103 km
southeast of WOLF in Syracuse (which has a 225 degree tower) but the
applicant cited this provision of the rules, and also made
measurements on a test station showing the conductivity on the direct
radial towards WOLF at 1 or less, rather than 4 as shown on the M3
map.   By the way, before WCDO's application was filed, I had
petitioned for a new Class A FM allotment to Sidney -- but, in those
days the FCC sat on FM rulemakings for at least a year -- so the AM
applicant was able to throw up the test station antenna, run radials,
then apply for and build his station before I even had an opportunity
to file for the FM construction permit.  At that point, I decided not
to bother, as the market couldn't support two independent stations.

Mark

On Sat, Apr 24, 2010 at 11:02 PM, Jeff Glass <Xmitters at aol.com> wrote:
> Hello,
>
> Anyone here know how long the channel allocation rules have been around for class C AM stations?
>
> I have done the calculations for existing class C AM stations, and two out of the two pairs of existing stations come up short spaced.
>
> I used the FCC web Java script as well as a couple of Great Circle programs suggested by the membership here. I'm therefore confident that the distances are OK. I also deliberately picked two stations to study that have the same ground conductivity as per FCC map in 73.190, so I don't have to mess around with an Equivalent Distance calculation since the ground conductivity is the same as per the map.
>
> I used actual antenna RMS for the two stations and when that came up close spaced, I used the minimum acceptable RMS of 241 mv/m @ 1 km as per the FCC rules. I then followed 73.37(b) and assumed both stations putting out 250 watts (120.5 mv/m effective field) and still ended up with a short spaced situation.
>
> The geographical spacing comes out to 132 km. The required cochannel spacing is the radius of the .025 mv/m contour plus the radius of the .5 mv/m which comes to 154 km required spacing.
>
> I don't recall there ever being a short-spaced situation with AM class C, do you? My understanding is that these rules are pretty old.