[BC] FCC Approves proposed AM MoM Rules

[Cowboy]

On Sunday 28 September 2008 09:04 pm, Thomas G. Osenkowsky wrote:
>  Bear in mind that at 1 km or 1 mile the pattern may
>  not be completely formed. As a matter of fact, when
>  considering 'proximity effect' it may never be 100%
>  fully formed.

 May not ?
 There are reasons I prefer tune points about 12 miles out.
 ( whenever possible, which isn't always )

>  Also consider two towers where the complete null is
>  in line with the towers. Theory tells us the field ratio
>  must be 1:1 to have a complete null. As you move
>  away from the array the distance from the farthest
>  tower is greater than the nearest. So, the required
>  field ratio is no longer exactly 1:1.

 Almost.
 Theory tells us the ratio is 1:1.
 As one moves farther from the towers, the difference in
 the distance from each becomes smaller, and smaller, at
 some point becoming practically insignificant.
 From that point, and beyond, the 1:1 ratio holds.
 In this classic case, consider a measuring point that is
 a distance from the nearest tower equal to the spacing
 between the towers. Since the distance to the far tower
 is twice the distance from the near tower, twice the field
 voltage is required to produce equal voltage at the measuring
 point, a power ratio of 4:1. Not at all the 1:1 conventional
 wisdom tells us.

>  FCC Rule sinusoidal assumptions define an array as
>  a point source. In practice, it is not.
>  
>  Tom Osenkowsky, CPBE

 Only because the FCC isn't capable ( desire ? ) of the level of
 math required for close in measurements, and the purpose of
 directional arrays is the prevention of interference at distance,
 not a mile or two from the emitter.
 To do otherwise, one has to 'splain it to them in detail.

 Oh, sure, we *can* determine how far above the "limit" close in
 readings need to be in order to produce far field radiation within
 the limit, but we have two answers. One point alone is invalid, as
 it won't tell us which of the two possibilities we have.

 This is why we tend to disregard close in measurements, and also
 why readings inside of about 2 miles are completely worthless for
 directional adjustment.
 We do make close in readings 2 miles and less Non-D, but that's 
 for determining the radiation "efficiency" of the system only, close
 enough that ground conductivity is not a significant factor, usually.

 The worst case I've ever seen, was a Non-D in Fort Worth where the
 conductivity is *SO* bad its effects could be seen 100 feet beyond
 the ends of the ground radials, but this is unusual.

 On the other hand, a certain array where the radials cross the
 Des Plains river, the far measurements when plotted had all the
 consistency as if we'd used the conductivity graph as a turkey
 shoot target. Mr. Rackley and myself spent some days collecting
 data, washing out proximity effects, and such to construct an
 FCC acceptable "proof" out of readings ALL less than 4 miles.
 Even running a partial on that array in the future will take an
 "engineer" with a level of skill beyond that which many stations
 choose to hire these days, or at least commanding a rate that
 many stations choose not to pay.

-- 
Cowboy