Re: Dan on WCRN

[Dan.Strassberg@att.net]

The 10% skywave is the signal level that the skywave exceeds no more than 10% 
of the time. The 10% RSS is used to establish an AM station's NIF (nighttime 
interference-free grpundwave service contour. The NIF contour is 20 times the 
RSS of the interfering 10% skywaves. In calculating the RSS of the 10% skywaves 
(root-sum-squared, perhaps better known as the square root of the sum of the 
squares), the FCC refers to the 50% RSS and the 25% RSS values. The 50% RSS 
(counter-intuitively the smaller of the two numbers) is the RSS you get by 
dropping out all contributors whose value is less than 50% of the highest one. 
The 25% RSS is the number you get by dropping out all contributors whose value 
is less than 25% of the highest one.

There is another 50% number used in calculating nighttime service areas. This 
is the 0.5 mV/m 50% skywave contour of Class A (formerly clear channel) AMs. 
Only Class As (in New England, only WBZ and WTIC) receive protection of their 
nighttime skywave service. The service is protected to the 0.5 mV/m 50% skywave 
contour--the contour that receives a 0.5 mV/m nighttime signal from the station 
in question at least 50% of the time. 

The FCC also refers to the RSS field of AM directional arrays. This is the 
square root of the sum of the squares of the field contributions of each of the 
towers in the array. The more meaningful number for field strength of a 
directional AM signal is the unattenuated RMS (root-mean-squared) field at 1 km 
(from the electrical center of the array). You get this value by taking the 
square root of 1/72 of the sum of the squares of the field strengths at 5-
degree increments around the compass.

The FCC and designers of AM DAs use the ratio of an array's RSS to RMS as a 
figure of merit for the array design. The ratio is never less than 1 and the 
closer it is to 1 the better. A ratio of 2.5 or more is considered unacceptable 
because it indicates that huge emounts of reactive power circulate in the 
transmission lines and ground system. If an array has an RSS/RMS of 1.25:1 
(happens to be the ratio for WCRN's proposed three-tower night array), the real 
plus the reactive power input to the array will be 1.25 times the radiated 
power, or for a 50-kW station, 62.5 kW. To the extent that the ground is an 
imperfect conductor, it dissipates some of the reactive power, so that the 
transmitter must output more than the nominal antenna-input power.
--
dan.strassberg@att.net
617-558-4205
eFax 707-215-6367

>  Dan, you mention 10% skywaves. Could you 'splain further what that means; is 
> there a 25% skywave; 50%?