Tuesday, May 24, 2011

Permit granted for new AM station

Rowe, New Mexico: 1420KHz:
Permit granted for new station.
2,500 watts daytime
103 watts nighttime
non-directional all hours
35-29-51N/105-40-20W, about 25 miles southeast of Santa Fe.

This station must propose a rather unusual antenna system...

The FCC requires new AM stations to have a minimum nighttime power of 250 watts, or a minimum nighttime signal of 141mV/m at 1km from the antenna. This station obviously proposes a lot less than 250 watts, but a signal of 441mV/m. How they are able to do this with a non-directional antenna I have no idea.

1 comment:


Hi Doug,

Over the last couple of years I have been writing a program to tabulate and graph the AM stations in the FCC database. Through a lot of work, comparison, and experimentation with FCC and my own formulas, this is the way I have come to understand how the FCC performs calculations for field strength.

RMS Theoretical values are computed from a formula of course. As you note, the values are calculated for a distance of 1 kilometer. The FCC formula used generates accurate millivolt-per-meter values (as published) for multiple tower arrays. It is not accurate for single tower arrays, in that the mV/m value calculated in the single tower case is always based on a 1 KW output power. Hence, the only single tower stations having accurate, published 1 kilometer mV/m values are 1 KW stations. As an example in the other direction, check the FCC's published figures for my local WHAM-1180 station out of Rochester, NY. This 50KW station shows a calculated value at 1 kilometer of only 376.59 mV/m. Now of course this cannot be correct for a 50KW station, as a 1 KW station running a quarter wave (.250 wavelength) monopole has an exact calculated figure of 305.768 mV/m at 1 kilometer.

376.59 mV/m would, however, be correct for a 1 KW station using the same single tower antenna that WHAM uses (a .492 wavelength antenna).

To accurately calculate the mV/m figure for WHAM (or any other single tower station), the following additional formula must be used:

(Power in KW, distance in KM):

mV/m = rms x SQRoot(Power/Distance)

Thus in WHAM's case:

2662.89 = 376.59 x SQRoot(50/1)

Or, 2662.89 mV/m.

Following through for the new station in Rowe, New Mexico, which only runs 103 watts at night:

141.5 = 440.9 x SQRoot(.103/1)

Or, 141.5 mV/m, just over the minimum required figure of 141 mV/m.

I checked the tower stats for the new station in Rowe. It is a top-loaded affair. The main section (A) is 60.4 meters tall, or .286 wavelength. The top load (what they call section B) is effectively 69.6 meters or .329 wavelength. So, the total effective height is .615 wavelength. This is the reason for the seemingly high figure of 440.9 mV/m (remember the FCC calculation result is based on 1KW, not 103 watts). The extra apparent antenna height over one-quarter wave (.250) raises the base 305.768 mV/m figure, resulting in the calculated value of 440.9 mV/m.

The FCC calculation (used to arrive at the 440.9 mV/m figure), which is the RMS Theoretical value, is actually quite complicated. It is a small program in fact.

Hope this helps.