Signal Patterns

Stephen from San Diego, CA, a reader of Mediumwave Oddities - Transmitter Power, wrote in with the following question:

"What stations actually are authorized with a HIGHER power at night than they are allowed in the daytime? I'll mention one - 760 KFMB San Diego, CA, about 7 miles from me. In the daytime they're only allowed a paltry 5KW omnidirectional, but at night they step up to 50KW."

An interesting question. And you mentioned signal patterns, all-important in the differences between daytime and nighttime mediumwave coverage.

I tweaked my database program Radio Data MW and came up with the following results:

73 US stations run higher power at night than during the day.

KFMB-760, your San Diego station, is the only one which runs 50KW at night and has a lower daytime power. KFMB uses one omnidirectional tower for daytime broadcasting. Power is set at 5KW during daytime broadcasting hours and 50KW at night. KFMB uses three towers at night to get that big 50KW signal out. But the pattern this time is pointed right out into the Pacific Ocean, to the southwest. San Diego's KFMB cardioid pattern has a deep, broad null at 60 degrees, nicely positioned for protection of Detroit Michigan's WJR-760 (50KW) and its western coverage area. Equally important, big gun KKZN-760, Thornton, CO (also 50KW), near Denver at 49 degrees is also protected, as well as a host of smaller US and Mexican stations.

KBRT-740 "K-Brite", a 10KW station in Avalon, on Santa Catalina Island is only a mere 85 miles to the northwest of San Diego's KFMB. It has a three tower array in use both day and night, pointing its signal due east to cover the Los Angeles and Long Beach areas, a scant 26 miles away. KBRT drops its power to a gasping 113 watts at night. Being nearly "co-channel" with KFMB at only 20 KHz removed, inspecting KFMB's pattern plot reveals that that the broad cardoid lobe to the northwest pumps an astonishing effective radiated power of 95KW up a 294 degree vector right at KBRT on Catalina. With a salt water path the whole way, that should be a lot of signal.

The next station with a big signal at night and a smaller one during the day is WLIB-1190 running 10KW daytime and 30KW nighttime, in New York, NY. WLIB uses three towers for daytime broadcasting and four towers at night. Both patterns are generally towards the east, with the nighttime pattern more focused and skewed a little more to the southeast. During the day it competes with WBIS-1190, Annapolis, MD (10KW), and WSDE-1190, Cobleskill, NY (1KW). WLIB's daytime pattern exactly bisects these two challengers on a line right out into the Atlantic Ocean. At night, WLIB only has to contend with New Yorker WSDE, as the Annapolis station is off the air.

The station with the greatest power differential, day to night, is WLAN-1390, Lancaster, PA. They run a mere 18 watts daytime and 1100 watts nighttime, both to a single, omnidirectional tower. Their nighttime power is a remarkable 61.1 times more than their daytime power!

WLAN's main competitor is WZHF-1390 (5KW), in Arlington, VA, a mere 92 miles away to the south-southwest. How does an 1100 watt station co-exist with a 5KW station at night when they are only 92 miles apart? Examining the tower pattern plots, you will find that WLAN in Lancaster, PA is exactly in a deep notch of WZHF's nighttime pattern.

Shown are nighttime pattern plots for San Diego's KFMB-760 and Avalon's KBRT-740, generated by Radio Data MW.