KMTI AM 650 kHz Directional Broadcast Antenna Array Information and Photos

KMTI is located almost exactly in the center of Utah. It is owned by the Sanpete County Broadcasting Company. Its programming is primarily country music, news and sports. Its signal is weak at the salt Lake City receiving site, both because it is 116 miles away and because it is behind 11,877 foot (3,620 meter) Mt. Nebo and other mountains.

KMTI transmits with 10 KW of power in the daytime, but reduces power to 900 watts at night to reduce interference to other stations that share the same frequency. It uses the directional antenna array shown below both day and night, but the RF phasing and power ratio between the two towers is changed at night to modify the radiation pattern and further reduce nighttime skip interference to more distant co-channel stations.

A March 4, 2005, photo looking north-west at the KMTI antenna array. The tower on the right is almost exactly straight east (91.5 degrees from north) of the tower on the left. An antenna tuning unit is located at the base of each tower behind the white fences. The transmitter building is in the center between the two towers.

Antenna Details
Each tower is 80 electrical degrees (336.5 feet, 102.6 meters) tall. They are tuned to quarter-wave resonance by the combination of guy-cable shunt capacitance and adjustable tuning reactance in antenna tuning units at their bases. The towers are physically spaced 165 electrical degrees (694 feet, 211.53 meters) apart.

Daytime Operation
In the daytime the phase of the RF current in the east tower leads the phase of the current in the west tower by 87.50 electrical degrees. The power supplied to the east tower is 1.75 times the power supplied to the west tower. With the 10 KW total authorized daytime transmitting power this produces a main antenna lobe field strength intensity of 961.08 mV/m at a distance of 1 kilometer from the center of the array.

Nighttime Operation
After local sunset and prior to local sunrise the phase of the RF current in the east tower leads the phase of current in the west tower by 22 electrical degrees. The power supplied to the east tower is 1.105 times the power supplied to the west tower. This produces deep radiation nulls both the east and west of the antenna. With the 900 watt authorized nighttime transmitting power the resulting radiation pattern has a main lobe field strength of 267.20 mV/m at a distance of 1 kilometer from the center of the array.

• Additional KMTI Antenna Information and Photos
• Antenna Photo Gallery