Antenna Tower 4 Has No Base Current

Engineering problems in the broadcast field can range from defective components, operator errors, snakes, or a combination of factors.

Some years ago, I was an engineering consultant for several local radio stations. One night, I received a call from a station because one of the towers in the directional antenna had no base current. Base current is measured by inserting an ammeter in series with the antenna. AM radio stations use directional antennas to increase their coverage area. When a station applies for a license, the application must show that the new station will not cause interference to any existing station. With a non-directional antenna, the station's coverage is limited by the distance to the nearest co-channel or adjacent channel station.

It is possible to create lobes and nulls in the station's coverage by erecting two or more antennas. This occurs because the signal in any direction is the vector sum of the power from each of the towers. The position of the nulls and lobes is determined by the orientation of the antennas, the spacing between antennas, and the phase relationship of the power being applied to each antenna. The depth of the nulls and size of the lobes is determined by the ratio of the power to each antenna.

The phase and power to each antenna is controlled by a phasor located next to the transmitter. Most stations have a different pattern at night because of the change in propagation after sunset. In the 1980s and prior, the operators were required to check the phase and power ratios by remote metering every hour. The operators were also required to go to each tower and measure the base currents at every pattern change. Stations were then required to make field measurements every week to verify that the nulls actually existed to the extent required by the license.

I received a call from the chief engineer when the station changed to night pattern and tower 4 had no base current. I arrived at the station just after midnight so I would be prepared to work at the 1:00 a.m. sign-off. I decided to work my way from the tower back to the transmitter to find the problem. I grabbed a couple of short clip leads and a 47-ohm carbon composition resistor and then I proceeded to tower 4, which was about a quarter mile from the transmitter building.

As I approached the tower, I verified by the flashing red light that the tower was physically standing. When I arrived, I pointed my flashlight at the base of the tower to verify that the pipe was connected to the base of the tower, and entered the doghouse. Inside the doghouse was a matching network. It is very rare for a tower to be exactly 50-ohms with no reactance. It is therefore bad practice to connect a tower directly to the coax because a mismatch will cause losses in the coax and can lead to excessive voltage, which may cause the coax to arc over. The matching network consisted of tapped coils and fixed mica capacitors.

I made a careful physical inspection of the matching network, making sure everything was tight. I looked at the mica capacitors for cracks in the porcelain or leaking tar. Everything looked fine. Next, I removed the shorting bar between the coax and the tuning network. I connected the 47-ohm resistor across the coax and returned to the transmitter shack.