Metering an AM Transmitter
Posted on January 28, 2013
My station uses an outdoor AM transmitter which has a “control point” provided by a simple box with two switches and a LED. The box transfers DC power to the outdoor transmitter from the wall wart source by means of one switch and the LED indicates power on. The other switch is an audio mute switch.
The original oscillator in this transmitter used a rather poor grade crystal so it was necessary to use a crystal oven to keep in on frequency as the temperature changed. I wanted some indication of the heater cycling so I connected an analog DC milliammeter in series with the transmitter power lead at the control box. With this I could tell the duty cycle of the heater and monitor it for correct operation.
The crystal oscillator has been replaced with a PLL and the heater is no longer used but I left the meter in the circuit to indicate the total current going to the transmitter.
Some time ago (years perhaps) someone, I believe it was Carl Blare, posted the idea of building a metering panel to connect to transmitters. The proposed parameters to meter were more than just the gross current but I am limited to this because there are only three conductors going to the transmitter (power, audio, and ground).
Over time I have grown accustomed to glancing at the meter and noting the current is 107 mA. But yesterday I noticed the pointer wasn’t where it usually is and was indicating a current of 95 mA. The signal sounded fine but something had changed. The change was that the measured field strength was down due to snow on the loading coil. It turns out that there is a slight but definite relationship between this current and the field strength due to the decreased power output into the antenna system as it was detuned by the snow. Today, the snow is gone, the current is back to 107 mA, and the signal strength is normal.
Monitoring the transmitter total DC input current can be a simple way to tell if something has changed even if what has changed is not immediately known. This current reading will alert to many problems which could develop. If for no other purpose the meter indicates the power is on so maybe I don’t need the LED anymore, but it looks nice so I’ll keep it.