Simple Remote Antenna Current Meter
By Rick W7RNB
November 20, 2016
Using an RMS, thermistor current meter at the antenna coupler was my first line of defense. However after reading Ralph’s (W5GJV) article about a remote current sensor and meter in the radio room seemed very appealing and smart with the rainy Northwest weather. Also my good friend John N7MWV, who had been in broadcasting for many years provided me with excellent input and inspiration to build this metering circuit. Giving it some thought, I began to formulate a plan by which a T300-2 toroid would be used for the sensor and a large format meter could be calibrated as the indicator. Below is an explanation of how this was done.
Here is a picture of the sensor, as built, using a T300-2 Amidon toroid. It is packaged up in a box made of clear plastic that I purchased and had cut 4 inches wide, 6 inches tall and 2 inches deep. This will keep out the weather. I centered a whole for the antenna lead to go through and to keep it in the center of the toroid. It is lined with ½ Teflon heat shrink to minimize water getting inside the box.
There are three wires, all of Teflon for resisting the weather and stained so as not to break during handling and hook up. A layer of electrical tape was put onto the toroid so as not to nick the enameled wire during installation. The green wires go to the sensor turns, 37 turns close wound #24 enamel wire was used. Then they were covered with a single layer of electrical tape to isolate them from the Faraday shield installed over them. The green wires were soldered to the bitter ends of the coil and sealed with silicone to keep the water out.
Talking about the Faraday shield, Ralph, W5GJV, brought up the point that the coil needed to be insulated from the RF field to be accurate. A 1 inch wide piece of copper tape was installed over the coil but the ends don’t touch, an 1/8 inch spacing between them will keep the tape from forming a pick up coil. Then the white wire was soldered to the copper tape and was taken to hard earth ground.
Here is what the sensor looks like installed at WI2XJQ. The bottom of the box is also setting on a piece of Plexiglas, with a little opening to allow for air flow and to reduce any condensation collection. Notice the antenna lead wire to the base of the vertical section passes through the two holes in the box and is centered on the toroid. Then the three wires are connected. Ground to the ground field and the two green wires hooked to Teflon coated ‘bell wire’ and brought back to the radio room for hook up to the metering circuit.
Here’s my meter – nothing fancy just reads the current coming back from the sensor. On the top is the shaft of a 1K ohm 2 watt pot used for calibration of the meter. The circuit is very simple. The sensor leads are brought into the meter box, where 4x 1n4148 diodes are configured into a bridge, then on the output is a .01uf capacitor and a 47mf electrolytic capacitor and then directly into the 1K pot configured as a rheostat. Then the output is sent to a 100ohm resistor and then to the hand selected resistor setting the range of the meter / calibration. Check out the block diagram below.