A Remote Reading RF Ammeter for High Voltage Antennas

by W5JGV

An Easy way to Monitor your Antenna Current from the shack

It's always a bit of a hassle to properly tune a low frequency antenna, since it seems that the settings you determined with your "cold" measurements are always a bit different from the "real world" settings you finally wind up with then you tune 'er up "hot." The ultimate solution for this, of course, is to do everything you can with the antenna, loading coils, and variometer to maximize the antenna current. After that, bringing the VSWR at the transmitter down to a reasonable level is a matter of matching impedances and reactances. But that's another story...

RF ammeters are becoming hard to find, and quite expensive. Although I do have several of these handy instruments on hand for testing, I thought it would be nice to have something I could leave attached to the WC2XSR/13 antenna system. What could I build?

Since I have installed a motor driven variometer control which I operate from the comfort of the shack, I wanted to have a remote reading RF ammeter so that I could read the antenna current from the shack as well. I also wanted to design it so that I could move the readout to the antenna tuner while I was making major adjustments to the tuning system. I decided to make it operate over a fairly wide range of RF current for versatility. Since the WC2XSR/13 antenna system generates upwards of 25,000+ volts, high RF voltage capability was also required.

Here's the front of the remote readout meter box.

The RF sample is fed to the circuitry inside the box through the BNC connector just visible on the far right side of the box. The meter is a Radio Shack 15 VDC meter, which has had it's external scaling resistor changed so that the meter reads 0-10 volts. A new scale meter was made by scanning the original meter faceplate, editing it in PhotoImpact, and then printing the edited image. After a little trimming and some adhesive, the meter looks just like a factory item.

The meter range switch selects between full scale readings of 1.5, 3, 6, 15, and 30 RF Amperes. A meter shorting position (OFF) is also provided for transport.

CIRCUIT DESCRIPTION:

An RF sample is picked up by transformer T1. The magnitude of the sample voltage is proportional to the current flowing through the primary of T1 (The antenna current.)

The sample voltage is carried from T1 to the remote meter circuits by a coaxial cable. The secondary winding of the transformer is shunted (burdened, in current transformer tech-speak) by one of the load resistors R6 through R10, which are selected by S1-B. These resistors, in combination with the leakage reactance of T1, serve to limit the maximum voltage generated by the secondary winding of T1 to a little over 8 volts. Because the voltage developed by the secondary winding increases with increasing current, the higher the current range selected by switch S1, the lower the resistance of the burden resistor will be.

Resistors R1 through R5 serve to allow full-scale calibration of the instrument on each range. They are selected by S1-A.

Capacitors C1 and C2, diodes D1 and D2, and capacitors C3 and C4 form a voltage doubler circuit. (C1 and C2 are shown as separate capacitors, because that's what I had on hand. A single 0.68 uF capacitor would do just as well as the two 0.33 uF capacitors shown in the diagram.)

Diodes D1 and D2 are high-speed Schottky diodes, which have a low forward voltage drop. By using these diodes and designing the circuit to provide 10 volts to the metering circuit at full scale, virtually all of the diode linearity errors are compressed into roughly the first 1 1/2 minor divisions on the meter scale. This means that for all practical purposes, it is not necessary to hand-calibrate the meter.

Capacitor C4 charges up to provide an average DC voltage value of the RF sample. The higher the capacity of C4, the slower the meter response will be.

Capacitors C3 and C5 bypass residual RF. They should be disc capacitors.

Because the RF loss of most cables at 166.5 KHz is low, there is almost no calibration error with as much as 100 feet of cable running between the units. In my system, I am using a 100 foot run of CAT-5 cable for the motor drive controller for the variometer, and I am using a spare pair in the cable to feed the RF sample from the pickup transformer to the meter box. It gives me the same reading as does the same length of RG-58/U.

Wow! That's a BIG ferrite core!

Yes, it is, but it's needed for the high voltages in the antenna system. The core is a T-520-26 core, and measures 5.2" OD X X3.08" ID X 0.8" thick. Because it is 26 material, it does not represent a large reactance at 166.5 KHz. That means that it will be more or less "invisible" to the antenna system.

The primary of the transformer is a single pass through of a length of 3/8" diameter polished and lacquered copper tube. The combination of the large radius of the copper tube and the air gap between the copper tube and the inside of the core prevents corona discharge and damaging RF arcs.

The secondary of the pickup transformer is a 30 turn RF pickup coil which is wound under the aluminum tape seen on the right side of the core. You can see the thin twisted copper wires from the pickup coil running over to the upside-down BNC chassis connector (with the rusted locking nut) at the lower right of then picture. The two yellowish colored plastic insulated wires leading from the pickup coil are the ground wires for the aluminum tape shields. (See PDF file for details.)

Note that the aluminum tape is grounded to prevent electrostatic RF pickup from the copper pipe, however, the tape must be carefully cut so that it does not form a short circuited turn through the core.

The core itself is attached to the plastic bracket using plastic tie-wraps. Do not use a metal panel - it will increase the parasitic capacitance between the base of the antenna and earth, and increase the chance of RF corona and arcs.

Although it would have been just as effective to simply flatten the ends of the copper pipe and drill a hole through the flattened end, I thought it would look nicer to make a clamp to bring the wires off the pipe. That also would allow me to slip the pipe out of the plastic mounting brackets should I ever have the need to do so. Some flattened 3/8" copper refrigeration tube was used to make the clamp. After flattening the tube, it was wrapped around a steel rod into a "U" shape. Then the ends of the "I" were carefully squeezed in a vise until the flat ends almost - but not quite - touched.

This end-on view shows the clamp screw and nut that holds the clamp to the end of the pipe. Note that the copper ends of the clamp do not quite circle the pipe. The gap lets the clamp hardware tightly squeeze the clamp to the pipe.

A view showing some details of the plastic brackets holding everything together. If you look closely, you can see the thin brownish gap in the aluminum foil shield placed over the RF transformers' secondary winding. There is a similar gap in the aluminum tape shield which is placed between the secondary winding and the core itself. Also note the BNC connector and the RG-58/U coaxial cable that carries the RF sample back to the remote meter.

Here's the setup I use for web monitoring of the antenna amperes and the transmitter frequency. The webcam looks at these instruments 24 hours a day. See that dim little pilot lamp in the top of the picture? It supplies all the light necessary for the camera to do its job.

Here's what it looks like on the web

Click HERE for a live web view!

73, Ralph W5JGV

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