A Clamp-On RF Current Probe

by: Lyle Koehler, KØLR

The original version of this article appeared in the April 1994 LOWDOWN

Here's a sensor that lets you measure currents in your LowFER, MedFER,or ham antenna and ground system without breaking the circuit. You simply clamp the pliers-like probe around the wire you want to measure. An advantage of a current probe over a simple relative field strength meter is that the measurements are highly repeatable and can provide a fairly good indication of how your antenna system compares to others.

A similar current probe is described in the October, 1992 issue of RADIO COMMUNICATION. However, it is intended for use in the 2 to 30 MHz range, and is made from a large toroid core that must be carefully broken or sawed in half. The probe described here uses a split rectangular ferrite core and has a useful frequency range from 100 kHz to more than 10 MHz. All of the electronic components (including the ferrite core) and most of the hardware such as screws, nuts, spacers and sticky-backed hook-and-loop ("Velcro") tape are available from Radio Shack.

Like the clamp-on ammeters used for 60-Hz circuits, this probe is based on the current transformer principle. When you clamp the probe around a wire, the wire becomes the single- turn primary winding of a transformer. If the secondary winding is loaded with a low resistance R, the RF voltage across R is equal to R times the primary current divided by the number of turns on the secondary winding. A diode detector circuit converts the RF voltage into a DC output which can be read on a standard multimeter.

Figure 1 shows suggested construction details for the clamp-on probe. The pliers can be constructed from 1-inch wide pieces of thin (1/8 to 1/4 inch) plywood or plexiglas. Attach the ferrite core to the pliers assembly with 4 small strips of sticky-backed Velcro or double-sided adhesive foam tape. Two pegs made from 1/2-inch spacers and 6-32 screws press against the outside of the core halves or, to be exact, against the winding. There is enough flexibility in the Velcro fastener so the core halves can align themselves when they are squeezed together. It's also easy to remove the core and reposition it if the alignment is incorrect. If you use double- sided foam tape, it will require considerably more care when you initially stick the core halves in place. The core halves do not have to be perfectly aligned side to side and front to back, as long as there is no air gap between them. There are lots of spring arrangements that can be used to provide the clamping action -- a fat rubber band does the job quite well. The kind of rubber band used in supermarkets to hold bunches of broccoli together is just about right for this application. If you don't like broccoli you'll have to improvise.

Wire size for the transformer winding isn't critical. I used #26 enameled wire. It bends easily around the core and fits in the gap that is provided by the Velcro or foam mounting strips. Wrap a layer of electrical tape around the core where the windings will go to keep the sharp corners from scraping the insulation. If metal spacers are used on the pliers assembly, the spacers should also get a wrap of tape to keep them from shorting the windings. Figure 1 shows the 19- turn secondary split into a 10-turn winding and a 9-turn winding, so that the secondary is somewhat equally distributed between the two halves of the core. When the entire winding is on one half of the core, the insertion impedance of the probe is slightly higher than with a split winding. An ideal ammeter should introduce zero impedance into the circuit it is measuring. This sensor adds a series impedance of less than half an ohm to the circuit under test at 200 kHz. At 2 MHz, the impedance is still under 1 ohm, but it climbs to about 1.6 ohms at 5 MHz and 6.3 ohms at 20 MHz. With a constant current in the primary winding, the DC output voltage is nearly constant from 200 kHz to 20 MHz, and is down about 5 per cent at 100 kHz.

Figures 2 and 3 are calibration curves for three different values of DC voltmeter input resistance. For the best linearity, the readout device should be a high-impedance voltmeter such as a digital multimeter with a 10-megohm input resistance. Between 50 and 500 mA, the RF current is approximately equal to the high-impedance voltmeter reading divided by 10. Use the 100k curves for a 20,000 ohm per volt (50 microamp) meter with a 5-volt full-scale range, and the 10k curves for the same type of meter with a 0.5-volt full-scale range. Note: when using a solid- state analog or digital multimeter near a transmitting antenna, the meter itself may detect RF energy and give inaccurate readings. Check to make sure that the meter reads zero with the current probe in the vicinity of the antenna but not clamped around a conductor.

Since this article first appeared in 1994, I've noticed that the Radio Shack split rectangular cores don't always have the same dimensions. This may have an effect on the calibration, so it's best not to take the curves of Figures 2 and 3 as gospel. However, with any luck at all the calibration will be accurate to within about 10 per cent.

To get an accurate antenna current measurement without detuning the antenna, it is best to measure the current on the transmitter side of the loading coil. This may not be possible; for example if the loading coil is part of the final amplifier circuit. Make sure that the antenna wire passes through the current probe without actually touching the probe's core or windings. You can minimize detuning effects by using short leads between the probe and voltmeter, and by mounting everything on an insulated support.

Besides using the current probe for tuning your antenna and for checking periodically to see if anything has changed, you can use it to estimate antenna system losses and effective radiated power. The total of all losses in the antenna system is P/I², where I is the antenna current and P is the power output of the final amplifier. Assuming 80 per cent efficiency, P is simply 0.8 times the DC power input. The effective radiated power is I²R_r, where R_r is the radiation resistance of the antenna. R_r is usually unknown, but you can make an educated guess. For a LowFER antenna, R_r is probably between 0.03 ohms (for a straight 15-meter base-loaded vertical) and 0.1ohms (if you've optimized the height and top hat dimensions using the interpretation that the height of the vertical section plus the radius of the top hat can't exceed 15 meters). The corresponding range of R_r for a MedFER antenna is approximately 0.1 to 0.3 ohms. My LowFER antenna current is over 200 mA in winter, but my MedFER antenna current is only 35 mA. That gives me an effective radiated power of something like 4 milliwatts on LF, and perhaps 0.4 milliwatts on MF!