RF current meter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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I recently needed to measure RF currents. With nothing handy for this purpose, I quickly put together this simple but effective tool. It is not only easy to assemble, but also very cheap.
The scheme of the probe is shown in Fig.1. With the values of the elements indicated in the diagram, the sensitivity with a deviation of the indicator of 0,2 mA per half scale is 10 mW (50 Ohm path) in the frequency range from 1,8 to 30 MHz. When using a more sensitive indicator (for example, 50 μA), the sensitivity of the probe can be increased. Germanium diodes are used as VD1...VD4.
Ris.1
Design
The size of the case depends, first of all, on the dimensions of the measuring head. For ease of use, I recommend such a case size so that the device is comfortable to hold in your hand. L1 - one turn of wire passed through a snap-on ferrite core.
Use epoxy to secure the snap core to the top of the case. To improve adhesion, clean and roughen both surfaces to be glued - the body and the core. Drill holes in the body from both ends of the core - the ends of L1 are passed through them. To create the one turn required for L1, pass the wire through the core, then through the holes, and connect its ends to a full-wave diode bridge. I note that the whole circuit has no connection to the body (ground).
Using
This meter is used very simply - the core is snapped around the wire to be tested, and the optimal deviation of the arrow is set with the sensitivity adjustment knob. The probe can be used to study the distribution of RF currents in antenna elements, unshielded feeder lines, guy wires, and other conductors.
If you are plagued by RF currents at the radio, a current probe can be used to resonate the station's ground wire. Snap the probe around the ground wire, connect a variable capacitor between the ground wire and the connector on your equipment (you can use a capacitor from a radio receiver) and turn the station on transmission, using such power that the galvanometer needle deviates noticeably. After that, by rebuilding the capacitor, achieve the maximum deviation of the galvanometer. It is possible that you will need to experiment to find the correct value of capacitance for series resonance in the ground wire.
If the operating frequency changes, the capacitor must be readjusted. I used this procedure at a friend's house in Iowa and completely eliminated the problem of RF currents that were causing hum to the computer when going on the air.
Author: S.SPARKS (N5SV), translated by A.Volsky; Publication: N. Bolshakov, rf.atnn.ru
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