## Measuring very low R

It sometimes happens that while repairing old antennas or searching for an antenna fault, the question of tube connections quality arises. A few tenths of Ohm in traps or in between tubes can already be a problem and lead to high losses.

Measuring very low resistances is not easy with common multimeters, and if you have them it's sometimes difficult or risky to carry them on field. Often, we are in situations where we need to measure low resistances without any special equipement.(contests, fieldday, etc...)

Even the lowest range of a digital multimeter is of no help to measure resistance under a few Ohms.

The following method described here allows to measure very low resistances, in particular contact resistances between tubes or any other metalic parts.

With our digital multimeters it is on the other hand easy to measure voltages of a few hundred milliVolts.Generaly, multimeters are much more precise as voltmeters. We wil use the Ohm's law to solve our problem.

We will make circulate a current in our contact and measure the voltage drop in our contact. With this method, we can detect resistances as low as a few milliOhms with a 3-4 digit multimeter !

In practice

All we need is a voltage source, a regulated variable power supply is perfect for this use. But any voltage supply like a fixed voltage PS or a battery will do the job. In the later cases, we only need a bit more mathematics as we can't adjust the voltage to a "round" value ! In all cases, in order to limit the current that will flow in the circuit. A resistance of a few Ohms will do the job. For an easier calculation, use a round value, for example a 12 Ohm for a 12V voltage, that will make a 1A current. In this later case, it will have to dissipate 12W, so use an adequate sized resistance.

For example, if we measure 0.2V voltage drop in our contact, and we have a 1A current , that means the resistance is 200 milliOhm !

The larger the current, the lower the resistance you will be able to measure. A 1A current is often enough, but i have already used much more to detect very small contact resistances.

With the help of this simple method, i have solved a lot of problems on antennas that have been damaged and corroded with the years and for which a visual checkup didn't give any result.

You can also use this method to measure any low resistances in other situations, but be careful not to apply a voltage or current that could destroy your circuit !

I hope this article will be helpfull for you. I'ld be glad to here from you.

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