via HACKADAY: Your Multimeter Might Be Lying To You

Multimeters are indispensable tools when working on electronics. It’s almost impossible to build any but the most basic of circuits without one to test and troubleshoot potential issues, and they make possible a large array of measurement capabilities that are not easily performed otherwise. But when things start getting a little more complex it’s important to know their limitations, specifically around what they will tell you about circuits designed for high frequency. [watersstanton] explains in this video while troubleshooting an antenna circuit for ham radio.

The issue that often confuses people new to radio or other high-frequency projects revolves around the continuity testing function found on most multimeters. While useful for testing wiring and making sure connections are solid, they typically only test using DC. When applying AC to the same circuits, inductors start to offer higher impedance and capacitors lower impedance, up to the point that they become open and short circuits respectively. The same happens to transformers, but can also most antennas which often look like short circuits to ground at DC but can offer just enough impedance at their designed frequency to efficiently resonate and send out radio waves.