Direct-read Impedance Using a Scope

Measure complex input impedance of a network, antennas or device using only a Dual-trace oscilloscope and an AC/Audio/RF energy source. This approach is good for Z's (impedances) much less than 1K Ohm.

This technique is usable from DC through as high a frequency as your Oscilloscope, DC source/AC/Audio/RF generator and circuit construction techniques will allow.

I came upon this simple (but functional) approach on very short notice and out of a desparate need to 1) find the resonant frequency of a matching network and 2) measure the impedance of a quarter wave vertical ground plane antenna at 1.06 MHz (at a commercial broadcast radio station). Using this approach (dual trace scope and the network shown below) also beats using a bridge (like the GR-60) when quickly looking for resonance in a network or antenna (the bridge has to be 'tuned' or balanced each time when the frequency is changed).  The change in phase and magnitude between Reference and Test ports can easily be seen to change when changing the RF generator's frequency - and the resonant point can easily be seen.

Note: A Vector Voltmeter (such as an HP Model 8405) may also be substituted for the Dual-trace Oscilloscope.

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