Analyzing my 30m dipole using Oscilloscope + Function Generator

I followed W2AEW’s video to analyze my antenna.

After building my 30m PSK-31 beacon, I needed to analyze the 30m dipole I have set up in my backyard to make sure it was either close enough to 50 ohms, or build a matching network to tune it before I could transmit into it. I don’t have an antenna analyzer like a MFJ-259 or a nanoVNA, so I followed W2AEW’s notes to use my oscilloscope, function generator, and a small little circuit to analyze my antenna.

The 30m dipole is only about 10 feet off the ground, and is slightly less than 14m in total length. (I accidentally cut off a small part of it) Hence, I expected its radiation resistance to be lower than the theoretical 73 ohms (because it’s close to the ground), and slightly more capacitive because it is shortened.

I built up the resistive bridge, which has no more than three 50 ohm resistors (I used smd ones), 2 1k resistors, and a BNC connector soldered onto a small piece of copper clad board:

Simple enough so far. I then connected my antenna to the BNC connector, connected the function generator to the left-most part of the board (the other ground clip attached to ground at the right-most part of the board), and probed the two legs of the 1k resistors. Make sure your oscilloscope probes are set to 1x and not 10x, or else you won’t see anything. The probes are terminated with 50 ohms at the scope (although this does not seem to be crucial). My scope doesn’t have built in 50 ohm termination, and I don’t have 50 ohm pass through terminators, so I just used BNC tees with 50 ohms on one end.

Then I set my signal generator to output a 10.148MHz sine wave, and made the measurements of both traces: peak voltage and time delay (used to calculate phase offset). Doing the calculations as shown in the notes, I found that my antenna had a complex impedance of 35.7 - 90.2j ohms. This is capacitive, and matched my initial predictions.

To counteract the capacitive reactance of the antenna, I placed a ~0.5uH inductor in series with the antenna. This made the yellow and blue waveforms stack on top of each other, meaning the capacitive reactance was cancelled out. I was then able to use my beacon with this antenna.