Charting an antenna

Recently, I’ve been working on weather satellite reception for the Russian Meteor-M No. 2-4 satellite, now that the NOAA APT satellites have been decommissioned.

I’ve decided to go with a simple half-wave dipole antenna for an initial design. I built the antenna last week, and have also found an old telescopic antenna in the vault that was the right size.

Tuning setup

Tuning an antenna, in a nutshell, refers to changing the physical characteristics of an antenna for it to match some characteristic impedance at a target frequency. Usually, we want the antenna impedance to match to 50 or 75 ohms at the frequency we want to receive on, since our cable and downstream components are also matched to 50 or 75 ohms respectively.

Setting up the VNA requires setting the start and stop frequency to sweep over, and calibrating the VNA. To do this, we need to present an open load, a shorted load, and a 50 ohm load. I then hooked up the antenna, and got this on the smith chart:

Smith chart from 100MHz to 150MHz

We can see that from 100MHz to 150MHz, we get an impedance of 48-j0.000000000138 at 138MHz — almost a perfect real 50 ohms at 137.9MHz, which is our target. Without any tuning, the antenna is a match, which is almost laughable how lucky that is.

Now, we just have to wait for a clean overhead pass of the satellite. From there, we hook up our antenna to an SDR, record the IQ data in SDR++, and let SatDump decode the image!