TDR with the NanoVNA
chrisprosser
The performance of my satellite downlink station is nowhere near as good as I would like. I been doing some various troubleshooting measures to try and figure out what is going on. In my previous post, you can see some of my explorations about antenna tuning. In this post I took my first pass at exploring the feedline situation. It’s been up for at least five years and wanted to make sure nothing untoward had happened to it.
To help measure it, I ran a TDR test on the feedline. This is not a perfect measurement as I left the antenna attached, which messes up some things. But it gives you the opportunity to measure where impedance changes occur in the feedline. Here’s what the resulting plot looks like:
At first I was terribly confused thinking my feedline was terribly broken. It’s important to note that the current NanoVNA-Saver software doesn’t present TDR the way most people expect. It stacks the impedances of the various segments, so you shouldn’t read that big flat segment in the middle as being at 90 ohms.
Breaking it down by segment I get the following: short stub that adapts SMA to UHF connector; 10 feet of LMR 400 equivalent; a big impedance jump as the signal travels through a 3 inch barrel connector through a door; 30 feet of LMR 400 outside; another barrel connector; 10 more feet of LMR 400; then the antenna.
What did I learn in the end? It looks like my various lengths of cable are fine. Most importantly I learned that UHF connectors have a surge impedance of around 35 ohms instead of the 50 ohms we are looking for. That causes reflections and increases the return loss of your feedline. Wikipedia article on the impedance of UHF connectors: https://en.wikipedia.org/wiki/UHF_connector
So where does that leave me?
This showed I didn’t have any problems in my feed lines other than the ones I caused myself by using UHF connectors and patching together shorter lengths of cable. My next steps to evaluate the system performance will be to measure the SWR and return loss of just the feedline without the antenna connected. This means I’ll replace the antenna with a 50 ohm calibration standard to measure return loss; then replace it with a dead short to measure insertion loss. More detail here: https://www.tek.com/blog/improving-vna-measurement-accuracy-quality-cables-and-adapters
I’ve been having a strong desire to replace every UHF connector with a Type N connector. These measurements are an attempt for me to quantify what, if any, practical improvement I would see from that change. I fear that my real problem is that I live in a city and there is just far too much noise around me.