Part 1 – Checking the Velocity Factor of the Coax Cable
I have a homebuilt Collinear Antenna on one of my two ADS-B receivers. It’s a 8 element antenna, built without any real effort of making sure that all the elements where the right length. So it’s about time to try to do it properly.
To build a good antenna, you would preferably use a Vector Network Analyzer so you can do proper measurements of your antenna design. With it you can easily check stuff like SWR and optimize the impedance of the antenna at the frequency it’s intended to be used at. In my case 1090MHz for ADS-B. But if you don’t have the money to spend, you have to improvise. I’m going to build a 12 element Collinear Antenna out of cheap RG-58 coax cable. But to calculate the right length of each element, we need to know the Velocity Factor, or the signal speed through the cable compared to light traveling in vacuum. So let’s do the measurements.