The goal of this demonstration is to explain the importance of a matched network and the role of transmission lines (coax) for an HF Antenna matched network. This network is designed for the 20-meter band in the HF domain of the radio frequency region of the electromagnetic spectrum.

Consider you have an HF antenna load, which is positioned on a tower. The tower height is a consideration as a feed coax line will be connected to the antenna from the bottom (roughly) of the tower. Secondly, another coax line will be connected from the base of the tower to the radio station.

The reflection coefficient is the measure for an impedance matched network. A matched network will mean that loss will be minimal. SimSmith is a free tool that is useful for smith chart matching. In SimSmith, the load (left), transmission lines (as mentioned in the previous paragraph) and the radio are plotted on the smith chart.

The length chosen for T1 was chosen at 18.23 feet, which gives a clear shot for an impedance match towards the center using a stub transmission line.

We now add a shorted stub between both coax lines and adjust the length of the excess line until the impedance is matched at the radio station.

As shown above, the the excess length on the stub is about 6′. Plotting the SWR shows that the system is matched well for the whole band, meaning that this station is set up well for an HF radio broadcasting station for extra class amateur radio broadcasters.