Antenna shunt-coil as featured in my article in the 2014 May edition of QST, on building a 17 meter high performance antenna for your mobile.
For matching high performance mobile HF antennas, this is the shunt coil that I build, and I have one of the same mounted in the coax at the feedpoint of my HF antenna mount on my mobile.
In fact, I use the same shunt coil with any one of my gearmotor antennas, or any one of my switchable band antennas that may reside on my mobile on any given day.
For matching any one of my HF antennas, a small amount of capacitive reactance taken from the antenna, along with the inductance of the coil, form a LC Network that changes up the antenna's low impedance to that of the 50 ohm feed line connected to the antenna.
This shunt coil has proven to work very well, and although shunt coils should not be enclosed, I use a enclosure with mine, as that keeps the coil out of the environment in the inclement weather of southern Alberta. Once the shunt coil is installed in place, the PL259 connectors of the coax connected to the Amphenol tee are wrapped with rescue tape.
The photo lists all the information required for you to build a shunt-coil for your mobile.
If you have any questions in regards to building this shunt-coil, send me an email.