An RF choke for a 1.8-30MHz coax power injector – LF1260 core

This article describes a prototype RF choke (RFC) for use in a power injector for 50Ω coax over range 1.8-30MHz. Power injector / extractors are often used to connect power and / or signalling on a shared common RF coax feed line to accessories such as remote antenna switches and ATUs.

Design criteria are:

  • Insertion VSWR of the RFC in shunt with 50+j0Ω < 1.1;
  • Dissipation < 2% of a 100W transmitter.

The core chose is a LF1260 ferrite suppression bead from Jaycar. It is a medium / high µ core readily available in Australia at $7.50 / 6.

 

Above is the prototype RFC wound with data cable wire for the purpose of measurement. In application it could be wound with 1mm enameled copper or PTFE insulated wire (Curie point is lowish at 120°+, but it still benefits from higher temperature insulation).

Above is a plot of measured impedance of the RFC. From that we can calculate the relative power dissipated in the core and InsertionVSWR in its application in shunt with a 50+j0Ω load.

Above is the calculated dissipation and InsertionVSWR based on impedance measurement.

The design is of a broadband choke, but it is obvious that it is has a broad self resonance around 6MHz. Using more or less turns changes the characteristic, as does substituting the core (dimensions and material).

A common mistake in making these things is to take an unknown ferrite core and pile as many turns on as possible in the mistaken belief that more turns equals higher inductance equals higher RF impedance. It is a naively simple model that is quite inadequate for this application, it can result is a RFC that is operating well above its first self resonant frequency and having quite low RF impedance.

A similar design using a Fair-rite core will be published in the coming weeks.