Overview
Ferrite braid chokes can be used to supress unwanted waves existing on the outer shield of coaxial cables. There are many ways which can cause surface waves on coaxial cables, e.g. the transition from a coaxial line to a symmetrical line, the coupling from other lines guided in parallel, or often also by radiated fields. In real life applications surface waves appear mainly in the frequency range from approx. 5 MHz to 200 MHz.
Sheath currents have common mode characteristics, therefore they are influenced strongly by line length and guidance as well as the spacing to the surrounding. Depending on the cable length and common mode impedance strong frequency selective resonances may appear. During fieldstrength measurements it is a good practice to touch the coaxial cable by hand and watch the measured results.
In case of handsensitive results the presence of surface waves is very probable.Because of the complex nature of surface braid waves there is only less information available from literature. Estimations can be made using the transmission line theory, assuming impedances depending on the location. Furthermore source and load impedances are a function of frequency and usually unknown.
Application:
For practical applications the use of surface wave chokes is an easy and usually efficient way to improve the reproducability. The surface wave choke is used at that location, which is under suspect to generate the braid current. The wanted signal path through the inner coaxial cable is of low loss, whereas the surface wave between the outer shield and the surrounding is attenuated by the inductance of the ferrite toroid. Typical applications for surface wave blocking chokes are e.g.:
- Fieldstrength measurements, especially with electrically small antennas
- Antenna measurements with Baluns of unknown or unsufficient common mode rejection
- Disturbance power measurement with absorbing clamps
- RF-voltage lift and unknown
- RF-ground conditions
- Hand sensitive measurements if the coaxial cable or the equipment housing is touched
