A SCREENED FERRITE ROD ANTENNA
(1995)

KLIK HIER VOOR DE NEDERLANDSE VERSIE


The small black ferrite rod antenna and the big one do both
have a screening for the electric field. But the small one
is much more sensitive then the big one. How is that possible?

Screening of a ferrite rod antenne for the electric field
Navtex is a kind of radio telex system working on 518 kHz for the transmission of safety and security messages for the shipping traffic. For my home made Navtex receiver, I used a big ferrite antenna with screening for the electric field. The screening was absolutely necessary. Without screening, the electric field of the high voltage supply of the PC monitor caused much radio interference in the reception of the Navtex signals. The screening exists of aluminium- and printed circuit board with a split in the bottom. Without that split, not only the electric field, but also the magnetic field is screened and the ferrite antenna does receive nothing anymore!


Without the split in the screening, also the magnetic field
will be screened. The copper on the blue bottom print is broken.

The problem
By coincidence I found out that the ferrite rod antenna was much more sensitive without the aluminium screening. The ferrite antenna was retuned to maximum during the experiments. Only when the space between the ferrite rod and antenna was increased to more then 10 cm, the screening did not influence the sensitivity anymore. But then the size of the antenna becomes very large: 20 cm in diameter! The reason of the degradation of the sensitivity is that the magnetic lines are only in the center parallel to the aluminium screening and not at the tips. Currents are induced in the screening and that is causing the losses.


The solution: Screening with flatcable! It is not
possible to induce currents in the flatcable.
The screening is unfolded here for the photo!.
The flatcable screening folded up here.

The solution
The solution was to use flatcable instead of aluminium plate. Flatcable is used in computers for the connections to harddisks etc. The wires of the flatcable are grounded at one side. At the other side, the wires should not make contact to anything, not even to the other wires of the flatcable! In this way, only the electric field is screened and not the magnetic field. As separate wires are used instead of a metal plate, it is not possible to induce currents in the screening. The flatcable is folded up around the ferrite antenna. The ends of the ferrite antenna are also screened with flat cable. That is not possible with a metal plate as you do have a shortened winding then at the end of the ferrite rod.


Schematic diagram, flatcable folded at the dotted line.
The ends are also screened!


The Navtex receiver with RA3AAE mixer
big diagram

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