1. Ferrite Materials
The reactance and RF resistance of short Straight leads may be increased by
placing ferrite beads over the wire. The attenuation or insertion loss
characteristics of cables may be improved by adding ferrite beads to internal
leads, with almost no increase in size and weight. Other applications of
ferrites and beads in RFI-related work include:
(a) Improved isolation between stages of electronic devices by placing
ferrite beads on filament leads, B+, and bias lines.
(b) Improved isolation between modules by putting ferrite beads on
Wires with a ferrite material sandwiched between two
layers of insulation will provide excellent interference suppression at high
frequencies. Details of the properties of these cables can be found in SAE
publication AIR 4465 (Design and Handling Guide For RF Absorptive Type Wire
and Cable (Filter Line)).
2. Tapes and Coatings
The amount of noise guided and conducted by power transmission lines may be
greatly reduced by wrapping the conductor with a thin, high permeability
metallic tape. The skin effect losses, present with any conductor carrying RF
energy, are greatly magnified by coating the conductor with a thin layer of high
permeability material. The effect due to the coating is large, in the range of
25 kHz to 50 MHz.
3. Special Coax
Special coax is available for use in severe radiation and leakage situations,
such as those encountered in high power pulse applications. Triple-shielded coax
should be used where pulse generators are separate from the transmitter or other
load. To obtain maximum performance from triple-shielded coax, the design of the
terminal ends should take advantage of the three shields.
4. Fiber Optic Cables
Fiber optic cable is an alternative to metallic electrical cable for data
transfer within an aerospace system. It is immune to both electromagnetic
interference (EMI) and electromagnetic pulse (EMP), and provides electrical
isolation and secure data transfer.
Twisted shielded pairs have a 5 dB/km loss at about 10 kHz, while coax
has a 5 dB/km loss at about 10 MHz. Both twisted pairs and coax have frequency
dependent losses which require equalization. Fiber optic cable, on the other
hand, has a low transmission loss of about 2 dB/km and is independent of
frequency. Furthermore, fiber optic cable does not suffer from problems
associated with mismatch as in conventional wired systems, and it is
relatively free of signal ringing, echoes, and crosstalk. Table 4-1 gives the
size-weigh advantage of fiber optic cables.