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Sheath Current Chokes

Serge Stroobandt, ON4AA

Copyright 2013–2016, licensed under Creative Commons BY-NC-SA

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Avoid sheath currents

Common-mode sheath currents on the coaxial feed line should be avoided because:

Origin of the common mode

The coaxial feed line is invariably located in the near-field of the antenna. This implies that large currents are induced on the sheath of the coaxial feed line. Most often, the common mode on the coaxial feeding cable is a continuation of the standing wave that exists over the length of most antennas. (Aperiodic resistive antennas and leaky-wave antennas are notable exceptions.)

This standing wave, in turn, results from the abrupt change in conductivity \(\sigma\) and/or permitivity \(\epsilon_{r}\) encountered at the antenna ends. Such an abrupt change is called “a boundary condition” in technical literature.

Being a standing wave implies that, for any given frequency, the common mode will show current minima every halve wavelength, starting from the antenna ends. Without a current balun and/or a sheath current choke, these current minima will repeat further down the coaxial feed line.

W2DU sheath current choke

The W2DU sheath current choke consists out of a large number of ferrite toroids that are slid over the sheath of a coaxial cable (Figure 1). Black heat shrink can be used to keep the ferrite toroids in place.

A W2DU sheath current choke prior to applying heat shrink.

A W2DU sheath current choke prior to applying heat shrink.

The device is often mistakenly called a current balun, which it is not because it does not force any currents to be equal. It merely prevents a current to flow on the sheath of a coaxial cable.

A ferrite material type is selected both for high permeability \(\mu_{r}\) and for maximal absorption over the desired frequency span. For HF and in order of preference, Fair-Rite® material 31 and Amidon™ material 73 and material 77 are all very appropriate. I procured yet a different kind of ferrite toroids from the German company DX-Wire, which I used for my home-built sheath current chokes. The company web site also publishes measurement results for different choke designs.

Optimal placement

At its lowest employable frequency, a sheath current choke typically inserts only about 500Ω impedance in series with the sheath of the coaxial cable. This means that sheath current chokes are ineffective at places along the feed line where the common mode wave has a current minimum, because the common mode impedance there will be high (typically about 2kΩ). Sheath current chokes are most effective at common-mode current maxima (Figure 2); i.e. at a distance \(\frac{\lambda}{4}\) from the antenna ends and at odd multiples thereof: \(\frac{3\lambda}{4}\), \(\frac{5\lambda}{4}\), …

A coaxial feed line runs from a 3-element 20m-band Yagi-Uda antenna at a height of about 15m (50ft) to the ground. No balun is being used. The curved lines are a measure for the currents on the antenna and the outer sheath of the coaxial cable. Current maxima along the coaxial line are good locations for inserting a sheath current choke. Current minima should be avoided. A current balun at the center of the feeding element should also be used with this type of antenna.

A coaxial feed line runs from a 3-element 20m-band Yagi-Uda antenna at a height of about 15m (50ft) to the ground. No balun is being used. The curved lines are a measure for the currents on the antenna and the outer sheath of the coaxial cable. Current maxima along the coaxial line are good locations for inserting a sheath current choke. Current minima should be avoided. A current balun at the center of the feeding element should also be used with this type of antenna.

Practical considerations

  1. By all means, use a decently made, properly rated (frequency and power) current balun at your antenna feed point. This will be a first, broadband defence against common-mode sheath currents. For the sake of clarity; a current balun and a sheath current choke are two different devices. Both devices can be found combined in the same housing, but this is not very common.

  2. Current baluns tend to be less effective at lower frequencies because they produce their common-mode series impedance from inductance. Therefore, common-mode sheath current rejection may not be sufficient at the lower frequency bands. A sheath current choke may be placed at current maxima of the lowest frequency band, i.e.:

    • At \(\frac{\lambda}{4}\) distance measured from the antenna ends, where \(\lambda\) is the wavelength of the lowest frequency band, and/or
    • At odd multiples of this length, i.e. \(\frac{3\lambda}{4}\), \(\frac{5\lambda}{4}\), …

    This will normally also handle the common-mode wave rejection for the second lowest frequency band if this is a harmonic of the lowest.

  3. Sheath current chokes close to the antenna feed point will help to preserve the radiation pattern of the antenna. Sheath current chokes closer to the shack may help better against RFI.

  4. If you can, connect the coax sheath to earth at both ends and place the coaxial cable on or below ground.

Measuring choke impedances

Nowadays, testing a toroidal ferrite core is really easy; put a wire through it and connect it to the measuring stand of a calibrated vector network analyser (VNA). Prior calibration is done using two paralleled high-precision 100Ω resistors and the same wire without any ferrite clamp. At the current price point of the miniVNA Pro (Linux supported!) and consorts, there is no good excuse for any self-respecting ham not to own a VNA.

For a complete sheath current choke, the basic premise is to measure it like any other inductor. Measure the impedance between both ends of the coaxial screen at the lowest desired blocking frequency. This ofcourse is only possible when the coaxial cable is not much longer than its enclosing choke. Measuring between the two center pins of the coaxial cable also works.

Choke testing with the miniVNA Pro over a wireless Bluetooth connection. The grey ferrite clamp is a TDK ZCAT 1325-0530A, the black ones are no-name counterfeit (see text).

Choke testing with the miniVNA Pro over a wireless Bluetooth connection. The grey ferrite clamp is a TDK ZCAT 1325-0530A, the black ones are no-name counterfeit (see text).

Beware of counterfeit

Above-mentioned test procedure came in handy when I compared the quality of counterfeit no-name ferrite clamps against the original TDK ZCAT 1325-0530A (Figure 3). Chinese counterfeit was shipped despite ordering original TDK clamps from Wen Juntao, just_for_survive on eBay. Apart from the labelling, both type of clamps look the same. However, there are slight differences in the visual appearance and weight of the ferrite material. Impedance measurements with a calibrated miniVNA Pro revealed the Chinese knock-offs to be consistently under-performing (Table 1 & Figure 4).

Measured impedance of ferrite clamps – original TDK ZCAT 1325-0530A vs. counterfeit
\(f\,(\text{MHz})\) \(\lvert\overline{Z}_\text{TDK}\rvert\,(\Omega)\) \(\lvert\overline{Z}_\text{fake}\rvert\,(\Omega)\) \(\Delta\)
3.5 27.0 17.3 -36%
10 55.7 39.7 -29%
30 101.5 89.3 -12%
Measured impedance of ferrite clamps at HF; higher values are better. Green: original TDK ZCAT 1325-0530A; Cyan: no-name counterfeit

Measured impedance of ferrite clamps at HF; higher values are better. Green: original TDK ZCAT 1325-0530A; Cyan: no-name counterfeit

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