Waveguide Below Cutoff

by William D. Kimmel, P.E.

and Daryl D. Gerke, P.E.

Kimmel Gerke Associates, Ltd.

Waveguide shielding is not commonly needed in the commercial world where shielding needs are modest, but is a vital aspect of shielding where high attenuation is needed, notably in military environments.

Waveguide below cutoff is based on the fact that an electromagnetic field attenuates rapidly down a small diameter hole of sufficient depth, providing certain conditions are met. This fact allows small diameter holes to be made in conductive enclosures, as may be needed for ventilation, or as a pass through for non-metallic members.

Let’s take a look at the necessary requirements to satisfy the needs for a waveguide and how a waveguide can be used effectively.

Waveguide Requirements

First we need to give the necessary requirements for waveguide below cutoff. The cutoff frequency is given approximately by the relationship:

      fc = 7/d

where d is dimension of the longest path in the opening in inches, and fc is the cutoff frequency in GHz. Attenuation is minimal above the cutoff frequency.

Usually, the longest dimension is a diameter or a diagonal, but in the case of a slot, it is simply the length of the slot opening.

Second, the depth of the hole must be greater than the longest dimension of the opening, which would typically be a diameter or a diagonal. In the case of a seam, the longest dimension is simply the length of the opening, generally the distance between fasteners.

Having met this criteria, the attenuation is given approximately by:

       SE = 32t/d

where SE is shielding effectiveness in dB, t is thickness of material surrounding the hole and d is the longest dimension of the opening, often a diameter. t and d are dimensions in inches, or other consistent units, as this relationship is a ratio. Curiously, attenuation is fairly independent of frequency, once the “below cutoff” criteria has been reached.

As an example, let’s take a 1/8-inch hole of 5/8-inch depth, such as might be found in honeycomb:

SE = 32*5/1 = 160dB, ample for just about any shielding need.

Take care not to misinterpret the dimensions given (see Figure 1). The parameter “d”, so named because it represents a round hole with diameter d, is the longest dimension of the opening, not the shortest – if you have a slot 6 inches long and 1/16-inch high, the relevant dimension is the 6-inch slot.

So if we have a flange of one-inch depth, a distance between fasteners of 6 inches and a gap between two metal surfaces of 1/16 inch, we get SE = 32*1/6 = 5 dB – not to mention that we don’t meet the criteria of a depth longer than the longest dimension of the opening.

Basically, this says that the waveguide effect is negligible for most enclosure cases – in such a case, you are better assuming the shielding effectiveness of a slot antenna.

So How Does a Waveguide Compare with a Simple Opening?

We start with the shielding effectiveness of a simple hole:

       SE = 20*log(l/2L)

where l is wavelength and L is length of longest dimension of the opening. (As with the waveguide, the longest dimension is the key parameter.)

Continuing with the example we started above, let’s assume an opening of 1/8-inch diameter, or about 3mm. The wavelength is given by c = fl, or 300 = f (MHz)*l (meter), so we can compute attenuation in a table:

Compare this with the waveguide attenuation of 160 dB, as calculated above. As can be seen, attenuation for a slot antenna at the lower frequencies is very good, but once you cross over into the microwave range, the attenuation diminishes to the point where waveguide shielding becomes necessary.

Applications

Having met the criteria of a waveguide, let’s look at some of the common applications for a waveguide.

The honeycomb panel is widely used for shielding of ventilation openings in military-designed enclosures and in shield rooms. Most commonly found as an array of hexagon openings, rectangular openings are also in use. Honeycomb is particularly important in the radar frequencies (above 1GHz), where screen or perforated ventilation panels start to leak.

Honeycomb has surprising attenuation, actually higher than a solid metal panel of the same material, until way into the GHz range. Upon reflection, this is not surprising, as there is a much higher surface area in the honeycomb to produce the losses.

Controls and indicators. Inevitably, when we have a box, we need to have an interface between the electronics within and the outside world, specifically, controls and indicators. Figure 2 illustrates a potentiometer for a control and a fiber optic path for an indicator. Note that this requires dielectric material be used to penetrate the waveguide – if a metallic path exists, the waveguide effect is defeated.

Shield room pipe. The above technique can be extended to a shield room, where nonmetallic paths can be used to pipe data into or out of the shield room. As mentioned above, the data path needs to be non-metallic. Figure 3 shows the case where a data cable is passed through the waveguide. The cable serves as a parasitic antenna, picking up RF energy on one side, conducting it through the waveguide and re-radiating on the other side – basically destroying the shielding effectiveness. If it is necessary to pass a data cable through the pipe, the cable must be terminated within the pipe, commonly with copper wool. A better choice for cables is to terminate the cable at the shield boundary and avoid waveguide use entirely.

Summary

Waveguide below cutoff provides very high attenuation when prop- erly applied. The longest dimension of the opening needs to be kept small compared with the depth of the opening. Only dielectric materials are allowed to penetrate the waveguide. Any metallic penetration destroys the shielding effectiveness