The first major trade-off in SCIF mechanical design is the requirement for fluid-flow diameter in a hydronic system versus the space limitations imposed by an RF-shielded wall penetration.
Increasing the pipe diameter reduces head loss; however, it increases RF leakage. Constraining the diameter to account for attenuation of electromagnetic radiation decreases the performance of the hydronic loop. Effectively engineering a hydronic waveguide requires the solution to both fluid dynamics and electromagnetic fields.
In the case of high attenuation associated with ICD 705 and TEMPEST, the construction method for a standard pipe penetration is the intentional misuse of an unintended slot antenna. In this scenario, the solution is to modify the dimensions of the Hydronic Waveguide to allow for total shielding with no negative influence on system hydraulics.
The primary method of blocking RF propagation for mechanical penetrations is operating as a waveguide below cutoff. For circular sections, electromagnetic waves exponentially decay with distance between the upper and lower boundary sections of a circular penetration if the frequency of the electromagnetic energy is below the critical cutoff frequency (f_c).
For a circular waveguide, the cutoff frequency (f_c) for the TE11 mode is given by:
c: Approximate speed of light in vacuum is 3 × 10^8 m/s
p'_{11}: First root of the derivative of the Bessel function (approx 1.841)
a: Internal radius of the aperture {m}
epsilon_r: Relative dielectric constant of the medium in the bore
For an air-filled penetration (epsilon_r = 1), a simplified engineering approximation gives the following expression for the cutoff frequency in relation to the inner diameter (D in inches):
f_c (GHz) ≈ \frac{6.92} D (inches)
A 1.0-inch air-filled sleeve has a cutoff at 6.92 GHz. RF energy can directly penetrate the opening only if the cutoff frequency is exceeded.
Liquid media changes the equations quite drastically. Water has a relative dielectric constant (epsilon_r of around 80 at standard temperatures, which actually hinders the propagation of waves and lowers the cutoff frequency to nearly 0.894 relative units (sqrt(80) approx 8.94).
A typical 1.0” pipe filled with water loses its ability to stop signals above about 770 MHz. This pipe allows cellular, Wi-Fi, and even Department of Defense communications to transmit directly into the secure enclosure.
To reach a shielding effectiveness of greater than 100 dB at up to 18 GHz, the geometry must apply an electric length-to-diameter (L/D) decay loss from evanescent modes.
Maintaining a strict 3:1 or 4:1 L/D ratio and employing open, large-bore piping will lead to unacceptable spatial footprints in wall studs and ceiling plenums.