RF Pipe Penetrations (Waveguide Feed-Throughs)

What an RF Pipe Penetration Does

An RF pipe penetration carries a non-conductive utility - water, gas, compressed air or fibre optics through the wall of a shielded enclosure without breaking its shielding effectiveness.

It works on the waveguide-beyond-cutoff principle. A circular tube acts as a waveguide, below its cutoff frequency, that waveguide attenuates RF heavily. The core diameter sets the cutoff frequency - the smaller the bore, the higher the frequency it blocks. The core length sets how much attenuation you get.

As a rule of thumb, keep the length at least four times the diameter. So a penetration is not a hole in the shield. It is a sized tube that lets your utility pass while the geometry keeps RF in or out.

Find your size - cut-off frequency by diameter

Cut-off frequency for a circular waveguide:

• fc = c / 3.412r
• fc = cutoff frequency
• c = speed of light, 1.181 × 10¹⁰ in/s
• r = radius of the circular waveguide, in inches

General rule of thumb:

Make the waveguide length at least four times its diameter.

_{Larger bores lower the cutoff, so they shield less at high frequencies. Above ½", you will not hold 100 dB at 10 GHz, size to your highest frequency of concern.}

Fire suppression & VESDA pipe penetrations

Fire suppression and VESDA pipework are the most common reasons to penetrate a shielded room and the easiest place to lose your shielding effectiveness.

APC supplies pipe penetrations for water and gas fire-suppression feeds and for VESDA aspirating smoke-detection pipework. Each penetration passes the pipe while holding RF attenuation across the bore. Where conductive metal pipe runs to the penetration, fit a dielectric union. It breaks the metal path and keeps the single-point grounding that a shielded enclosure depends on. The penetrations are fully NPT threaded, so they connect to standard plumbing on the incoming side.

Dielectric unions

Dielectric unions (part numbers 551083–551091) maintain isolation from ground. Use wherever conductive piping meets the shield, so you do not create a hard ground that degrades shielding performance.

Honeycomb Inserts

Honeycomb inserts suit high-frequency or gaseous applications, where a single open bore would let too much RF through. The honeycomb subdivides the bore into many sub-cutoff cells, raising attenuation while still passing gas or air.