CPR Flange
How a CPR Flange Seals and Aligns a Waveguide Run
A CPR flange exists to solve two problems that a bare contact flange cannot: keeping the inside of a waveguide run dry and pressurized, and guaranteeing that two waveguide bores line up without a step at the joint. It does this by splitting the interface into a complementary pair. The grooved CPRG half has a shallow rectangular channel cut around the aperture that captures an elastomer gasket, plus two hardened dowel pins. The mating CPRF half is flat, with clearance holes for those pins. When the four bolts are torqued, the flat face squeezes the gasket into its groove, the dowels register the two bores, and the result is a sealed, repeatable joint that can be pressurized with dry nitrogen.
Pressurization is the whole point of the design. Operating a radar feed or an earth-station uplink at high transmit power inside humid air invites condensation, corrosion, and ultimately arcing inside the guide. Filling the run with dry nitrogen at 15 to 30 psig raises the breakdown threshold and excludes moisture, and the captive CPR gasket is what holds that pressure across every joint in the run. A correctly torqued joint with a fresh gasket leaks under about 1 standard cubic centimeter per minute, so a sealed run holds its charge for weeks between top-ups from a small pressurization panel.
Electrically, the goal is to make the joint invisible. The dowel pins keep the two apertures aligned within a few thousandths of an inch so there is no reflecting step at the seam, and the gasket sits in the recessed groove rather than intruding into the TE10 mode field at the aperture wall. A clean CPR joint adds only a small reflection, contributing on the order of 1.02 to 1.05 VSWR per connection, which lets several CPR-jointed sections cascade without the mismatch budget running away.
CPRG Versus CPRF: Always Mate Opposites
The single most common field mistake is bolting two like halves together. A CPRG-to-CPRG joint puts two grooves face to face with no flat compression surface, so the gasket is never squeezed and the joint leaks. A CPRF-to-CPRF joint has no groove at all, so there is nothing to retain a gasket. Every working CPR connection is one grooved half against one flat half, written in ordering shorthand by size and letter, for example CPR137G to CPR137F for a WR-137 joint.
Gasket Compression and Joint Reflection
Compression (%) = (tfree − tcompressed) / tfree × 100 (target 15% to 30%)
Pneumatic leak / hold time:
thold ≈ (Vrun × ΔP) / (Patm × Qleak)
Single-junction reflection from an aperture step Δ:
Γ ≈ Δ / (2 × b) → VSWR = (1 + |Γ|) / (1 − |Γ|)
Where t = gasket thickness, Vrun = internal guide volume, ΔP = gauge pressure, Qleak = leak rate, Δ = bore misalignment step, b = waveguide narrow-wall height. Example: WR-137 (b ≈ 15.8 mm) with a 0.05 mm step → Γ ≈ 0.0016 → VSWR ≈ 1.003 per junction.
CPR Flange Family Reference
| CPR size | Waveguide | Band (GHz) | Bolts | Pressurizable | Typical use |
|---|---|---|---|---|---|
| CPR-650 | WR-650 | 1.12 to 1.70 | 4 (+ intermediate) | Yes (CPRG/CPRF) | UHF / L-band radar feeds |
| CPR-430 | WR-430 | 1.70 to 2.60 | 4 (+ intermediate) | Yes (CPRG/CPRF) | L / S-band transmit runs |
| CPR-284 | WR-284 | 2.60 to 3.95 | 4 | Yes (CPRG/CPRF) | S-band radar, earth stations |
| CPR-187 | WR-187 | 3.95 to 5.85 | 4 | Yes (CPRG/CPRF) | C-band uplink plumbing |
| CPR-137 | WR-137 | 5.85 to 8.20 | 4 | Yes (CPRG/CPRF) | C-band feeds, test runs |
| CPR-112 / 90 | WR-112 / WR-90 | 7.05 to 12.4 | 4 | Yes (CPRG/CPRF) | X-band radar, comms |
Frequently Asked Questions
What is the difference between CPRG and CPRF flanges?
They are the two halves of a CPR pair. CPRG (grooved) has a channel around the aperture that holds a captive gasket plus two dowel pins; CPRF (flat) presents a smooth face that compresses the gasket. A joint always mates one CPRG against one CPRF, never groove-to-groove or flat-to-flat. The dowels align the bores within a few thousandths of an inch to keep VSWR low. Ordering shorthand reads as size plus letter, for example CPR137G against CPR137F.
Can a CPR flange hold internal pressurization?
Yes, that is its purpose. The captive gasket lets the run be pressurized with dry nitrogen, typically 15 to 30 psig, to exclude moisture and raise breakdown voltage on high-power transmit systems. A correctly torqued joint with a fresh gasket leaks under roughly 1 standard cubic centimeter per minute, so a sealed run holds pressure for weeks. Plain UG cover flanges and bare contact flanges have no groove and cannot be pressurized without an added gasket adapter.
What torque and bolt pattern does a CPR flange use?
Sizes from WR-159 to WR-650 use four bolts in a rectangular pattern (#8-32 up to 1/4-20), with the largest guides adding intermediate bolts. Tighten in a cross pattern over two or three passes; typical final torque is 12 to 25 inch-pounds for smaller sizes, higher for large guides. Even clamping matters more than absolute torque, since uneven force cocks the gasket and opens a leak path. The dowels handle alignment, so the bolts only supply clamping force.