Combating Insertion Loss in 39 GHz 5G Infrastructure

The Physics of 39 GHz Signal Attenuation

When we started machining custom components back in 2012, the requests crossing my desk were mostly for X-band and Ku-band systems. Today, the global rollout of 5G New Radio (NR) in the FR2 frequency bands has forced network engineers to confront a brutal reality of physics. Frequencies at 28 GHz and 39 GHz provide enormous contiguous bandwidth, allowing multi-gigabit data rates. However, moving electromagnetic energy at these millimeter wave frequencies from the baseband processing unit to the massive MIMO antenna array introduces severe, highly destructive insertion loss.

In traditional sub-6 GHz LTE networks, we relied heavily on standard coaxial cables (such as LMR-400 or RG-214) to transport RF power up the tower. At 2 GHz, a coaxial cable functions perfectly. The solid copper center conductor efficiently carries the signal, and the dielectric core provides stable impedance. But as we scale up toward 39 GHz, the fundamental physics of the coaxial cable begins to work against us.

The Skin Effect and Dielectric Heating

Insertion loss at high frequencies is driven primarily by two mechanisms. The first is the Skin Effect. As frequency increases, the magnetic fields generated by the alternating current force the electrons to the absolute outer surface of the conductor. At 39 GHz, the entire RF current is traveling in a microscopic surface layer less than 0.4 micrometers deep. The center mass of the copper wire is essentially dead weight. Because the electrons are squeezed into such a tiny physical area, the ohmic resistance skyrockets, converting massive amounts of transmitter power directly into wasted heat.

The second mechanism is Dielectric Loss. In a coaxial cable, the space between the center conductor and the outer shield is filled with a dielectric material, typically PTFE (Teflon). At 39 GHz, the alternating electric field causes the molecular dipoles in the Teflon to vibrate violently 39 billion times per second. This molecular friction creates extreme heat, absorbing the RF energy before it ever reaches the antenna.

The Waveguide Solution

To overcome the massive attenuation inherent in coaxial cables, I constantly advise clients building high-performance 5G macro cells and point-to-point backhaul links to abandon the center conductor entirely. We must transition to rectangular waveguides. A waveguide is simply a hollow, highly machined metal pipe. For 39 GHz applications, we typically specify the WR-28 waveguide standard.

A waveguide completely eliminates dielectric loss because the RF wave travels through pure air (or a pressurized nitrogen vacuum). It also severely mitigates conductor loss. Because there is no tiny center pin, the electrical currents flow along the vast, wide interior walls of the rectangular pipe, providing a massive surface area for the electrons and drastically lowering the ohmic resistance. When we test these in our shop, the difference in thermal performance is night and day.

Power Handling and Voltage Breakdown

Beyond insertion loss, modern 5G massive MIMO systems require substantial transmit power. A coaxial cable has a hard physical limit based on voltage breakdown. If the voltage of the RF wave arcs across the tiny gap between the center pin and the outer shield, it burns through the Teflon and destroys the cable. A standard 2.92mm coaxial cable at 39 GHz can typically handle only 10 to 20 Watts of continuous wave (CW) power before thermal or voltage failure.

A WR-28 waveguide has no center conductor to arc against. The voltage must cross the entire physical height of the hollow pipe to spark. Consequently, a WR-28 waveguide can safely handle thousands of Watts of peak power without any risk of dielectric breakdown. This makes waveguides absolutely mandatory for high-power radar installations and dense urban 5G macro sites.

Comparative Performance Thresholds

The decision to transition from coaxial to waveguide is ultimately an engineering trade-off between mechanical flexibility, cost, and RF performance. Coaxial cables are highly flexible and cheap. Waveguides are rigid, require precision CNC machining, and are difficult to route around tight corners without using specialized E-Plane and H-Plane bends.

Transmission Medium	Frequency Range	Attenuation at 39 GHz	Power Handling	Primary Drawback
Precision Coaxial (2.92mm)	DC to 40 GHz	1.5 to 2.0 dB per foot	~20 Watts CW	Extreme loss over distance; high thermal heat.
WR-28 Waveguide	26.5 GHz to 40.0 GHz	0.15 to 0.25 dB per foot	>1,000 Watts Peak	Rigid geometry; cannot pass DC power.
Dielectric Waveguide	30 GHz to 100 GHz	0.8 dB per foot	Low Power	Experimental; highly sensitive to external moisture.

Architectural Strategies for 5G Deployment

In the early days of cell towers, the massive radio units were located in a shed at the base of the tower. Long coaxial cables carried the signal hundreds of feet up to the antennas. At 39 GHz, running a 100-foot coaxial cable would result in over 150 dB of signal loss, meaning absolutely zero power would reach the antenna. Even running a 100-foot rigid waveguide is economically unfeasible; I've seen the raw material costs for machined brass, and it simply doesn't scale for a nationwide rollout.

To solve this, the telecom industry developed the Active Antenna Unit (AAU). The baseband processor is kept on the ground, but it sends digital fiber optic data up the tower instead of analog RF. The actual RF power amplifiers, mixers, and synthesizers are bolted directly to the back of the antenna at the very top of the tower. This reduces the required RF transmission distance from 100 feet to less than 6 inches.

For that critical final 6 inches linking the power amplifier output to the antenna feed horn, precision waveguide assemblies are heavily utilized. Even over short distances, saving 1 dB of insertion loss by using a waveguide instead of a coaxial cable means the power amplifier can operate using significantly less DC electricity. Across a massive nationwide network featuring millions of 5G cell sites, optimizing that final waveguide link translates into massive savings in municipal power consumption and reduced thermal cooling requirements.

Conclusion

The transition to 39 GHz fundamentally changes the rules of RF plumbing. We can no longer rely on the flexible convenience of coaxial cables for high-power or long-run signal chains. By understanding the severe limitations of dielectric heating and the Skin Effect, system architects must aggressively implement hollow waveguide structures to protect their link budgets, maintain signal integrity, and ensure the successful deployment of next-generation wireless infrastructure. If you're struggling with insertion loss in your FR2 systems, give us a call. This is exactly the type of problem we solve every day.