Air-Dielectric Cable
Understanding the Air Dielectric Cable
If you look at the massive 1,000-foot TV and radio towers near your city, you will see massive black pipes running all the way to the top. Those are not water pipes; they are massive RF coaxial cables. But if you cut one open, you won't see white foam inside like your home TV cable. You will just see empty space. These are Air Dielectric Cables.
The Threat of the Melted Foam
A standard coaxial cable uses solid white foam to keep the center wire from touching the outer wire.
If a massive TV station blasts 50,000 Watts of raw RF power through a foam cable, the foam will violently absorb the energy, heat up, and melt. The center wire will sag, touch the outer wire, and cause a massive, catastrophic electrical explosion that will destroy the multi-million dollar transmitter.
The Hollow Power Pipe
Air Dielectric Cables use physics to survive.
- The core is a massive, hollow copper tube. The outer shield is an even larger corrugated copper pipe.
- There is no foam. The center tube is held perfectly in place by a continuous, microscopic spiral of plastic, or tiny plastic discs placed every few inches.
- The cable is 95% empty air.
- Because air is physically incapable of melting, the TV station can safely blast tens of thousands of Watts of raw energy up the tower with virtually zero heat generation and zero signal loss.
Key Equations
An Air Dielectric Cable is an elite, high-power RF coaxial transmission line utilized in massive broadcast towers, military radar installations, and macro-cellular base stations. In...
Key specifications:
000 Watts | 95 % | 0.3 dB | 35 dB | 60 dB
S-params: IL=−20log|S21|, RL=−20log|S11|
Comparison
| Connector | Freq Max | Impedance | Power | Interface |
|---|---|---|---|---|
| SMA | 18 GHz | 50 Ω | 0.5 W | Threaded |
| N-Type | 11 GHz | 50 Ω | 5 W | Threaded |
| 2.92mm (K) | 40 GHz | 50 Ω | 0.3 W | Threaded |
| 1.85mm (V) | 67 GHz | 50 Ω | 0.2 W | Threaded |
| 1.0mm (W) | 110 GHz | 50 Ω | 0.1 W | Threaded |
Frequently Asked Questions
Why do Air Dielectric cables require air compressors?
To stop condensation. Because the cable is a massive, 1,000-foot hollow pipe hanging outside, the temperature changes violently from day to night. This causes liquid water to condense inside the pipe, which will destroy the radio wave. The tower operators bolt a massive industrial air compressor (a Dehydrator) to the bottom of the cable, constantly pumping ultra-dry, pressurized air into the pipe to force the moisture out.
What is HELIAX?
HELIAX is the incredibly famous, trademarked brand name of corrugated Air Dielectric (and high-end foam) cables manufactured by Andrew Corporation (now CommScope). In the RF engineering world, the brand name is so dominant that many engineers simply refer to any thick, corrugated cell tower cable as "Heliax," similar to calling a tissue a Kleenex.
Are Air Dielectric cables hard to bend?
Astronomically hard. Standard foam coax is flexible. An Air Dielectric cable (which can be up to 5 inches in diameter) is essentially a solid copper plumbing pipe. Bending it requires massive mechanical tools and extreme care. If the engineer bends the cable too sharply, the outer copper pipe will physically dent inward, permanently ruining the mathematical impedance of the cable and forcing them to throw it away.