A coaxial cable is designed to be a neutral pathway for radio frequency (RF) energy — carrying signals between a transmitter or receiver and the antenna without radiating on its own. But under certain real-world conditions, that cable can start acting like an antenna itself.

This unintended behavior can degrade performance, cause interference, and create compliance issues with FCC and EMC regulations. Understanding why this happens — and how to prevent it — is essential for any RF installation, from Wi-Fi and cellular systems to IoT, GPS, or 5G setups.

1. How an Antenna Cable Becomes a Radiator

Unintentional Radiation (Cable as Transmitting Antenna)

In a perfect coaxial cable, RF energy flows along the center conductor, with the outer shield carrying equal and opposite current that prevents external radiation.

However, several conditions can disturb this balance and cause RF currents to appear on the outer surface of the shield, effectively turning the cable into a radiating element:

• Compromised Shielding:
  Physical damage, poor manufacturing, or low-quality braiding allows the electromagnetic field to “leak” through. Once shielding effectiveness drops, part of the signal escapes, and the cable radiates like a long wire antenna.

• Impedance Mismatch:
  Using a 75-ohm cable (like RG-6) in a 50-ohm system (common in Wi-Fi, LTE, and IoT) causes reflections and standing waves. These mismatched conditions drive current onto the outer shield, resulting in radiation.

• Improper Grounding:
  If the cable shield isn’t grounded where it should be — typically near the device end — stray currents can circulate on the cable’s exterior. This unbalanced current can make the cable radiate like an unintended monopole antenna.

• Poor Connector Termination:
  Connectors that aren’t crimped, soldered, or sealed correctly can act as small radiators or impedance breaks. Even a slight discontinuity at a connector interface can create an imbalance that excites shield currents.

• Excessive Lengths:
  Long coax runs — particularly when approaching a quarter or half wavelength at the operating frequency — can behave as efficient radiators if unbalanced. For instance, a 2.4 GHz signal has a wavelength of 12.5 cm; a 1.5 m cable equals roughly 12 wavelengths, which can produce standing-wave patterns if not terminated properly.

2. The Receive-Side Effect: Cable as an Unwanted Antenna

Even when you’re not transmitting, the same physical principles apply in reverse.

A coaxial cable can pick up electromagnetic interference (EMI) or radio frequency interference (RFI) from its environment — especially when it runs through electrically noisy areas.

Common Causes:

• Proximity to Noise Sources:
  Power lines, motors, LED lighting, switching power supplies, and Wi-Fi access points can induce unwanted signals in nearby coaxial runs.

• Strong Nearby Transmitters:
  Cellular base stations, CB radios, or broadcast antennas can couple energy into the cable shield, introducing common-mode noise that travels along the cable into sensitive RF circuits.

• No Balun or Ferrite Choke:
  Without a choke or balun, there’s nothing to block those outer-surface currents. The result: interference and reduced receiver sensitivity.

Symptoms You Might See:

• Fluctuating signal strength or dropped connections

• Unexplained noise floor increase

• Poor return loss or standing-wave ratio (SWR)

• RFI issues in nearby electronic equipment

Essentially, when the coax acts as both a signal conduit and a receiving antenna, your system’s noise performance deteriorates — even if the antenna itself is working properly.

3. Practical Consequences of a Radiating Cable

A cable behaving like an antenna can cause a range of problems that often appear unrelated at first:

1. Signal Degradation:
   Re-radiation along the line alters impedance and phase relationships, reducing power transfer efficiency and distorting the transmitted waveform.

2. Interference to Other Devices:
   The stray RF field can couple into nearby cables or circuits, producing EMI compliance failures or audible noise in nearby audio systems.

3. Reflected Power and Heat:
   When mismatch-induced standing waves occur, part of the RF energy is reflected back toward the transmitter. That can overheat amplifiers, reduce effective radiated power, and shorten equipment life.

4. Regulatory Risk:
   Any unintended radiation from a cable might exceed FCC or CE emission limits, creating compliance issues in certified systems.

4. How to Prevent Your Cable from Acting Like an Antenna

a. Use Quality Coaxial Cable

• Choose double-shielded or triple-shielded types (e.g., LMR-400, LMR-240, or RG-8).

• Look for foil + braid combinations that achieve >90 dB shielding effectiveness.

• Avoid cheap “video-grade” cables (often 75 Ω) for 50 Ω RF applications.

b. Match Impedance

Keep all components — cable, connectors, and devices — at the same impedance, typically 50 Ω for RF/antenna systems. A mismatch greater than 2:1 VSWR (Voltage Standing Wave Ratio) will cause reflection and imbalance.

c. Proper Termination and Connectors

• Use high-quality connectors (SMA, N-Type, TNC, etc.) rated for your frequency.

• Make sure the braid and foil make full contact with the connector body.

• Avoid excessive bending near connectors; mechanical stress can loosen the shield contact.

d. Apply Ferrite Chokes or Baluns

• Ferrite beads or clamp-on chokes suppress unwanted common-mode currents on the outer shield.

• For directional antennas, a 1:1 current balun (choke balun) is often installed at the feedpoint to maintain balance and isolate the feedline from radiating.

e. Ground Strategically

• Ground the coaxial shield at one point only — usually at the equipment or lightning arrestor end.

• Multiple grounds can create loops that invite additional currents.

• Outdoor systems should include a lightning surge protector integrated into the grounding system.

f. Manage Cable Routing

• Keep coaxial runs away from AC power lines, switching power supplies, and other noise emitters.

• Avoid coiling excess cable length — coils can act as inductors or unintended choke antennas.

• Use cable clips or conduit to maintain consistent spacing and minimize mechanical stress.

5. Testing for Cable Radiation

If you suspect your coax is radiating:

• Near-field RF meter: Detects stray fields along the cable run.

• Clamp-on current probe: Measures common-mode currents on the outer conductor.

• SWR/Return Loss test: Unexpected peaks or dips may indicate radiating sections or impedance discontinuities.

These tools can confirm whether RF energy is staying confined inside the coax — where it belongs.

6. Summary: Keep the Cable a Cable

An antenna cable’s job is to deliver energy cleanly and quietly between your equipment and antenna.
But poor shielding, improper termination, impedance mismatch, or lack of grounding can turn it into a radiator or receiver in its own right.

That not only reduces system performance — it can cause interference, regulatory violations, and equipment damage.

Key Takeaways

• Use high-quality, well-shielded coax appropriate for your frequency range.

• Maintain impedance consistency (50 Ω) across all components.

• Add ferrite chokes or baluns to eliminate common-mode currents.

• Ground correctly and inspect terminations carefully.

• Periodically test your system’s SWR and shielding integrity.

With these practices, your coaxial cable will stay what it’s meant to be — a clean, efficient conduit for RF power — not an antenna in disguise.