Coaxial Cable Selection, Installation & Testing for FM Broadcasting

Coaxial cable is the critical link between your FM transmitter and antenna, and its selection directly impacts your station's effective radiated power, signal quality, and long-term reliability. Choosing the wrong cable or installing it improperly can waste a third or more of your transmitter power as heat and cause VSWR problems that damage equipment.

Why Coaxial Cable Matters

Every meter of coaxial cable introduces signal loss (attenuation) measured in dB per 100 meters. At 100MHz, common cable types have significantly different losses: RG-58 loses approximately 8.5dB/100m, RG-213 loses about 4.5dB/100m, and 1/2-inch corrugated (50-12) loses only about 2.2dB/100m. A 30-meter run of RG-58 at 100W input delivers only about 56W to the antenna, wasting 44W as heat. The same length of 1/2-inch corrugated delivers approximately 86W. For installations over 10 meters, always use 1/2-inch or larger corrugated coaxial cable. The cost difference is modest compared to the power savings and improved reliability over years of operation.

Cable Types for FM Broadcasting

• 1/2-inch Corrugated (50-12): The standard choice for most FM broadcast installations from 0–2000W. Features copper-clad aluminum inner conductor (4.8mm), foamed polyethylene dielectric (12.3mm), and annular corrugated copper tube outer conductor (13.9mm). Impedance 50±1 Ohm. Attenuation at 100MHz: 2.21dB/100m. Power rating at 100MHz: 3.94kW average. Available from Elecsky in lengths from 20m to 50m with L16 NJ or DIN L29 connectors.
• 7/8-inch Corrugated: For installations above 2000W or runs longer than 50 meters. Larger diameter reduces loss further and increases power handling. Attenuation at 100MHz: approximately 1.2dB/100m. Requires larger bending radius (minimum 120mm).
• RG-213: A flexible alternative for short indoor patch cables or test setups. Not recommended for permanent outdoor runs over 5 meters due to higher loss and lower shielding effectiveness compared to corrugated cable. The braided outer conductor allows moisture ingress over time.

Connector Selection

Connector choice depends on your transmitter and antenna interfaces. The L16 (N-type) connector is the most common for FM broadcast equipment up to 1000W, providing reliable 50-ohm connections with excellent RF performance up to 11GHz. The 7/16 DIN (L29) connector is rated for higher power (up to 2700W at 1GHz) and features a larger center conductor and more robust mechanical design preferred in professional installations. Always use connectors matched to your cable type with proper crimp or clamp termination tools; poorly installed connectors are a leading cause of high VSWR and intermittent failures. Elecsky provides pre-terminated cables with factory-installed connectors tested for impedance and VSWR performance.

Installation Best Practices

• Respect Minimum Bend Radius: 1/2-inch corrugated cable has a static minimum bend radius of 70mm for single bends and 120mm for repeated bending. Exceeding this kinks the outer conductor, creates impedance discontinuities that reflect power, and permanently damages the cable.
• Support the Cable Properly: Use cable hangers or clamps every 1–1.5 meters on vertical runs. Never let the full weight of the cable hang from the connector. Install a drip loop at the building entry point to prevent water from running along the cable into the building.
• Weatherproof All Outdoor Connections: Wrap all outdoor connectors with multiple layers of self-amalgamating butyl rubber tape followed by UV-resistant vinyl electrical tape. Apply in overlapping layers, stretching the rubber tape to activate the self-bonding. Start from below the connector and wrap upward to create a shingled effect that sheds water.
• Ground the Cable: Ground the outer conductor at the antenna, at the tower base, and at the building entry point using proper grounding kits. This provides lightning protection per electrical codes and improves overall safety.
• Test Before Commissioning: Use the PM-1B RF Power Meter to verify forward power, reflected power, and VSWR at the transmitter output after installation. A VSWR below 1.3:1 indicates a good installation. Values above 1.5:1 indicate a problem that should be investigated before full-power operation.
• Avoid Cable Coiling: Never coil excess cable in a tight loop, as this creates an inductor that can alter impedance and generate heat. If you need to manage excess length, form a large figure-eight pattern with at least 1 meter diameter or cut the cable to the exact required length plus a small service loop.

Pre-terminated vs. Field-terminated Cables

Elecsky offers pre-terminated coaxial cables with connectors factory-installed and tested, available in lengths of 20m, 30m, 40m, and 50m. Pre-terminated cables eliminate installation risk, ensure consistent performance, and include a test report verifying impedance and VSWR. For custom lengths or replacement connectors, field termination kits with the appropriate crimp or clamp tools are available. Field termination requires skill and the correct tools; an improperly terminated connector can cause intermittent VSWR problems that are difficult to diagnose. The PM-1B RF Power Meter is an indispensable tool for verifying any installation, whether pre-terminated or field-assembled.