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Choosing the right feedline is one of the most critical decisions in station design. Whether you are running low-power FT8 on 80 meters or pushing VHF/UHF weak-signal modes, feedline attenuation (signal loss) directly impacts your effective radiated power (ERP) on transmit and your system noise figure on receive.
1. Understanding Coaxial Impedance and Shielding
In modern amateur radio, 50-ohm unbalanced coaxial cable is the universal standard for transmitters, antennas, and feedlines. Coax relies on skin effect and electromagnetic containment within the dielectric shield.
- Center Conductor: Carries the primary RF signal (solid copper vs. stranded copper-clad steel).
- Dielectric: The insulating material separating the center conductor and shield (foam polyethylene vs. solid polyethylene). Foam provides lower loss and velocity factor closer to 1.0, but solid polyethylene is more rugged against moisture migration.
- Shield: Braided copper, foil, or a combination. Double-shielded (foil + braid) cable provides near-100% coverage, essential for preventing common-mode current and RFI ingress/egress.
| Cable Type |
Outer Diameter |
Dielectric |
Attenuation per 100 ft (HF / 10 MHz) |
Attenuation per 100 ft (VHF / 144 MHz) |
Attenuation per 100 ft (UHF / 440 MHz) |
Best Application |
| RG-174 |
0.100" |
Solid PE |
~2.5 dB |
~9.5 dB |
~17.0 dB |
Jumpers, QRP patch cables only |
| RG-58 |
0.195" |
Solid PE |
~1.5 dB |
~5.5 dB |
~10.5 dB |
Short HF mobile runs, low-power patches |
| RG-8X |
0.242" |
Foam PE |
~0.9 dB |
~3.1 dB |
~6.0 dB |
Short HF base station runs (<50 ft) |
| RG-213 |
0.405" |
Solid PE |
~0.6 dB |
~2.0 dB |
~4.5 dB |
Standard HF high-power runs, military spec |
| LMR-400 |
0.405" |
Foam PE |
~0.3 dB |
~1.5 dB |
~2.7 dB |
Gold Standard for VHF/UHF & long HF runs |
| LMR-600 |
0.590" |
Foam PE |
~0.2 dB |
~0.9 dB |
~1.7 dB |
Very long VHF/UHF tower runs |
Rule of Thumb: Every 3 dB of loss represents a 50% reduction in power. If you lose 3 dB in your feedline, a 100-watt transmission results in only 50 watts reaching the antenna.
3. Band-Specific Recommendations
- HF (1.8 MHz - 30 MHz):
- Loss is relatively forgiving at lower frequencies. RG-8X is fine for short runs (<50 ft) at 100W.
- For high power (1kW) or longer runs (>75 ft), RG-213 or LMR-400 ensures minimal thermal loss and high breakdown voltage.
- VHF (50 MHz - 144 MHz):
- Loss increases noticeably. Avoid RG-58 or RG-8X for long runs. RG-213 or LMR-400 is strongly recommended.
- UHF (420 MHz - 450 MHz and up):
- Signal attenuation is severe. LMR-400 (or equivalent low-loss foam dielectric cable) is mandatory for anything more than a few feet of feedline. Avoid solid-polyethylene RG cables entirely at UHF.
4. Connectors: UHF vs. N-Type vs. BNC
The physical connector is often the weakest link in a feedline system if improperly installed or moisture-compromised.
UHF Connector (PL-259 / SO-239)
- Frequency Range: Reliable up to ~300 MHz (though performance degrades above 150 MHz due to impedance bumps).
- Pros: Rugged, easy to solder, handles high power (kW+) easily.
- Cons: Not weatherproof by design; non-constant impedance (~40 ohms).
N-Type Connector
- Frequency Range: Excellent up to 11 GHz.
- Pros: Constant 50-ohm impedance, weatherproof with O-rings, low VSWR at VHF/UHF.
- Cons: Requires precision installation; more difficult to solder center pins on large cables.
BNC Connector
- Frequency Range: Good up to 4 GHz.
- Pros: Quick-disconnect bayonet locking mechanism; ideal for patch panels, test equipment, and low-power HF/VHF.
- Cons: Not suitable for high-power (kW) RF or heavy dangling feedline weights.
Summary Best Practices
- Match your connectors to your operating frequency: Use PL-259 for HF/VHF, and N-type for UHF and above.
- Weatherproof outdoor connections using coaxial sealing tape (butyl rubber) wrapped with electrical tape or UV-resistant heat shrink.
- Keep cable runs as short and direct as possible to preserve signal integrity on both transmit and receive.
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