Why does cable loss increase with frequency?

Two mechanisms. Conductor loss is caused by the skin effect: at higher frequencies, current flows in a thinner layer on the conductor surface, increasing the effective resistance. This loss increases as the square root of frequency. Dielectric loss is caused by molecular friction in the insulating material as it polarizes and depolarizes at the signal frequency. This loss increases linearly with frequency. At low frequencies (below about 1 GHz for most cables), conductor loss dominates. At high frequencies, dielectric loss takes over, which is why low-loss cables use air or expanded PTFE dielectrics instead of solid polyethylene.

How do I estimate total cable loss for my run?

Multiply the manufacturer's specified loss per unit length at your operating frequency by the total cable length. Add connector losses (typically 0.1 to 0.3 dB per connector for SMA, 0.05 to 0.15 dB for N-type). For a 30-meter run of LMR-400 at 2.4 GHz: loss per meter is 0.22 dB/m, so cable loss is 30 times 0.22 = 6.6 dB. With two N-type connectors: total is 6.6 + 0.2 = 6.8 dB. This means the antenna only receives 21% of the transmitter's power. At 5.8 GHz with the same cable: 0.36 dB/m times 30 = 10.8 dB, leaving only 8.3% of the power.

When should I switch from coax to waveguide?

When cable loss becomes intolerable. Above about 10 GHz, even the best semi-rigid coax (like 0.141 inch UT-141) has 1 to 2 dB per meter of loss. Rectangular waveguide at the same frequency has 0.05 to 0.2 dB per meter, 10 to 20 times less. The crossover point depends on cable run length: for short interconnects (under 30 cm), coax is fine up to 40 GHz. For antenna feeds longer than 1 meter above 20 GHz, waveguide is almost always required. The trade-off is that waveguide is rigid, heavy, and expensive, and each frequency band requires a different waveguide size.

Test & Measurement

Cable Loss

A 30-meter run of RG-58 at 2.4 GHz loses 18.6 dB, meaning only 1.4% of the transmitter's power reaches the antenna. Switch to LMR-400 and the loss drops to 6.6 dB (22% through). Switch to 7/8-inch hardline and it is 2.4 dB (58% through). Cable selection is not an afterthought; in many systems the feedline loss exceeds every other loss in the signal chain combined. At millimeter-wave frequencies, even premium coax becomes impractical, and waveguide or direct-mount amplifiers are the only options with acceptable loss budgets.

Category: Test & Measurement

Loss Mechanism: Conductor (√f) + Dielectric (f)

Spec: dB per meter at frequency

Where Your Signal Disappears

Cable Loss Comparison at Key Frequencies

Cable Type	Diameter	Loss at 900 MHz	Loss at 2.4 GHz	Loss at 5.8 GHz	Typical Use
RG-58	5 mm	0.28 dB/m	0.62 dB/m	1.10 dB/m	Lab bench, short jumpers
RG-213	10 mm	0.13 dB/m	0.26 dB/m	0.46 dB/m	Amateur radio, HF/VHF
LMR-240	6 mm	0.14 dB/m	0.25 dB/m	0.42 dB/m	Short WLAN, indoor runs
LMR-400	10 mm	0.07 dB/m	0.13 dB/m	0.22 dB/m	Outdoor antenna feeds
7/8" Hardline	22 mm	0.03 dB/m	0.05 dB/m	0.08 dB/m	Cell tower, broadcast
WR-90 waveguide	23 × 10 mm	N/A	N/A	0.04 dB/m	X-band radar feed

Calculating Total System Loss

Total feedline loss:
Loss_total = (α_cable × Length) + (N_connectors × Loss_connector)

Example: 50 m cell tower run at 1.9 GHz:
LMR-400: 0.10 dB/m × 50 m = 5.0 dB + 2 × 0.1 dB (N-type) = 5.2 dB
7/8" hardline: 0.04 dB/m × 50 m = 2.0 dB + 2 × 0.05 dB (7-16 DIN) = 2.1 dB

The 3.1 dB difference means the hardline delivers twice the power to the antenna. For a 20 W transmitter: LMR-400 delivers 6.0 W; hardline delivers 12.3 W. That is a 3 dB coverage radius improvement.

Common Questions

Frequently Asked Questions

Why does loss increase with frequency?

Conductor loss rises as √f (skin effect forces current into a thinner surface layer, increasing resistance). Dielectric loss rises linearly with f (molecular friction in the insulator). Below ~1 GHz, conductor loss dominates. Above that, dielectric loss takes over, which is why premium cables use air or expanded PTFE dielectrics.

How do I estimate total loss?

Multiply dB/m at your frequency by cable length, then add connector losses (0.05 to 0.3 dB each depending on type). A 30 m LMR-400 run at 2.4 GHz: 0.22 × 30 + 0.2 = 6.8 dB total. Only 21% of power reaches the antenna.

When to switch from coax to waveguide?

Above ~10 GHz, even semi-rigid coax loses 1 to 2 dB/m. Waveguide is 10 to 20× lower loss (0.05 to 0.2 dB/m). For runs over 1 meter above 20 GHz, waveguide is almost always required. Trade-off: rigid, heavy, frequency-specific sizing.

Related Terms

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System Planning

Cable Loss Budget Calculator

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