How does cable loss before the LNA affect system noise figure?

Cable loss between the antenna and LNA adds directly to the system noise figure on a 1:1 dB basis. A 3 dB cable loss before the LNA degrades the system noise figure by exactly 3 dB. For a receiver with 1.5 dB LNA noise figure, adding 3 dB cable loss raises system NF to 4.5 dB, reducing sensitivity by 3 dB. This is why tower-mounted amplifiers and mast-head preamplifiers place the LNA at the antenna, eliminating cable loss from the noise figure calculation.

How do you calculate cable loss at a specific frequency?

Cable loss scales approximately with the square root of frequency: Loss(f) = Loss(f_ref) * sqrt(f/f_ref). For LMR-400: 1.5 dB/100ft at 450 MHz, 3.9 dB/100ft at 2.4 GHz, 6.0 dB/100ft at 5.8 GHz. Total loss = (loss_per_length * length) + connector losses. Each N-type connector adds 0.1-0.3 dB; each 7-16 DIN adds 0.05-0.15 dB. Temperature increases loss by about 0.1-0.2% per degree Celsius.

What cable types minimize loss for different applications?

Short runs under 3 meters: RG-316 or RG-58. Medium runs 3-30 meters: LMR-400 provides 3-6 dB/100ft at cellular frequencies. Long tower runs 30-100 meters: 7/8-inch corrugated foam at 1-2 dB/100ft. Lowest loss at microwave: waveguide (WR-90 for X-band at 0.11 dB/m). The decision balances attenuation, cost, weight, bending radius, and wind loading.

What is Cable Loss in Link Budgets? | RF Path Attenuation

Definition & Link Budget Impact

Cable loss in a link budget represents the total RF signal attenuation contributed by all coaxial cables, waveguides, jumpers, and connectors between the transmitter output and antenna input (transmit side) and between the antenna output and receiver input (receive side). This loss directly reduces effective radiated power on transmit and degrades receiver sensitivity on receive.

On the transmit path, cable loss reduces EIRP by the loss amount: a 40 dBm PA with 3 dB cable loss delivers only 37 dBm to the antenna. On the receive path, cable loss before the LNA adds directly to system noise figure. Every decibel of pre-LNA cable loss degrades receiver sensitivity by one decibel. This asymmetry drives the practice of placing LNAs at the antenna (tower-mounted amplifiers in cellular, mast-head preamps in amateur radio) and accepting higher transmit cable loss, which only costs transmitter power rather than sensitivity.

Key Formulas

Cable Loss vs. Frequency:

Loss(f) = Loss(f_ref) × √(f / f_ref)

LMR-400: 1.5 dB/100ft at 450 MHz → 3.0 dB/100ft at 1800 MHz

System Noise Figure with Cable Loss:

NF_sys = L_cable(dB) + NF_LNA(dB)

3 dB cable + 1.5 dB LNA NF = 4.5 dB system NF

EIRP Reduction:

EIRP = P_tx − L_cable − L_conn + G_ant

40 dBm − 3 dB − 0.5 dB + 15 dBi = 51.5 dBm EIRP

Cable Loss Comparison

Cable Type	Loss @ 900 MHz	Loss @ 2.4 GHz	Loss @ 5.8 GHz	Best For
RG-58 (5 mm)	12.0 dB/100ft	21.0 dB/100ft	36.0 dB/100ft	Short lab jumpers
LMR-400 (10 mm)	2.7 dB/100ft	3.9 dB/100ft	6.0 dB/100ft	Wi-Fi, cellular jumpers
1/2" foam (LDF4)	1.2 dB/100ft	2.0 dB/100ft	3.2 dB/100ft	Cell tower feeders
7/8" foam (LDF5)	0.68 dB/100ft	1.1 dB/100ft	1.8 dB/100ft	Tower main feeders
1-5/8" hardline	0.38 dB/100ft	0.65 dB/100ft	1.1 dB/100ft	Broadcast, high-power

Practical Application

A cellular base station at 1900 MHz uses a 30-meter (100 ft) run of 7/8-inch foam cable from the ground-level radio to the tower-top antenna. Cable loss is 1.1 dB, plus two 7-16 DIN connectors at 0.1 dB each, totaling 1.3 dB. On transmit, the 46 dBm PA delivers 44.7 dBm to the 18 dBi antenna for 62.7 dBm EIRP. On receive, the 1.3 dB cable loss before the tower-mounted amplifier (TMA, 1.2 dB NF, 15 dB gain) raises system NF from 1.2 dB to 2.5 dB. Without the TMA, cable loss feeds directly into the base station receiver (3.5 dB NF), making system NF = 4.8 dB. The TMA saves 2.3 dB of sensitivity, equivalent to 30% more coverage area.

Frequently Asked Questions

Cable loss effect on noise figure?

Adds 1:1 to system NF. 3 dB cable + 1.5 dB LNA NF = 4.5 dB system NF. Fix: mount the LNA at the antenna (TMA or mast-head preamp) to eliminate pre-LNA cable loss contribution.

How to calculate loss at frequency?

Loss scales with √f. LMR-400: 1.5 dB/100ft at 450 MHz → 3.9 dB at 2.4 GHz. Add 0.1-0.3 dB per N-type connector, 0.05-0.15 dB per 7-16 DIN.

Which cable for which application?

Short (<3 m): RG-58. Medium (3-30 m): LMR-400. Tower (30-100 m): 7/8" foam. Lowest loss: waveguide or 1-5/8" hardline. Balance loss vs. cost, weight, and bend radius.

Cable Loss (Link Budget)