Why downconvert at the antenna?

Ku-band signals (10.7-12.75 GHz) suffer extreme cable loss in coaxial cable: approximately 30-50 dB per 100 m at 12 GHz vs only 5-8 dB at the L-band IF (950-2150 MHz). By placing the LNB at the dish feed, the signal is amplified and downconverted before cable transmission, preserving SNR over long cable runs.

What is inside an LNB?

An LNB contains: a feedhorn/probe collecting the focused satellite signal, a low-noise amplifier (NF 0.3-0.7 dB), a local oscillator (9.75 GHz for low band, 10.6 GHz for high band), a mixer for frequency conversion, an IF amplifier, and a bandpass filter. Power (13/18V DC) and band selection (22 kHz tone) come from the receiver via the coax.

Universal vs standard LNB?

A universal LNB covers both Ku low band (10.7-11.7 GHz, LO=9.75 GHz) and high band (11.7-12.75 GHz, LO=10.6 GHz), selected by a 22 kHz tone from the receiver. A standard LNB covers only one sub-band with a fixed LO. Universal LNBs also switch polarization (H/V) via 13V/18V DC, serving all four combinations through DiSEqC commands.

Active Components

Block Downconverter

A Block Downconverter (BDC), commonly called an LNB (Low Noise Block downconverter), converts an entire satellite frequency band to a lower intermediate frequency (IF) for distribution over coaxial cable. In Ku-band DTH (Direct-to-Home) systems, the LNB at the dish feed converts 10.7-12.75 GHz to 950-2150 MHz IF. This preserves signal quality over long cable runs where Ku-band losses would be prohibitive (30-50 dB/100 m at 12 GHz vs 5-8 dB at L-band).

Category: Active Components

NF: 0.3-0.7 dB

Understanding Block Downconverters

The "block" refers to converting an entire frequency block at once, not individual channels. All signals within the input band are translated to the IF band simultaneously. Channel selection happens in the indoor receiver (IRD) after the IF signal arrives via coax.

Modern universal LNBs use two local oscillators: 9.75 GHz (low band) and 10.6 GHz (high band), switched by a 22 kHz tone. Polarization (H/V or L/R) is selected by 13V/18V DC on the coax. DiSEqC protocol enables multi-satellite switching.

Frequency Conversion

IF = f_RF − f_LO

Low band: 10.7 GHz − 9.75 GHz = 950 MHz
Low band: 11.7 GHz − 9.75 GHz = 1950 MHz

High band: 11.7 GHz − 10.6 GHz = 1100 MHz
High band: 12.75 GHz − 10.6 GHz = 2150 MHz

LNB Type Comparison

Type	Input	LO	IF Output	Outputs
Universal	10.7-12.75 GHz	9.75/10.6	950-2150 MHz	1-4
Wideband	10.7-12.75 GHz	10.4	290-2340 MHz	1-2
C-band	3.4-4.2 GHz	5.15	950-1750 MHz	1
Ka-band	18.3-20.2 GHz	Various	950-2150 MHz	1-2

Common Questions

Frequently Asked Questions

Why downconvert at antenna?

Ku-band cable loss: 30-50 dB/100 m. L-band IF: 5-8 dB/100 m. LNB amplifies and converts at the dish, preserving SNR.

What's inside an LNB?

Feedhorn, LNA (0.3-0.7 dB NF), LO, mixer, IF amp, BPF. Powered via 13/18V DC on coax. Band selected by 22 kHz tone.

Universal vs standard?

Universal: dual LO (9.75/10.6), covers full Ku. Standard: single LO, one sub-band. Universal also switches H/V polarization via DC voltage.

Related Terms

← Block Diagram Block Error Rate →

Satellite RF

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