Published on 10/13/2026
Add an LNA when receiver-chain noise and feeder loss limit reception, not simply when an RSSI number looks low. A suitable low-noise amplifier near the receive antenna can reduce the effect of downstream cable loss, but it cannot recover signal-to-noise ratio already lost before its input.
Imagine a rooftop receive antenna feeding a radio through a cable with 3 dB loss. Putting 20 dB of gain beside the radio increases its input level, yet the cable has already degraded the front end. Moving the same amplifier before that cable changes the noise budget, not only the signal level.
This article compares those placements using an explicitly illustrative 290 K, matched-system calculation. It does not recommend an LNA for every antenna feed or claim a measured improvement on a particular installation.
On This Page
Gain tells you how much a stage increases signal power. Noise figure characterizes added noise relative to the standard reference condition. A 20 dB gain stage amplifies existing input noise as well as the desired signal and adds its own noise; it does not make a dirty signal clean by raising its amplitude.
| Parameter | What to ask | Why it matters |
|---|---|---|
| Gain in dB | How much gain across my band? | Offsets downstream loss, but can overload later stages |
| Noise figure in dB | What is the NF at my frequency and bias? | Sets the amplifier's noise contribution |
| Input/output compression | What are the blocker and output limits? | Strong unwanted signals can compress the chain |
| Band and matching | Is it intended for this frequency and impedance? | A generic booster is not a broadband cure |
Key idea: At the 290 K reference temperature, an ideal matched passive loss has noise figure equal to its insertion loss in dB. This standard noise-figure statement is not a universal dB-for-dB prediction of actual antenna-system SNR under every noise temperature.
For a weak-signal, noise-limited receive chain, place the low-noise first stage before substantial feeder loss when the installation allows it. Gain ahead of the cable reduces the contribution of the cable and receiver to the input-referred noise figure. Loss ahead of the first amplifier cannot be divided down by that amplifier's later gain.
| Order from antenna | Likely advantage | What remains to check |
|---|---|---|
| LNA, cable, receiver | Lower input-referred noise contribution from feeder | Power, weather protection, blockers and gain limits |
| Cable, LNA, receiver | Easy amplifier access indoors | Feeder loss already precedes first gain |
| Short low-loss cable, receiver | No extra active stage to power or overload | Whether the existing receiver noise performance is sufficient |
Keep any necessary antenna-side lead short and low loss. A masthead amplifier with a long lossy pigtail in front of it gives away part of the placement benefit. Measure or estimate that first lead as part of the front-end budget.
For three matched stages, noise factor Ftotal = F1 + (F2 - 1)/G1 + (F3 - 1)/(G1 x G2). The F terms are linear noise factors and G terms are linear power gains, not dB entries. Convert with F = 10^(NF/10) and G = 10^(gain/10), then return to dB with NFtotal = 10 log10(Ftotal).
Step 01
List the complete chain
Include cable, filters, switches, the LNA and the receiver in their physical order. A loss stage has gain below 1; at 290 K its noise factor is the reciprocal of that gain.
Step 02
Use frequency-specific numbers
A nominal amplifier gain or cable loss measured at a different frequency can hide the actual result. Keep datasheet conditions and operating bias consistent.
Step 03
Check noise and level budgets separately
First calculate input-referred noise. Then calculate signal and blocker levels at every stage. A favorable noise figure is useless if the receiver is driven into compression.
Specify the loss you can afford, not only the cable name
Send SigmaRF your frequency band, connector interfaces, installed length and insertion-loss target. Every assembly is VNA-swept for VSWR and insertion loss before shipment.
Specify a Custom AssemblyAn LNA can reduce usable performance when strong in-band or out-of-band signals overload the amplifier or receiver. Gain is not free dynamic range. Check compression, linearity and the strongest credible blocker, including local transmitters, before choosing a gain value.
Filtering before the LNA can protect it from out-of-band blockers, but that filter loss raises the input-referred noise figure. Filtering after the LNA avoids placing its loss first, yet cannot prevent the LNA itself from overloading. The right order is a measured trade-off between noise and blocker rejection.
Do not insert a receive-only LNA in a transmit path. For a transceiver, use equipment explicitly designed for the switching and transmit power, or keep the receive amplification in the correct separate path.
An active GNSS antenna may already contain an LNA. Adding another amplifier without checking the antenna output, receiver input range and cable loss can exceed the receiver's allowed gain budget. Follow the receiver and antenna specifications rather than stacking gain stages by habit.
Where DC bias travels on the coax, verify antenna supply voltage, current, cable DC resistance, bias-tee compatibility and whether intervening protectors or adapters pass or block DC. RF insertion loss and DC voltage drop are different checks. Weather sealing and strain relief remain important around a masthead device.
Use SigmaRF's RF Link Budget Basics for the signal-power ledger and its Coaxial Cables for GPS/GNSS Antennas guide for active-antenna feeds. This guide focuses on amplifier position and cascaded noise, not antenna range promises.
Illustrative matched chain at 290 K: cable loss 3 dB, LNA gain 20 dB with 1 dB NF, and receiver NF 5 dB. Assume the gain and NF apply at the chosen operating frequency and ignore additional interfaces.
Both orders have the same 17 dB net gain before the receiver: 20 dB amplifier gain minus 3 dB cable loss. Their noise figures differ because the first-stage order differs. These are calculated reference-condition results, not field SNR measurements or guaranteed sensitivity gains.
Should an LNA go before or after the antenna cable?
In a noise-limited receive chain, before substantial cable loss is usually better for input-referred noise performance. Check power, filtering, environmental protection and overload limits before choosing that placement.
Can an amplifier recover signal lost in coax?
It can raise signal level and reduce the relative impact of later receiver noise. It cannot reverse SNR degradation that occurred before its input or remove interference already mixed with the signal.
Is 20 dB LNA gain always better than 10 dB?
No. More gain suppresses later noise contributions but can overload the receiver or the LNA output. Choose enough gain for the noise budget while preserving the required blocker and signal headroom.
Does a 3 dB cable add 3 dB noise figure?
For an ideal matched passive cable at the 290 K reference temperature, its standard NF equals its 3 dB loss. Actual system SNR depends on source noise temperature and the rest of the chain.
Do active GPS antennas need another LNA?
Not automatically. Active antennas already include gain. Use the antenna and receiver gain limits, cable loss and bias requirements to decide whether any additional amplifier is needed.
Should a filter be before or after the LNA?
A pre-LNA filter can stop out-of-band overload but adds loss before gain. A post-LNA filter contributes less input-referred noise but cannot protect the LNA itself. Check both noise and blocker budgets.
Shorten or improve the feeder first where practical. If receive-chain noise still limits the application, calculate an LNA option with the real frequency, gain, NF and blocker levels. Placement matters because signal gain and input-referred noise are different budgets.
SigmaRF VNA-sweeps every assembly for VSWR and insertion loss before shipment. Provide the band, installed length, connectors, DC-bias needs and maximum feeder loss when specifying an assembly. SigmaRF covers requirements from 100 MHz to 40 GHz through Vadodara volume and Cleveland-area precision facilities; suitable builds can have lead times as short as 2 days.
Specify the loss you can afford, not only the cable name
Send SigmaRF your frequency band, connector interfaces, installed length and insertion-loss target. Every assembly is VNA-swept for VSWR and insertion loss before shipment.
Standard AssembliesCustom AssembliesTalk to an Engineer