Why an LNA Won’t Fix Your Receiver’s Dynamic Range
“Put an LNA in front of your receiver — it’ll make weak signals louder.” Tempting, but incomplete. A Low Noise Amplifier will not improve your receiver’s dynamic range. On HF especially, it often makes things worse.
What Dynamic Range Really Means
Dynamic range is the difference between the weakest signal your radio can hear and the strongest it can handle without distortion. The weak end is about noise figure (NF); the strong end is about linearity — usually expressed in terms like RMDR, BDR, and IP3.
| Scenario | Can Your Radio Handle It? |
|---|---|
| S1 DX alone | Yes — if the NF is low enough and the band noise dominates |
| S9+60 AM broadcast alone | Yes — if your front-end has strong blocking/linearity |
| Both at once | Only with excellent dynamic range |
• NF (Noise Figure): How much noise the receiver itself adds. On HF, band noise is so high NF often doesn’t matter.
• RMDR (Reciprocal Mixing Dynamic Range): Receiver’s ability to hear weak signals close to very strong ones without phase noise burying them.
• BDR (Blocking Dynamic Range): How strong a nearby signal can be before it blocks weaker ones.
• IP3 (Third-Order Intercept Point): A measure of linearity. Higher IP3 = fewer spurious IMD products when strong signals mix.
• ADC: Analog-to-Digital Converter. In SDRs, defines how much range your digitizer has before clipping or distortion.
• SNR (Signal-to-Noise Ratio): Desired signal power compared to noise — the real-world measure of readability.
• IM (Intermodulation): False signals generated when strong stations mix in a non-linear stage (mixer, LNA, etc). Example: 2 strong AMs can create a phantom third signal elsewhere on the dial.
Why Adding an LNA Can Make Things Worse
- It amplifies everything — weak DX, strong locals, and noise together.
- It reduces headroom, pushing strong signals closer to overload.
- It can create intermodulation (phantom signals from signal mixing).
- It raises the noise floor if you’re already band-noise limited.
Lower Headroom
That S9+60 AM station just became +70 into your radio. Mixers and ADCs distort, AGC pumps, and spurs bloom.
More Intermodulation
Intermodulation occurs when two or more strong signals mix inside a non-linear device, producing false signals at sum and difference frequencies. LNAs are rarely as linear as a good receiver front-end, so they can make IMD worse.
Raising the Noise Floor
If you already hear band noise, more gain just raises both the signal and the noise together. No extra readability.
F_total = F1 + (F2 − 1)/G1 + (F3 − 1)/(G1·G2) + …This shows how the noise figure of the first stage (F1) dominates if its gain (G1) is high. On VHF/UHF, masthead LNAs help because coax loss is large. On HF, sky noise is already so strong that reducing F_total barely changes SNR — but overload risk rises sharply.
When an LNA does make sense
LNAs are essential in active receive antennas like loops and probes. There the LNA buffers the tiny, high-impedance element before coax loss and drives the line. Without it, the antenna wouldn’t function.
If Not an LNA, Then What?
- Filtering: notch MW/AM/FM to tame strong offenders.
- Choking: suppress unwanted return/common-mode currents.
- Antenna design: prioritize balance, placement, and RDF for receive.
- Receiver quality: pick rigs with strong RMDR, BDR, and IP3 specs.
Final Takeaway
An LNA is not a magic bullet. On HF, it usually makes things worse. The right tools are filters, chokes, strong front-end design, and antennas optimized for noise rejection. Save LNAs for where they belong: inside active RX antennas, or masthead preamps at VHF/UHF/SHF.
Mini-FAQ
- Will an LNA help me hear weaker HF stations? — Only if you are truly noise-limited, not overload-limited.
- What is intermodulation? — False signals generated when strong stations mix in a non-linear device, creating phantom frequencies.
- Why do active RX antennas always have LNAs? — To buffer tiny signals and overcome feedline loss before it buries them.
- What does NF mean on HF? — Almost nothing. Atmospheric noise dominates; LNAs won’t lower it.
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