TECHNICAL NOTE / LNA / SENSITIVITY
Weak-signal reception depends on the antenna, external noise, feed losses, preamplifier, filtering, receiver and interference environment. A very low amplifier noise figure is useful only when the complete receive chain is engineered correctly.
Noise figure and first-stage loss
A passive loss ahead of the LNA directly degrades the receiving noise figure by approximately the amount of that loss in dB, under matched conditions. This is why placing an LNA close to the antenna can be valuable—particularly on higher UHF and microwave bands.
Gain is not sensitivity
A 30 dB gain preamplifier does not create a 30 dB improvement in signal-to-noise ratio. Excess gain may overload the receiver with nearby signals, especially in a dense contest environment. Consider dynamic range, filters and system noise as well as the LNA data sheet.
Examples from the station equipment discussions
The station has used or evaluated a WA2ODO 6-meter preamp, a Microwave Components 2-meter GaAsFET LNA, DEMI 222/432 units and AG6EE/DEMI options at 1296/902 MHz. Any cited laboratory figures belong to the individual device and test configuration; they are not a measured end-to-end station noise figure.
System checklist
- Sequence RF relays and amplifier keying so the LNA is isolated before transmit RF appears.
- Calculate insertion loss through relays and protective components ahead of the amplifier.
- Keep bias power clean and verify fail-safe relay positions with loss of power.
- Check strong-signal handling, not just a low noise-figure specification.
See Transmit/receive sequencing → for practical design safeguards.
Use Friis, not an LNA sticker
For cascaded receiving stages, calculate linear noise factors rather than simply adding noise figures in dB. The idealized cascade relation is Ftotal = F1 + (F2 − 1)/G1 + (F3 − 1)/(G1×G2) + ..., where all F and G terms are linear ratios. A cable or filter ahead of the LNA is part of stage one. If it has 1 dB loss near 290 K, its noise factor is approximately 1.259 and can degrade system NF by roughly 1 dB before preamp effects are considered.
Benchmark the entire receive path
- Measure reference noise with a quiet termination and an appropriate calibrated setup.
- Measure loss from antenna to LNA input, including protection relays, filters, arrestors and short jumpers.
- Record gain and NF at the frequency in use; manufacturer or bench-reported LNA specs alone do not determine system sensitivity.
- Check for compression, intermodulation, gain instability and oscillator-related spurs during a strong local signal.
- Compare receive levels on a reproducible source, such as a known beacon or calibrated injected signal, before claiming a dB improvement.
See Relay and Sequencing for TX survival safeguards. A low-noise preamp provides no advantage if RF damage, gain overload or relay insertion loss defeats the receiver.