Transmitters are easy: almost any modern radio puts out a clean 100 watts. Receivers are where radios differ, and where the specifications are written to be flattering. These are the numbers that predict how a radio behaves on a crowded band.
Sensitivity is rarely the problem
MDS (minimum discernible signal), often quoted as noise floor in dBm, says how faint a signal the receiver can hear. Any current radio hears far below the atmospheric and man-made noise arriving at your antenna on HF, so sensitivity almost never limits you below 30 MHz — your noise environment does. See RFI and noise.
Above 50 MHz, where the sky is quiet, sensitivity and preamplifier noise figure start to matter, and a mast-mounted preamp ahead of the feedline loss can be worth several dB.
Dynamic range is the problem
Dynamic range describes what happens when a strong signal is near a weak one:
- Blocking / gain compression — a nearby strong signal desensitizes the receiver so the weak one
- Third-order intermodulation (IMD3) — two strong signals mix inside the receiver and produce a
- Reciprocal mixing dynamic range (RMDR) — how much the receiver's own local-oscillator phase
fades.
false signal where neither exists. The figure of merit is IP3 (third-order intercept, in dBm); higher is better, and the useful test is at 2 kHz spacing, not the flattering 20 kHz.
noise smears a strong nearby signal across your passband. On a contest weekend this dominates, and it is the number that separates good radios from great ones.
The ARRL Lab and the Sherwood receiver tables publish these consistently measured; a manufacturer's brochure often does not.
Filters and the front end
- Roofing filters sit early in the signal path in a superheterodyne receiver and limit how much
- Direct-sampling SDRs convert the antenna signal to digital immediately. Their limits come from
- DSP bandwidth, notch and noise reduction operate after the damage is done. They make listening
- Preselectors and band-pass filters ahead of the receiver help enormously at a multi-station
energy reaches the later stages. A narrow (300–600 Hz) roofing filter is the single biggest contest-band improvement in a traditional radio.
the analog-to-digital converter's bit depth and the front-end preselector rather than crystal filters, which is why a good SDR can be superb but overloads abruptly rather than gracefully. See software-defined radio.
more comfortable; they cannot recover a signal that overload has already buried.
club site or Field Day setup where another transmitter is 100 feet away.
AGC, attenuators and using what you have
The two most underused controls on any radio are the attenuator and RF gain. On a loud band, 20 dB of attenuation costs you nothing you could hear anyway and moves the whole receiver back into its linear range. Setting AGC decay short for CW and long for SSB, and turning off noise blankers that are pumping on a strong signal, will improve reception more than most hardware purchases.