I asked Gemini some questions and got the following.
The source is here.
The official factory specifications for the input sensitivity of the Hallicrafters
SX-28 (and the subsequent wartime SX-28A) Super Skyrider change depending on the
frequency band you are operating on. Because it is a single-conversion superheterodyne
receiver designed in 1940, performance naturally tapers off at the upper limits
of its 550 kHz to 43 MHz range.
The baseline sensitivity figures are typically broken down as follows:
Factory Sensitivity Specs
Why the Sensitivity Varies Across Bands
The architecture of the SX-28’s front-end explains this performance curve:
Single Preselection (Bands 1 and 2): For the standard AM broadcast band and the
lower intermediate frequencies (550 kHz to 3.0 MHz), the receiver bypasses the
first RF amplifier stage. It uses only a single 6SK7 RF preselector stage because
atmospheric noise at these lower frequencies is inherently high enough that
ultra-high receiver sensitivity isn't necessary.
Double Preselection (Bands 3 through 6): From 3.0 MHz all the way up to 43 MHz, the
receiver automatically engages both RF amplifier stages (double preselection). This extra
gain is injected to maintain the 2 μV sensitivity floor and to combat image
rejection issues common to shortwave frequencies.
The Upper Limit Drop-off (Band 6): By the time you reach 10 meters and
above (21–43 MHz), the efficiency of the vintage octal tubes (like the 6SK7 or
the 6AB7 used in later production runs) drops off due to internal tube capacitance,
and stray circuit inductances take a toll. This is why the sensitivity spec
loosens to 4 μV.
Real-World Benchmarks
On a modern test bench, a properly recapped and aligned SX-28 will routinely
beat these factory specs on the lower shortwave bands, often dropping well
below 1 μV for an audible CW or AM signal. However, if you are restoring one,
achieving that 4 μV target on Band 6 is a prime indicator that the notoriously
complex front-end RF coil box is perfectly aligned.
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Last modified: Wednesday, 1 Jul 2026