If you try to tune in a station on these channels, you will get extra distortion or interference generated by the HF-225 itself. The 4 kHz filter was selected for the measurements, being the filter than SW listeners are most likely to use for broadcast listening. The receiver's built-in synthesiser generates a lot of noise between 30 and 70 kilohertz, way down in the very low frequency part of the spectrum. This masks most of the signals coming in. But from 100 kHz up to 30,000 kHz, covered continuously on this set with no breaks, the sensitivity is remarkably uniform.
Sensitivity is also dependent on the chosen bandwidth filter.
The following measurements were taken at 12.1 MHz. The receiver offers an analogue signal strength meter which turns out to be very accurately calibrated, far more than just a tuning gimmick. A push-button attenuator is provided, offering around 17 dB attenuation when measured at 12 MHz. It consists of a resistance-network, which can be shorted out of circuit by a diode. After the initial attenuation is seen on the S-meter, there is no further indication that the attenuator is switched in.