Technical Topics

摘要

Recent 'TT' items have noted that the actual performance of many of the factory-builttransceivers and receivers has advanced only little in the past quarter-century despite the impressive specifications of dynamic range etc. It has been recognised that the most demanding HF reception conditions apply to the 7MHz band in Europe during the evening, when the number of very strong broadcast transmissions just above 7100kHz appearing at the input of the receiver reach the first mixer with little attenuation. It should be appreciated that the effect of multiple strong signals is additive, and all strong signals can contribute to the intermodulation products, which are sufficiently numerous to appear as 'noise' creating an artificially-high noise floorfor the receiver. Even with the older 'classic' valve receivers with up to three gang-tuned signal-frequency tuned circuits in front of a valve mixer, relatively strong 7MHz broadcast signals will inevitably appear at the mixer input, but the situation is significantly worse for modern receivers, particularly in general coverage receivers with minimum RF filtering. Even good sub-octave or amateur-band-onlyfilters (covering the full American band of 7000 to 7300kHz) will allow through the 7MHz broadcast signals unattenuated. Much the same situation applies, for example, to the 15MHz broadcast signals when receiving the 14MHz band, orthe 21.5MHz broadcast signals on 21MHz.