Preselection

 
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überarbeitet am 21.10.2010

An antenna, which is not selective, will supply the signals of the entire high frequency spectrum, from long, medium ans short waves with their high signal levels from local broadcast band transmitters strong signals in the 49m shortwave bands in central Europa to the faintest SSB signals from a Pacific island to the antenna input socket of the receiver. The receiver has the job of selecting a tiny bit out of the whole spectrum and bring the informations transmitted out of the speaker to the ear of the listener.

A highly amplified broadband signal leads to overloading of the receiver front end, which will lead to highly distorted audio and appearing of intermodulations, "ghost signals" in unexpected locations on the shortwave dial. These will lead to confusion, instead of the messages of Togo's Radio Lomé, You will hear the transmission of the French service of Radio Moscow.

For this reason, engineeers developed circuits which will cut out only a small segment of the whole high frequency spectrum and let this get passed to the RF amplifier stages.

So called band - pass filters will cut off frequencies over or under it's frequency, in many communication receivers You will find a filter to keep the strong signals from the medium-wave band or from signals above 30 MHz away from the receiver RF amplifier and mixer stages.

More sophisticated possibilities to restrict the HF signal to a desired segment consist of manually or electronically switched band-pass filters, manual or automatic preselector circuits, some of them are even motor-driven.

In most modern communication receivers, the preselection consists of electronically switched band-pass filters, there are no controls to be adjusted by hand any more. In some older receivers, like the Collins 51-S1, the preselection ist realised mechanically with complex gears. In the Grundig Satellit 600/650, You find a motor driven automatic preselector. After having typed in a new frequency, You can observe, how the motor will work and the pointer of the preselector will be moving across the dial like magic. Sometimes, it will need a bit of manual tweaking.

Many of the older communication or "world band" receiver have a manual preselector: In these sets, the preselector must be tuned roughly to the appropriate frequency marks (for receiving the "Deutsche Welle" at 6075 kHz set it to 6) and then be adjusted under observation of the signal-strength meter excursion to maximum. With an incorrect preselector setting, You won't hear anything and get the impression, Your radio might be "deaf".

In the modern receiver conception, it is considered as obsolete to leave the operation of the preselection circuit to the listener.
But I would personally rather use a receiver with a manually adjustable narrow preselector, which will give good results on poor signals in proximity of "powerhouse" station the to "enjoy" the annoying noise and language jam of a cheap "World band radio" without decent preselection. Classical receivers using a manually adjustable preselector are the Sony ICF-6800W, the Drake R4 sets or the FRG-7.