|Radio and Electronics (DED Philippinen, 66 p.)|
|6. RECEPTION OF RADIOSIGNALS (AM - TYPE)|
The origin of any signal processed in a radio receiver is the signal picked up by the aerial.
So the radiowave is an electromagnetic wave the reception (picking up) can be achieved generally in two different ways.
RECEPTION OF THE MAGNETIC PART OF THE RADIOWAVE:
This can be achieved by a so called FERRIT ROD AERIAL. Such an aerial consists of a ferrite rod around which the one or more coils of copperwire are wound. The advantage of this type of aerial is, that it needs only little space and therefore it can be built inside the cabinet of even rather small radios.
But keep in mind: the bigger the ferritrod is, the more powerful is the received signal.
The reception of this aerial is depending very much on its position in relation to the received signal. It is receiving best, if the rod is hit perpendicular by the waves. This fact explains, why a receiver with that kind of aerial can have very different output power at the same spot, if the receiver is turned into another direction.
RECEIVING THE ELECTRICAL PART OF THE RADIOWAVE:
This can be achieved by aerials of different construction.
If the radio is stationary we can improve the reception by a long piece of or just a so called TELESCOP-AERIAL (as you find with most of the small portable radios). In generally one can say: as longer the aerial as better its reception.
While the aerials shown above are mostly used for long- and medium-wavebands, we have to use another type for higher frequencies.
The so called dipol-aerial consists of: either two wires constructed as fig. 36a shows (an OPEN DIPOL) or a loop of a wire constructed as fig. 36b shows (a FOLDED DIPOL).
Both types work best, if they have a length of about one half of the waves which are intended to be received. But even though we talk in such a case of ADJUSTED or TUNED aerials, this does not mean, that this aerial is only able to receive one single radiostation. All aerials are able to receive a rather wide range of frequencies with reasonable results.
This is on the one hand a big advantage (we don't need a special aerial for each different radio station we want to receive). On the other hand it causes problems for the radio technician, because we will always find at the terminals of the aerial a lot of different incoming signals, which are mixed together.
The above stated fact makes already clear what has to be done first with the signals found at the terminals of the aerial: we cannot just amplify the signal coming from the aerial, because under this condition we would hear an awful mixture of sounds of all the radioprograms transmitted in the surrounding of the radio.
We have to make sure first, that only the signal of the desired station will be processed during the next stages of our radio receiver.
This process of barring the other stations out and letting through only one station is called FILTERING. In which block the filtering is achieved, will be explained in the next chapter.