Hello All, its the 27th of Oct. 2014

We discussed receivers. A bit on transmitters with vacuum tubes. But before we proceed, we have to covered two topics. Electromagnetic waves(EMW) and modulation.

EMW

     Also called radio waves. It is a sine wave which goes positive then negative in one cycle. With tubes, it became possible to build oscillators and with crystals, it became possible to create an oscillator with a fixed frequency. Tubes not only created oscillators but also served to amplify the signal.

     The TX coil is like a primary coil in a transformer. The oscillator creates how many cycles are generated in a  second. Our  wall AC supply for example has an oscillation of 60 Hertz, very low and hardly at radio waves frequency. The MW broadcast band ,AM, is around 1 Mhz. The CB radio is at 27Mhz. Today the new band is at 2.4 Gigahertz.

     The receiver coil is like a secondary coil in a transformer. So, what the TX generates, the RX can still pick it up at great distances but in a very minute form, in microvolts. In a transformer, the coils are not in resonance and current is induced to the secondary via  the metal core. In radio, there is no core but space. The coil being in resonance permits the RX coil to "hear" at great distances.