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I'm building a small radio transmitter based on a 555 timer. In my circuit a 555 timer in astable mode is used as an oscillator. kind off like : http://www.instructables.com/id/555-AM-Transmitter/

From what i understand a 555 Timer in astable mode outputs a "Pulsed DC" signal (HIGH LOW HIGH LOW, but never goes negative).

And a "AC" signal is needed to drive an antenna for electromagnetic radiation to occur, Is this correct?

Or can a "Pulsating DC" signal just as well be used for radio transmission? In the instructables project the "Pulsed DC" is directly used to drive the antenna. How does this work?

thanks in advance,

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  • \$\begingroup\$ Your pulsed signal has both AC and DC components. It is the AC component that get transmitted over the antenna. The DC component only leads to a steady field. \$\endgroup\$ – Bart Sep 21 '17 at 8:56
  • \$\begingroup\$ If you look at a OPamp circuit, the input signal can be both AC coupled or DC coupled. Essentially, the same is happening, the DC coupled signal, just has a DC bias at ex. +2.5 Volts. The voltage is never negative (relative to ground), but fluctuates at a positive bias (say between 0 and 5 Volts). \$\endgroup\$ – Andreas HD Sep 21 '17 at 9:20
  • \$\begingroup\$ This is pretty much how most "Class C" RF power amplifiers work. Note that the pulses - square waves - are rich in harmonics. They are fed to the antenna via a narrowband filter which eliminates the harmonics - and the DC too. \$\endgroup\$ – Brian Drummond Sep 21 '17 at 9:30
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And a "AC" signal is needed to drive an antenna for electromagnetic radiation to occur, Is this correct?

If you are building a radio transmitter as per the title of your question then there are a few things you need to get right: -

  1. You need to ensure you only transmit at allowable powers at allowable frequencies
  2. You need to restrict the bandwidth of your transmitted carrier so that harmonics don't upset other radio systems at harmonics of your basic carrier wave.
  3. You need to respect that an antenna presents an impedance that is quite variable to wideband signals and this can screw up the ability to produce wide band RF (see also 1 and 2)
  4. You need to have an antenna that is designed for your output stage - you need to observe whether your antenna design is balanced (dipole) or unbalanced (monopole)

The AC content from the 555 is what an antenna is interested in - the fact that there might be a standing DC voltage on it that represents that average value of your pulsed waveform is of no-importance to the antenna.

In the instructables project the "Pulsed DC" is directly used to drive the antenna. How does this work?

Quite badly and quite possibly illegally.

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