Damped waves by electromechanical vibrator. Bandwidth considerations and more
Their frequency is determined by the resonance of the LC and the dumped pulses occur at the frequency of oscillation of the relay switch.
The LC provides some form of tuning to a single frequency, but if additional band-pass filters are used after this LC, I believe that the output signal would be quite clean.
I believe that the use of high-Q LC improves the frequency stability.
Also the RF frequency does not depend on the variations of the relay oscillation, but only on the LC resonance and Q.
Why then damped wave emission is prohibited for transmitting, if these countermeasures are in place?
The fundamental frequency is set by the LC resonance, but there is some "modulation" by the switching. So you do NOT get a clean continuous wave, and instead a complex signal with a lot of unwanted/uncontrolled frequency components.
Well, why not just include an output band-pass filter to cope with these? Then all you get will be RF signal that is modulated by the "tone" of the relay switching. I find the frequency to be quite stable when using one of these LW/MW loopsticks, with a frew turns primary, as "transformers".
Think of the class-E transmitters, they are driven with a square wave and they output a square wave. The only thing to bring them to legal bandwidth limits is the LPF after the switching device.
I do not say it is the same, as these do not produce a damped wave, but why not just filter the unwanted frequencies in the electromechanical case?
The modulation you refer is caused by the pulse rate of the relay, and it may be desirable as a tone (when morse code is used) if the relay can switch fast enough. Are there any other by-products caused by this in the RF part?
If the filter is more complex than building a "clean" osciallator, it's just inefficient.
Of course it's not just "a tone". Expect a fence of sidebands at n*switching frequency, plus other frequency components from the dampening.
Here's an example for a time shape (Gaussian pulse) and the related spectrum:
../imgqa/eboard/Antenna/rf-uhugalrgfhn.jpeg
Indeed, but it can be done that way, won't this work?
Thank you for the diagram, this is helpful to see what is going on. I see these signals are very close together, hm...
This is the basic spark gap transmitter. If you consider your train of damped oscillations, they can be thought of as a CW with some sort of exponentially decaying modulation. So in the output spectrum there will be the carrier frequency with sidebands that are multiples of the modulation frequency. Because the frequency of the modulation is so close to the carrier they will not be able to be filtered out.
Frank
Thanks Frank, I had it like this in my mind indeed. It is exactly like this, but there is also a gap (big one) between the carrier decaying pulses, as the Q of the LC can never be that high to sustain oscillations until the next (1KHz) modulating pulse comes in.
The diagram of the previous post helps visualizing it.
Here is an interesting experiment I did yesterday:
I made a tuned circuit for LW. Then I connected a crystal (one of these vintage higher power types) to one end of the tuned circuit and took the output from the crystal to the FFT analyzer.
All harmonics of the fundamental were disappeared! The crystal does a very good job in filtering.
Also, since the single tuned circuit can never have such a high Q as the crystal and it's bandwidth is much higher, even slight miss tuning (out of the crystal frequency) does not play a major role (if it is slight) and it seems the main frequency is determined by the crystal filter and it is stable of course.
The way the crystal filters reminds me this circuit
http://qrp.gr/refosc/index.htm but of course only the tuned LC is before the crystal.
However I doubt if the crystal can filter the so close to the "carrier" signals you refer to. A single crystal bilter is very narrow (a few 10s of Hz) but maybe not enough for these so close to the carrier signals.