Can any of the guru's here explain in simple language what a "damped wave" is, as in transmitter technology, and a specific example of how one can be produced as an experiment?
Thank you,
Robert
Can any of the guru's here explain in simple language what a "damped wave" is, as in transmitter technology, and a specific example of how one can be produced as an experiment?
Thank you,
Robert
High resolution ground penetrating radar requires a single EM transition to be radiated. Single EM transition is easy enough to generate. Just take a step function, differentiate and fed it to the antenna. The problem is that the antenna will resonate unless heavily loaded. This is sometimes accomplished by constructing the dipole out of resistive elements. To overcome huge losses antenna is fed with a high voltage. The mechanical analogue would be that of plucking a guitar string while simultaneously holding a finger on it to dam any vibrations. The receiving antenna must also be lossy else it will ring like a bell. If someone knows how to radiate a single EM wave with a period of 1 to 3 ns I'd like to hear about it.
-- Boris Mohar
Classically, this was a sine wave that started big and decayed exponentially, V = sin(w*t) * e^(-t/tau).
Specifically, this is what came out of a spark-gap transmitter. Every once in a while, sort of randomly, a charging capacitor would reach enough voltage to break over a spark gap. On the other side of the gap was a resonant L-C tank, coupled to the antenna. Each spark would charge up the tank capacitor, and the LC would ring, making the damped sine. The final antenna drive was a randomish series of damped sinusoids. It was a fairly broadband signal and sounded like a raspy buzz to an AM receiver. The modulation was CW, Morse code, of course.
Some modern UWB systems transmit damped sines, too.
John
One of my customers makes a ground-penetrating radar, to look for mines. The antenna is a sort of horn, made of two tapered metal strips glued to a carved styrofoam block. It looks sort of like a biggish horn tweeter, narrow at the back and flaring out towards the radiating end. We make the fast differential impulse generator to drive it, pretty much gaussian pulses about 200 ps wide. See pic...
We tried using a pair of antennas, one driven from our pulser and the other connected to a sampling scope. The received waveform looked sort of like the transmitted one, with a bit of ringing. We could bounce it off sheets of aluminum and stuff, pretty cool.
As far as I can tell, lots of people have built gp radar mine detectors, but none have worked very well.
John
Nice for close up work. You need lower frequency and more power for big stuff.
The problem with detecting plastic mines using GPR is the dielectric constant of the surrounding soil is similar to that of the mine so that there is very little reflection.
-- Boris Mohar
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