Damped wave transmitter?

[Robert Stevens]

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

 

[Boris Mohar]

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.

 

[John Larkin]

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

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

 

[John Larkin]

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.

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...

http://www.highlandtechnology.com/DSS/T200DS.html

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

 

[Boris Mohar]

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...

http://www.highlandtechnology.com/DSS/T200DS.html

Nice for close up work. You need lower frequency and more power for big
stuff.

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

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.