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PostPosted: Fri May 24, 2013 4:06 pm 
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Joined: Fri May 24, 2013 3:34 pm
Posts: 4
Hi everyone,

I'm a beginner at GPR use and I am having some problems with all the parameters that are involved in the choice of the antenna. My main problem right now is about the frequency of the antenna. My doubt is what is the difference, in terms of resolution and depth, between Center Frequency and Bandwidth.

I have an antenna with 200 Mhz to start with. Lets say hypothetically that his antenna has a 10-1400Mhz Bandwith. My question is, which of the parameters will define the depth and resolution achieved using this antenna. If I use it at 900Mhz will I have the same results as if I used a 900 Mhz center frequency antenna? If not, what is the use (or purpose) of using a 200Mhz center frequency antenna at a different frequency, 900Mhz for example?

Ty for the help.

By the way, sorry for my bad English, this is not my native language.


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PostPosted: Wed May 29, 2013 2:16 pm 
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Joined: Tue Nov 30, 2010 3:21 pm
Posts: 30
hi titolivioerculino,
I would suggest that you take a look at these two documents to get a better grasp on the relationship between the center frequency and the bandwidth of the GPR antenna:
http://www.geoscanners.com/pdf/gprbw.pdf
http://www.geoscanners.com/pdf/antcomp.pdf

Now, let's see your hypothetical examples where you declared that you have two antennas with center frequencies 200MHz and 900MHz. The center frequency of the antenna should always be in the middle of the bandwidth of the antenna, so we can say that it fairly represents all of the frequencies in the spectrum. Furthermore we are not interested in the part of the spectrum that has the frequencies with magnitudes weaker more than -10dB. If we use this specified point and the formula from the first document in the first link we ideally get the effective bandwidth of the 200MHz antenna to be between 100 and 300MHz. The same calculation for the 900MHz antenna will provide (ideally) a bandwidth between 450MHz and 1350MHz.
From this you see that these two antennas are very different - their effective bandwidths are not overlapping and we can presume that the results coming from them are going to be very different.

As a good evaluation value for the resolution we can always calculate one half of the wavelength of the center frequency (because it is a good representation of the entire bandwidth) in the material and say that we can expect to "see" objects which are bigger than that size.

When it comes to depth of penetration we can calculate that the material attenuation is mostly depending on the conductivity for all the frequencies between a certain low frequency and before the frequency is high enough that the effects of water relaxation come in to play. This zone of "equal" material attenuation for all frequencies stretches approx. between 15MHz and 1500MHz. However we know that, in general, lower frequency antennas penetrate deeper than the higher frequency antennas. The reason for this is in the scattering attenuation inside the material. The scattering attenuation is the result of the signal reflecting and refracting (never to return to receiver) around objects and particles in the material and since higher frequency antennas "see" more smaller objects the dispersion of the energy occurs faster.

I hope that this rambling makes since to you and that it will help you continue your research.


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