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 Post subject: Homemade GPR
PostPosted: Mon Oct 26, 2015 1:04 pm 
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Joined: Wed Oct 21, 2015 7:57 pm
Posts: 2
Dear all,
I am a newbie in GPR and doing a project to build a low frequency gpr. I have done some reading and drawn the following plan to construct that.
-- I have acquired a pulse generator that can generate 5 V 10 ns width of pulse at 500 kHz repetition frequency.
-- As my Gaussian pulse width is 10 ns so centre frequency is 1/10 ns = 100 MHz.
-- I have built two dipole antennas with centre frequency of 100 MHz and bandwidth is 96 MHz - 104 MHz. I have tested the antenna with a network analyzer.
-- Now I am transmitting pulses using one antenna and using another antenna to receive it. I have kept 1/lambda space between the two antennas. I am using a 1 GHz bandwidth digital oscilloscope to capture the a-scan.

Could you please tell me if my procedure of building the gpr is correct? I will eventually build a higher frequency (500 MHz) system with shorter dipole antennas, but is my initial setup correct?

Please let me know if I have missed anything or doing something incorrectly. Do I have to consider some other issue?

Thanks in advance.


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 Post subject: Re: Homemade GPR
PostPosted: Tue Oct 27, 2015 7:37 am 
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Joined: Wed Apr 29, 2009 5:52 am
Posts: 96
Location: Zagreb, Croatia
Dear howladem,
some of the GPR electronics fellows could probably give you a more detailed answer, but in a lame term perspective, I believe that you have an obvious problem here.
GPR antennas are and should be UWB (ultra wide band) devices, so unlike in the communication world, you want your antenna to have as wide bandwidth as possible to improve the data. From that perspective feeding the antenna with the generator or a simple dipole may not be the best choice for the antenna design.
Narrow banded antennas (in GPR world) are bad, because they have a tendency to be very "ringy" (horizontal banding in the data). That means that most of the energy is used for "useless" repeating of a single frequency (and harmonics related). The penetration will be limited and any responses (reflections) will be overwhelmed with the ringing, so you will have data that is terrible to interpret.
One way of evaluating the quality of a GPR antenna is by looking at the bandwidth.
Your antenna
Width(MHz)=(Fh-Fl)=104-96=8MHz
Fc=(Fh+Fl)/2=(104+96)/2=100MHz
Bandwidth(%)=((Width/Fc)*100)=((8/100)*100)=8%

Example commercial 100MHz antenna
Width(MHz)=(Fh-Fl)=156-62=94MHz
Fc=(Fh+Fl)/2=(156+62)/2=109MHz
Bandwidth(%)=((Width/Fc)*100)=((94/109)*100)=86%
I hope this helps you a little bit in your work.

_________________
"If the only tool you have is a hammer, you will see every problem as a nail."


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 Post subject: Re: Homemade GPR
PostPosted: Tue Oct 27, 2015 10:54 am 
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Joined: Wed Oct 21, 2015 7:57 pm
Posts: 2
bekic wrote:
Dear howladem,
some of the GPR electronics fellows could probably give you a more detailed answer, but in a lame term perspective, I believe that you have an obvious problem here.
GPR antennas are and should be UWB (ultra wide band) devices, so unlike in the communication world, you want your antenna to have as wide bandwidth as possible to improve the data. From that perspective feeding the antenna with the generator or a simple dipole may not be the best choice for the antenna design.
Narrow banded antennas (in GPR world) are bad, because they have a tendency to be very "ringy" (horizontal banding in the data). That means that most of the energy is used for "useless" repeating of a single frequency (and harmonics related). The penetration will be limited and any responses (reflections) will be overwhelmed with the ringing, so you will have data that is terrible to interpret.
One way of evaluating the quality of a GPR antenna is by looking at the bandwidth.
Your antenna
Width(MHz)=(Fh-Fl)=104-96=8MHz
Fc=(Fh+Fl)/2=(104+96)/2=100MHz
Bandwidth(%)=((Width/Fc)*100)=((8/100)*100)=8%

Example commercial 100MHz antenna
Width(MHz)=(Fh-Fl)=156-62=94MHz
Fc=(Fh+Fl)/2=(156+62)/2=109MHz
Bandwidth(%)=((Width/Fc)*100)=((94/109)*100)=86%
I hope this helps you a little bit in your work.


Thank you. So you think my antenna is the issue. In that case can I use a 200 MHz centre frequency antenna with 100 - 300 MHz bandwidth bowtie antenna for the same 10 ns width pulse signal??


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 Post subject: Re: Homemade GPR
PostPosted: Tue Oct 27, 2015 3:12 pm 
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Joined: Wed Apr 29, 2009 5:52 am
Posts: 96
Location: Zagreb, Croatia
hi howladem,
I just noticed the potential problem with having a narrow banded antenna - this I am certain would make your attempt futile.
Your second question should be answered by an electrical engineer with deeper insight on the design issues - I do not have enough knowledge on the subject to be helpful.
Bowtie antenna with ~100% bandwidth should be a good starting point and, if I am not mistaken, I believe that most GPR manufacturers try to use that particular design. Keep in mind that, unfortunately, selection of a good antenna element design is just a scratch on the surface when it comes to making a GPR solution.
But, then again, what is life without a good challenge, right?

_________________
"If the only tool you have is a hammer, you will see every problem as a nail."


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