Very far-fetched pre-project...

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Externet
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Very far-fetched pre-project...

Post by Externet » Sun Sep 10, 2006 12:24 pm

Hello all.
Again giving some headaches to you all:

I have some plain 150 MHz transceivers (2m) of around 50 Watt, can spare one to play with.
Also, a fish finder / depth sounder, with its LCD, a typical used for fishing with the transducer that attaches to the side of a boat.

A marriage...

During the echosounder burst transmission, to trigger the VHF into transmission; and when it listens for the echo, feed the VHF reception echoes into the echosounder transducer.

Dragging the contraption VHF antenna on soil, to use as ground penetrating radar. -A poorman's GPR-
The antenna would have to be built to suit single directionality downwards, the timing of the echosounder would need tailoring for the much shorter propagation times, etc. (or simply delay the VHF received signal fed to the transducer to fit the echosounder timing)

The LCD to display the particularities under the soil and their depths...

Could such animal work ? :shock:

Miguel

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Post by jwax » Sun Sep 10, 2006 3:38 pm

You can come up with some dandy's! :grin:
I'd guess 150 MHz won't penetrate more than inches in soil/rock. Then there is the water attenuation/reflections. Hmm.
The echosounder is useless in this application due to its ultrasonic range of frequencies, but the display may be useful for this project-displaying "disturbances" vs. depth.
I'd research ground penetrating radar and see what freqs they use. Also check into the latest metal detectors- maybe just what you're looking for!

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Post by Chris Smith » Sun Sep 10, 2006 3:42 pm

GPR, echo locators, and metal detectors all work on different principals.

GPR requires the right frequency pointed downwards, strong, and directional [ant] as well as a locator usually made from the use of a scope and amplifier taking back the signal and all of its discrepancies,... and deciphering all of its signal, thus receiving a band width much larger than simply detecting the ping of a single signal and then discriminating all of this EM on the way back into a full picture, not a scope glitch.

Metal detectors send a basic strong wide pulse out and simply detect that it has been altered by the metal,... and the field that got energized by this wave front and the bounce of EM interacting with the field that metals throw off.

Echo locators work on sound, and like the GPR read much more than the single wave that is disturbed. They decipher the bounce into a full picture like display. [Different frequencies]

Yes, all similar, but all are different. Sounds like a lot of R&D?

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Post by Externet » Sun Sep 10, 2006 4:36 pm

Hey jwax... ¿wouln't that be neat? :grin:

Don't guess. 150 mhz is right on the range, easily beyond 1 metre depth at 50 W; higher frequencies impair resolution and lower frequencies penetrate deeper. (honesly I would not like to dig deeper :smile: )
Wet soils absorb the RF; dry soils allow readings.

I could use my 140W 30 MHz linear brick too, but have no exciter. Other then a whole bulky Kenwood TS440. I think I have a 450 MHz 25W too.
And the power supply is not much, for a very short ping every second. A motorcycle battery would do it.

Yes, of course the echosounder would be just to use its graphing.
Naaaah. No metal detectors... something more challenging, to bypass the GPR unafordability of the real world.

If in your interest, check :
http://www.sensoft.ca/faqs_frame.html
For shallow knowledge.

Miguel

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Post by jwax » Sun Sep 10, 2006 5:27 pm

Appears interpreting those squiggles is the tricky bit. I'm surprised 150 MHz has that much penetration, but now it sounds doable. Also sounds like a helluva cool project! I'd think N & V would print your article about that!

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Post by Chris Smith » Sun Sep 10, 2006 6:14 pm

Real GPR is often in the terrea Htz [and higher] , not the Mhz.

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Post by bigkim100 » Sun Sep 10, 2006 6:43 pm

I second the thought, sounds like a hell of a project...that WOULD be great in N&V Mag if it was practicala as doing any sort of landscaping or digging can be a nightmare not knowing what is underground. I researched basic ground penetrating radar for my friends that work in a graveyard, and found that the cost for even the most basic of units is sky high.
Kim..The man with the cute little girls name...and Frankensteins face and body.

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Post by jimandy » Sun Sep 10, 2006 7:49 pm

If you're interested in GPR, besides the characteristics of RF passing through soil (and reflecting) you have to get into the science of tomography involving some very sophisticated DSP coupled with display technology. Explains why those GPR units are so damned expensive.
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Post by rshayes » Sun Sep 10, 2006 8:03 pm

This would not be a very happy marriage.

The depth sounder is designed to work with signal that propagate on the order of kilometers per second. Radio waves propagate at about 300,000 kilometers per second.

A depth sounder whose timing and display circuits can resolve 1 foot using sound waves in water would be able to resolve about 100,000 feet (about 19 miles) using radio waves. That's more digging than I would care to do.

It would also be difficult to transmit a short enough pulse with a normal tranceiver and the transmit-receive switching is probably not fast enough (a few nanoseconds).

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Post by Robert Reed » Sun Sep 10, 2006 9:35 pm

"Well Ollie, you've really done it now"(ala Laurel & Hardy). I think if you get into this project, you will spend the rest of your life getting out of it. Considering that the radar mile is 10. 8 microseconds (the term holds true for any RF signal and is round trip), and your intended targets are - say a few meters to 100 meters, The round trip time will be extremely short ( on the order of 10 to 200 nanoseconds). This would take some very sohpisticated timing circuitry to say the least in order to properly capture and display the echo. Even conventional radar has a minimal range that is dictated by transmission clutter(many meters ). The same also holds true for conventional sonar operating at 200Khz ( 2 feet min. for sonar). For starters you would need a very short burst of RF and this has to be well damped to eliminate residual ringing and this will reduce the power you may otherwise have attained. Also as RS Hayes pointed out the switching diplexer would probably be a nightmare to design and we are only getting started. The timing circuits in the sonar would be WAY to slow for the high speed operation involved here. The echo circuits output data would have to be dumped into some sort of memory and then clocked out at some super slow speed in order for the sonar display to properly handle and display it-another nightmare. And it goes on and on. There is a reason that GPR is so expensive, because it takes a lot of sophistcated circuitry to accomplish it. I admire your guts for even thinking of tackling such a project and if you even come up with a "lo-budget" version, there will be lot of people on this forum applauding you for your efforts!

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Post by stevech » Sun Sep 10, 2006 9:41 pm

Chris Smith wrote:Real GPR is often in the terrea Htz [and higher] , not the Mhz.
yep. marine depth finders are, as you know, sonic rather than RF.

I don't know about terra Hz for GPR. I do have some work in process with ultrawideband (UWB). Due to FCC regs, it hardly penetrates drywall.

GPR that the military/police use have special regulations.

150MHz is a licensed band so don't transmit there illegally or interfer with public safety or aircraft. That freq. doesn't propagate in soil much at all.

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Post by jwax » Wed Sep 13, 2006 9:05 am

For an entirely different approach, these guys use ultra wideband pulses to create thru-wall pictures of people on the other side.
http://www.timedomain.com
Perhaps their technology would make a ground penetrating imager, and you could still use your radios for chatting! :grin:

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Post by Chris Smith » Wed Sep 13, 2006 10:20 am

The military GPR is in the higher frequencies for two good reasons. First they have the money and technology, and second, they don’t look just a few feet under ground, from the ground.

Even some of the Polar Ice expeditions are in the higher range, on the ground and airborne.

The GPR units such as the spy planes and satellites use, go down way more than 20 feet to look for weapons, underground shelters, and command centers, etc.,... hence the higher frequency which equates out to more power purposefully utilized.

Smaller energy objects usually penetrate further, such as UV, X-rays, and Gamma rays.

Same goes for RF, within appropriate degrees.

Also most military GPR units are Airborne.

There are certain frequencies for certain materials and having the band width allows multiple sweeps for many materials and depths.

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Post by Chris Smith » Wed Sep 13, 2006 10:22 am

Terra Hz usually sees through walls.

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Post by Dean Huster » Thu Sep 14, 2006 3:43 pm

Regarding the sound-vs-radio aspect of resolution, radio/light/electricity travels at the rate of approximately one foot per nanosecond. I always used this fact to give students an idea of speed relationships in digital circuitry (risetimes, propagations delays, memory access times, etc.

Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

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