When Will They Make A Multi Frequency Detector and Fractal Coil ?

[Functional]

It just seems to be a natural thing to apply the same concept of fractal antenna design to the coil of a metal detector that they use in cellphone antenna's. I know they make multi frequency metal detectors, but I'm talking about a metal detector that can transmit multiple frequencies in a sequence, over a period of nanoseconds, using a fractal type of, (antenna), coil design.
See: http://en.wikipedia.org/wiki/Fractal_antenna

A detector based on such a design would avoid the limitations of "fixed" frequency detectors and depending on the operator's selected settings, a person could choose to go for depth and target size, rather than settle for whatever the normal detector frequency is capable of detecting. Rather than settling for either a low, or high frequency MD with depth, or target size limitations imposed by the detectors design, the user would be the one deciding what they wanted to search for and where.


Just a thought.

F.

 

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[mikeofaustin]

Just guessing, but I would think a fractal antenna is the same as a "loaded" antenna. Meaning, it doesn't serve any purpose other than to get smaller and less range, but at least it can fit in the form of a 'whip' antenna (short and stubby). I think fractal antenna hasn't made it main stream because it is just that.

It all has to do with wavelenght. The short the wavelenght (higher the frequency), the small the penitration. The longer the wavelength, then you'd have to swing your coil at a very sloooow rate to get good depth.

Think of it as radar. The very long range radar has to 'round go very slowly to get long distances (watch the weather channel radar for example). But, a short range radar can get back information much faster. That's why airports have multiple radar horns. a long range (slow), a medium (intermediate), and a short range (quick feedback). Another example would be HAM enthusiasts. When people talk around the world, they're on a very low freq. But, short range, they can go much higher freq for a better sound quality.

It's just the nature of the beast.

 

[Carl-NC]

Fractal antenna concepts can't be used in an induction metal detector because the search coil doesn't work like an antenna.

- Carl

 

[jimmyjiver]

Hi,, Excellent call Carl....... ......JJ

 

[Treasure finder]

Best metal locator is still a shovel.
Rich

 

[olepossum]

when will they come up with xray vision glasses so all i have to carry is a lesche trowl and a nail apron in stead of a md a trowl a digger a probe a find bag a trash bag and head phones and a gallon of water on my back just to find 250 in clad a few good relics and a wal mart sack full of trash

 

[Functional]

Fractal antenna concepts can't be used in an induction metal detector because the search coil doesn't work like an antenna.

- Carl

I've heard that said, but nobody has explained it. I'd always thought that a MD. (like the Gold Bug-2 for example), that uses the lower end of the radio spectrum at the 71kHz frequency, was a radio transmission and reception in the VLF range (Very Low Frequency)? It would seem logical, to have radio waves used for metal detection, as I do know that gold and other metals are effected by radio waves. Not long ago there was a report of a medical procedure on TV involving gold nano particles being injected into cancerous tissue, then an RF transmission is used to cause the particles to heat up, destroying the cancerous cells. Which makes me wonder if the heat generated was unusually higher when gold is involved and would this mean a thermal imaging camera could be used to detect gold heated by exposure to RF waves?

Getting back to the topic at hand. I find it hard not to wonder what energy form is being emitted and received by an MD, if it isn't radio waves.

F.

 

[Functional]

Just guessing, but I would think a fractal antenna is the same as a "loaded" antenna. Meaning, it doesn't serve any purpose other than to get smaller and less range, but at least it can fit in the form of a 'whip' antenna (short and stubby). I think fractal antenna hasn't made it main stream because it is just that.

It all has to do with wavelenght. The short the wavelenght (higher the frequency), the small the penitration. The longer the wavelength, then you'd have to swing your coil at a very sloooow rate to get good depth.

Think of it as radar. The very long range radar has to 'round go very slowly to get long distances (watch the weather channel radar for example). But, a short range radar can get back information much faster. That's why airports have multiple radar horns. a long range (slow), a medium (intermediate), and a short range (quick feedback). Another example would be HAM enthusiasts. When people talk around the world, they're on a very low freq. But, short range, they can go much higher freq for a better sound quality.

It's just the nature of the beast.

Fractal antenna design isn't quite the same as normal antennas. Quote: "A fractal antenna's response differs markedly from traditional antenna designs, in that it is capable of operating with good-to-excellent performance at many different frequencies simultaneously."

See: http://en.wikipedia.org/wiki/Fractal_antenna


The fractal antenna is only one of the many different types of radio frequency antenna listed on this page:
http://en.wikipedia.org/wiki/Category:Radio_frequency_antenna_types

F.

 

[Charlie P. (NY)]

VHF detectors have wavelengths in the seven or eight meter range. Radio waves around 1 meter. Cell phones are 2.4GHz - closer to radar waves in the centimeter wavelangths. That's why I am curious why moving a few Hz and setting up multiple frequencies is thought to help. I would think a single, well modulated frequency and well tuned receiver at any given single frequency would be better at soil penetration. The FCC limits us to transmit at 0.5 watts or so, why not concentrate it? Multiple frequencies might just fuzzy it all up. You want to bounce a signal off the moon? Send a lazer (extremely tuned single frequency transmission). You want to bounce a signal off the backside of the earth? Send a single very precise ELF pulse.

The NAVY does it all the time to contact subs.

 

[Functional]

Self-similarity and origin symmetry are shown to be the key geometric constraints in the determination of frequency independent properties of antennae. Fractal antennae with origin symmetry meet these criteria of the extended version of Rumsey's principle. Frequency independence is not achieved by self-similarity alone.

Not being an electronics guru, I would say that that sounds reasonable.

Self-complementarity plays no role in frequency independence, but does aid in smoothing out impedance variations for coarsely iterated frequency independent antennae.

I'd guess that the electronics involved would play a part in frequency variations, (tuning), in much the same way that I tune my CB radio antenna based on the reading I get from my SWR and signal strength meter, (adjustments being made for best results for specific frequencies)?

New families of practical designs arise from these geometric insights, which need not follow the usual constraints of angle-defined structure of the original Rumsey's principle, or am I incorrect?

Sounds good to me. I just wondered if anyone had actually had any results related to applications involving MD's. Traditional coil design doesn't seem to have had any significant outward change in appearance or ability in many years, (with the possible exception of increased performance of the PI detectors). I did find a couple of references to "walk through" type security metal detectors that are supposed to be using a fractal design antenna, but there weren't any pictures detailing them.


F.

Links of interest on the topic:
http://www.worldscinet.com/fractals/07/0701/S0218348X99000098.html
http://www.fractenna.com/nca_faq.html

 

[Functional]

VHF detectors have wavelengths in the seven or eight meter range. Radio waves around 1 meter. Cell phones are 2.4GHz - closer to radar waves in the centimeter wavelangths. That's why I am curious why moving a few Hz and setting up multiple frequencies is thought to help. I would think a single, well modulated frequency and well tuned receiver at any given single frequency would be better at soil penetration. The FCC limits us to transmit at 0.5 watts or so, why not concentrate it? Multiple frequencies might just fuzzy it all up. You want to bounce a signal off the moon? Send a lazer (extremely tuned single frequency transmission). You want to bounce a signal off the backside of the earth? Send a single very precise ELF pulse.

The NAVY does it all the time to contact subs.

Different frequency for different soil conditions, (like ferrous materials), moisture content and salinity of any water present, all have an effect. In a perfect world, it wouldn't matter which metal detector you used, or what frequency it used, but reality can be intrusive. From what I've read of various metal detectors, they can operate at frequencies as low as 5 khz and has high as 100 khz. (I think my old Compass Magnum is an 80 khz unit, but I'm not certain off hand.) I figure that it would be nice to have a single MD that could be used on all types of soil, with all levels of moisture content, while searching for various sizes of deposit, without needing an arm full of MD's to do the job.

F.

 

[Carl-NC]

I've heard that said, but nobody has explained it. ... I find it hard not to wonder what energy form is being emitted and received by an MD, if it isn't radio waves.

Detectors use an AC magnetic field. Look up electromagnetic induction and eddy currents.

That's why I am curious why moving a few Hz and setting up multiple frequencies is thought to help.

Induction phase responses (or time constants) vary quite a bit over the VLF band.

The FCC limits us to transmit at 0.5 watts or so...

Sorry, not true.

 

[Treasure finder]

Interesting topic,
I was a microwave radio repairman in the Army in the 60's and this discussion
is a bit above me. On multi-Frequency detectors I have heard that frequencies
around 1.785 KC (Early Whites GEB) were great for iron artifacts, freqs from
13.77 KC to 71 KC were used on gold detectors, and frequencies around 100 KC
had a diminished response to iron. If so, how about sending out 3-5 frequencies
with signal strength meters or bar graphs so you can compare and maybe know
what you are getting.
Also Accurate Locators has a combination Pulse Induction and magnetometer
detector so that you can really discriminate iron, since magnetometers only
respond to magnetic fields.
Rich

 

[Carl-NC]

If so, how about sending out 3-5 frequencies with signal strength meters or bar graphs so you can compare and maybe know what you are getting.

White's V3 does this.

 

[Treasure finder]

Thanks Carl,
Great forum, Ask and you shall receive. I guess I was smart too late to
get the invention money.
Rich