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Re: Uhuh, and they're restricted to certain bands...
Posted by: Mr.Bill
Date: January 29, 2006 03:40PM
They are because they don't fall within any restrictions. Top this off the US FCC did change it's regulations a few years back also. It may not be OK in Canada, but it's OK here in the US.

If you do yourself a search on this forum, and other forums you will see that has been brought up many times before, it's starting to get old hat. Go take a look see, there's plenty of info to look over.

Carl M. from the GeoTech forum had addressed this question last I believe, with a good explanation. Do a search.

Mr. Bill

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A couple of years back
Posted by: willy
Date: January 29, 2006 06:51PM
is when I think I last read about it on a forum and I think 500 milliwatts is the number which was bandied about. Who knows, it could even have been wrong back then. ..Willy.

Copy of NC-Dave's post from below on this subject
Posted by: Mr.Bill
Date: January 29, 2006 07:20PM
Transmit Power Levels and the FCC
Posted by: NC-Dave (65.7.241.57)
Posts: 15
Date: January 29, 2006 05:19PM


Radio transmitters and transceivers are categorized by the FCC here in the USA as "Intentional Radiators". Any such radio device is regulated as to it's maximum power level and it's transmitted harmonics. Even a radio receiver has to pass a very stringent set of tests to make sure that it does not radiate unwanted energy. Receiver's usually radiate some energy from their conversion oscillators and can be harder to get through their compliance testing than a transmitter.

Metal detectors are not intentional radiators. They do not come under the same rules as radio's. Most metal detectors operate at audio frequencies. Stereo loudspeakers also transmit some of their signal through the air They too do not require RF compliance testing.

The coil size required to efficiently transmit a signal at metal detector frequencies would be incredibly large. Antennas are made in wavelengths or fractions of a wavelength such as a quarter or a half wavelength.

The wavelength in meters = 300,000,000/frequency in Hz. OK, so for a metal detector operating at 5000Hz (5KHz) the the wavelength is 60,000 meters. A half wave antenna would be 30,000 meters from end to end. That is 18.64 miles while a nice little quarter wave mobile whip would extend to just over nine miles above your car. You can now see why your ten inch loop is not going to cause too much of a problem.

Plus, also.................
Posted by: Eric Foster
Date: January 30, 2006 12:28AM
most coils have a conductive shield around them, which attenuates any em signals that are outside the useful band that the detector needs to operate. This applies both to signals radiated and received.

Eric.

.N/T
Anonymous User
Date: January 30, 2006 03:41AM

(This message does not contain any text.)

Edited 2 time(s). Last edit at 02/22/2006 06:32PM by vlad.

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Re: Plus, also.................
Posted by: Prospector Al
Date: January 30, 2006 11:53PM
Everything that has been said here about the interference caused by a metal detector at its operating frequency and possible harmonics is true...

However, the situation changes dramatically when a microprocessor is used in the detector to manipulate the received data and to display the results on a screen.

I used a spectrum analyser to look at the RF output of a detector whose uP clock oscillates at 4 MHz. There was a strong fundamental and harmonics all the way into the FM band!

Fortunately, metal detectors are mostly used far away from sensitive equipment...

I'm thinking about selecting a frequency that falls into the ISM band.
(Industrial, Scientific and Medical)

If anyone knows about FCC regulations that apply to metal detectors,
I'd appreciate a link...

Prospector Al

Re: Plus, also.................
Posted by: Carl-NC
Date: February 01, 2006 09:45PM
"If anyone knows about FCC regulations that apply to metal detectors,
I'd appreciate a link... "

There are no FCC regs that apply specifically to metal detectors. The old "100mW" limit is a myth. Detector coils are horribly inefficient "antennas" and do not support far-field EM transmission.

Detectors do have to meet the "unintentional radiator" limits that all electronics must meet, and that is largely self-regulated. Read through FCC 47 CFR Part 15 for the gory details.

- Carl

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Re: Plus, also--FCC Regulations
Posted by: Prospector Al
Date: February 01, 2006 10:00PM
Hi Carl,

Thanks for the info. You're right--nothing applies to metal detectors as such. The closest reference is "cable locators".

It appears that if they operate below 9 kHz, the power input could be as high as 10 W.

That's enough to operate even the most powerful hand-held detector.

I believe Eric's detectors work at 10 kHz. Nobody is likely to complain, and if anyone does, he only needs to drop the frequency a kHz...

Happy Hunting,

Allan (Prospector Al)

Re: Plus, also.................
Posted by: 5900_XL-1
Date: November 09, 2015 12:37AM
Quote
Carl-NC
"If anyone knows about FCC regulations that apply to metal detectors,
I'd appreciate a link... "

There are no FCC regs that apply specifically to metal detectors. The old "100mW" limit is a myth. Detector coils are horribly inefficient "antennas" and do not support far-field EM transmission.

Detectors do have to meet the "unintentional radiator" limits that all electronics must meet, and that is largely self-regulated. Read through FCC 47 CFR Part 15 for the gory details.

- Carl

A 100mw regulation for ground work is a pure joke.

Yea, detector coils are horrible antennas. There's tons of techy advances to be done on metal detecting but it just isn't worth the R&D expenses to the manufacturer..

Again, the 100mw spec is a pure joke IMO.

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