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How Quiet is Goldscan 5?

A

Anonymous

Guest
I found that out years ago when using a 24" loop-except others with detectors were screaming at me to go somewhere else-most emphatically!
 

Eric Foster

New member
You can, but it's not something I would recommend. GS5 seems to cope well enough in electrically noisy environments, without having to resort to drastic measures. Better to turn the Range (RX gain) control down.

Eric.
 

Mr.Bill

New member
What your citing does not apply in this case.

If you do a search here on this forum, and other forums, you will find a lot of post on this. :)

Mr.Bill
 

gatorguy

New member
You crank the Threshold back not the Gain. Run the Gain as high as you can always, if you turn the Threshold back to the edge of the silent zone you will not lose much depth, but the deeper you go into the silent zone you will start to loss some good depth. Don
 

willy

New member
and detectors don't seem to be. That, in itself, would be grounds for restricting the output. If a 5 watt phone can comunicate for miles, a 5 watt metal detector pumping out that much noise across the entire AM band for miles around would probably lead to them being banned pretty quickly. But then again, maybe I'm wrong.. anyone elso know? ...Willy.
 

Eric Foster

New member
No PI metal detector could even be heard one mile away on an AM radio. Most of the energy is at low VLF frequencies, with harmonics up to 500kHz, if you are lucky. A detector coil is very inefficient at radiating to any distance. String a long wire antenna to the live end of the coil, then you might cause some interference.

Eric.
 

willy

New member
So what are the restrictions on power output on a metal detector? In regards to antennas, mobile phones have tiny ones and get good range. My main thrust here is that, regardless of frequency, the govmt. usually sets output limits. Surely metal detectors are not exempt? ...Willy.
 

Mr.Bill

New member
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
 

willy

New member
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.
 

Mr.Bill

New member
Transmit Power Levels and the FCC
Posted by: NC-Dave <Send a PM> (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.
 

Eric Foster

New member
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.
 

Prospector Al

New member
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
 

Carl-NC

Member
"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
 

Prospector Al

New member
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)
 

5900_XL-1

Member
Carl-NC said:
"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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