Riddle me this!

[Cherry Picker]

These are probably questions for Carl, but let's give it a go. Not so much questions as statements needing a thumbs up or down.

It is my understanding that 10-12 kHz is probably the best overall frequency for a mono frequency detector. Multi-Frequency SMF/Multi-IQ/etc., works great for hunting high & low conductors, but when a particular target/metal is sought, then a mono frequency to fit the need is best. I think FCC put a cap on the transmitted power back in the 70s, and pretty much all metal detectors since have been maxed to this cap as far as transmitting power. Frequency is used to get the best results regulated by conditions and target conductivity.

With 10-12 kHz being a good all-purpose frequency, a lower frequency, say 2.5 kHz will have better ground penetration but is more prone to iron falsing. On the other hand, a higher frequency, say 40 kHz will be more sensitive to small low conductive targets but has less depth or ground penetration.

Does this sound right?

 

[oldkoot]

what you stated is pretty much how I understand it

 

[MasonDixonMding]

I can’t help you with your question, but your post makes me question why the fcc would need to put a cap on metal detector power.

I think FCC put a cap on the transmitted power back in the 70s,

 

[Cherry Picker]

I can’t help you with your question, but your post makes me question why the fcc would need to put a cap on metal detector power.

They put a cap on transmit power for the same reason they do on all such devices. Safety. High magnetic transmission can, and does, cause cancer. Even medical equipment output is regulated by the FCC.

 

[MasonDixonMding]

They put a cap on transmit power for the same reason they do on all such devices. Safety. High magnetic transmission can, and does, cause cancer. Even medical equipment output is regulated by the FCC.

They put a cap on transmit power for the same reason they do on all such devices. Safety. High magnetic transmission can, and does, cause cancer. Even medical equipment output is regulated by the FCC.

 

[MasonDixonMding]

Thank you for educating me.

 

[Rudy]

They put a cap on transmit power for the same reason they do on all such devices. Safety. High magnetic transmission can, and does, cause cancer. Even medical equipment output is regulated by the FCC.

I think this falls under the category of "it sounds plausible", but I can't find any FCC directive regulating the transmit power of a metal detector.

The magnetic field from the transmit coil is not an electromagnetic, or RF, wave. It doesn't really radiate into the ether, like a radio transmitter does.. That is why you can hunt with a buddy, provided you stay away from each other by just a few feet.

More likely, the transmit power is limited by:

1. Cost.
2. Weight.
3. Operating time between recharges or battery replacement.

 

[UT_Dave]

Disclaimer: I do not know!

Having read a lot of Carl's old posts on the subject, my take away is that for practical purposes, no the FCC isn't limiting coil TX power. But the three points above by Rudy are. And add in ground signal.

I really don't actually know. But I don't think it's FCC rules that are limiting TX "power"? But rather the law of diminishing returns and the math of how much juice you get for the squeeze of increasing power. You just don't get much for doubling, or quadrupling power and all the battery life, weight etc. costs that have to come along with it.

- Dave

 

[Carl-NC]

Rudy & UT_Dave have it about right. The TX field strength ("power" is not the right term) is limited by practicality. There is an FCC limit on field strength but we're not there yet. For a 10kHz detector it's 240uV/m at a distance of 300m, which you'd be hard-pressed to measure anything at that distance.

For frequency, 1kHz would be best for US silver coins, 2.5kHz for wheaties, and 15kHz for nickels. So, yes, 10kHz makes for a good compromise choice.

 

[metaladdict]

Back in the 80s a friend had a boosted cb radio that bled over everything radios tv etc!

 

[UT_Dave]

Back in the 80s a friend had a boosted cb radio that bled over everything radios tv etc!

Heck, back in the early 70s the Mallory ignition on the 460 in my Dad's F250 made TV's all up and down the block go fuzzy .

- Dave

 

[FLOOD]

I think this falls under the category of "it sounds plausible", but I can't find any FCC directive regulating the transmit power of a metal detector.

The magnetic field from the transmit coil is not an electromagnetic, or RF, wave. It doesn't really radiate into the ether, like a radio transmitter does.. That is why you can hunt with a buddy, provided you stay away from each other by just a few feet.

More likely, the transmit power is limited by:

1. Cost.
2. Weight.
3. Operating time between recharges or battery replacement.

FCC Part 15 covers radiated/transmit power for a host of portable devices operating in various frequency ranges. Part 22 is also applicable to detectors that employ wireless headsets due to their tx/rx operation as well. The metal detector companies, like all other device manufacturers, have to demonstrate compliance for transmission strength, spurious emissions and RF exposure limits. Not all of the specs are listed at as wattages. Some are spec’d at micro volts per meter, which can be converted to wattage, but there is definitely limitations on RF output for free air and closed loop transmitters, and wireless headsets. You can find metal detector company compliance reports for the FCC on the web.

 

[Rudy]

FCC Part 15 covers radiated/transmit power for a host of portable devices operating in various frequency ranges. Part 22 is also applicable to detectors that employ wireless headsets due to their tx/rx operation as well. The metal detector companies, like all other device manufacturers, have to demonstrate compliance for transmission strength, spurious emissions and RF exposure limits. Not all of the specs are listed at as wattages. Some are spec’d at micro volts per meter, which can be converted to wattage, but there is definitely limitations on RF output for free air and closed loop transmitters, and wireless headsets. You can find metal detector company compliance reports for the FCC on the web.

Part 15 is not applicable to the coil's transmit power. It has more to do with, for example, things like walkie talkies that operate in the frequency spectrum of Family Radio Service (FRS). Likewise, wireless headset power operate using RF, which are covered by FCC regulations. The original question was, specifically, about a metal detector's transmit coil power.

 

[Cherry Picker]

I used to think the limit was 5w but I believe I saw somewhere that it is 1w and has been maxed for many years.

I would assume that is per frequency, since if it was shared between all frequencies, then multi-frequency would mean less depth per frequency. This gives the marketers something to spin by claiming more power because of more frequencies.

 

[Carl-NC]

Part 15 is not applicable to the coil's transmit power.

No, Part 15 is exactly applicable to detectors. 15.209 specifically. A metal detector is considered an intentional radiator under 15.3o. In the US the limits in 209 start at 9kHz, meaning below 9kHz you can do whatever you want. Not so in the EU.

I used to think the limit was 5w but I believe I saw somewhere that it is 1w and has been maxed for many years.

It's not 5W or 1W or 100mW. I gave the correct answer in post #9.
Besides that, someone would need to explain how a metal detector could have a 1W transmitter.

 

[Rudy]

No, Part 15 is exactly applicable to detectors. 15.209 specifically. A metal detector is considered an intentional radiator under 15.3o. In the US the limits in 209 start at 9kHz, meaning below 9kHz you can do whatever you want. Not so in the EU.

With all due respect Carl, I don't think a practical. portable detector transmit coil would generate a field strength strong enough to measure anywhere near the limits allowed in the Table in section 15.211. If it did, it would be very difficult (impossible?) for two people to detect a few feet away from each other. It is for that reason that I said Part 15 is not really applicable to commercially available metal detectors.

 

[tnsharpshooter]

Manticore FCC info.
Enjoy.

0049 Metal detector supporting wireless audio, used for detecting buried metal objects Test Report Minelab Electronics Pty

Metal detector supporting wireless audio, used for detecting buried metal objects Test Report details for FCC ID Z4C-0049 made by Minelab Electronics Pty Ltd. Document Includes Test Report Test Report

fccid.io

 

[UT_Dave]

That's just for the BTE bluetooth used for the wireless headphones. It was posted on DP months ago and had a lot of us thinking Manticore would be BTE compatible since it uses that really nice Nordic chip.

- Dave

 

[FLOOD]

Metal detectors are covered by Part 15, as has been said, but the subsections of Part 15 are relevant to the wireless/Bluetooth transceiver, not the coil. Think of the Detector as a dual-function unit, with part of its functionality (wireless communications) covered by Part 15, but not the voltage delivery to the loop coil. Metal dectector wireless operations are covered under several subparts (15.209, 15.247), as is the internal antenna (15.203) Detectors that can be plugged in to a USB cable, or directly to an AC outlet, also have to comply with 15.207. Detectors that only operate on replaceable batteries don't need to comply with 15.207. Most of the wireless compliance testing is to ensure that the wireless channels operating in the 2.4 Ghz range are not radiating spurious emissions of varying frequencies outside of their channels; peak output power per channel is correct; channels are not bleeding over in bandwidth on either edge of a particular channel, causing interference to an adjacent channel, etc.

Many of the newer detectors are operating with internal Lithium batteries. They provide DC and AC voltages to the control box brains, which drives all of the detectors electronics, to include it's detection circuits, the wireless communications circuits and providing the voltage needed to drive the coil. Most of these detectors can operate anywhere from several hours to 10 or more hours before they need a recharge, depending on detector settings (like adjusting lights, frequencies, unit vibration, volume, bluetooth enabling, etc). We all know this...with respect to powering, for example, from what I've read in their literature, the Nokta Legend's wireless antenna circuit has an output power of approximately 2.6 mW...not very much, but, not much is needed because the headset is located a couple feet from the antenna.

When you take into account that an internal battery, like the Legend's 5050 mAh battery has a listed use life of 8-20 hours, and the fact that the battery is handling all circuitry functionality, the device is producing very low voltages and amperage to run the machine's circuits and components. It doesn't require a lot of current drive a coil to create a magnetic field.

I'm not sure why the question arose as to how much power a coil transmits...a metal detector control box generates very little, in terms of voltage, amperage or power with its internal batter/power supply. Given the increased functionality and circuitry designed into the newer detectors, and their operating time under load, the companies are delivering pretty good products to the user.

So, Part 15 doesn't directly apply to the coil's operation, but I'm certain that when detectors are going through design and R&D, the entire device is bench-tested and tested in an anechoic chamber to ensure that while it is on, any RF emissions generated by the device, whether intentionally, unintentionally or incidentally generated, do not exceed design limits or rules such as those established by the FCC.

 

[Cherry Picker]

Myself, I was going off what I know about RF transmitters, the FCC, and radio. I know that the FCC had a limit of 5w on standard CB radios. That is why I just assumed it would be 5w on detectors. Everywhere I look I still find 1w as the FCC limit. You also have to remember, this is the company that marketed the idea that FBS transmitted 28 frequencies, when now we know all along it was only 2. Whites could have done the same thing, but didn't.