Oscillator power!!!!

[Qiaozhi]

As far I know national authorities (national EMF control agencies) act (adopt EMF regulations) according recommendations of ICNIRP . There are many documents / recommendations which are than adopted in national regulations ( I am sure that in UK too):

http://www.icnirp.de/

This seems to be the problem. Several times people have mentioned a restriction on the power output of metal detectors, but no-one can ever point to a source for this information.

[pebe]

I did a search and found some information.
Try Googling "mpt 1570". There is a publication there by www.ofcom.org.uk

That gives maximum radiation levels from 9kHz upwards and the method of measuring them. There seems to be no regulation of frequencies below 9kHz.

[Aziz]

Hi pebe,

I did a search and found some information.
Try Googling "mpt 1570". There is a publication there by www.ofcom.org.uk

That gives maximum radiation levels from 9kHz upwards and the method of measuring them. There seems to be no regulation of frequencies below 9kHz.

Thanks for the information. It would be interesting to know, where the real limit is.

But it should generally be possible to make a very low frequency (<9 kHz) pulse induction metal detector with very high output power.

Aziz

[Tinkerer]

Hi pebe,



Thanks for the information. It would be interesting to know, where the real limit is.

But it should generally be possible to make a very low frequency (<9 kHz) pulse induction metal detector with very high output power.

Aziz

The 9 kHz limit seems to be favoring the VLF configuration. With a PI we get much higher frequency emissions at the switch off of the pulse.

One factor that helps, is that the pulsed emission frequencies are averaged for measurement.

Tinkerer

[Tinkerer]

Here is a pdf. for outdoors that maybe applies.

Tinkerer

[Qiaozhi]

I did a search and found some information.
Try Googling "mpt 1570". There is a publication there by www.ofcom.org.uk

That gives maximum radiation levels from 9kHz upwards and the method of measuring them. There seems to be no regulation of frequencies below 9kHz.

"IR 2030" is another interesting document. Look at Table 2.13 which relates specifically to metal detectors. ->
http://www.google.co.uk/#hl=en&sourc...d8ecc3630e400c

[Tinkerer]

"IR 2030" is another interesting document. Look at Table 2.13 which relates specifically to metal detectors. ->
http://www.google.co.uk/#hl=en&sourc...d8ecc3630e400c

Thanks for the link.

below is the table about metal detectors.



That part of an induction system designed or adapted to produce:–

(a) to produce a controlled magnetic field; and

(b) a predetermined recognisable signal when operating within that magnetic field; so as to be capable of use only within the frequency bands, and at a radiated level, not exceeding the maximum for such frequency bands, specified in the table below:-

Category Frequencies or Frequency Band Radiated level Channel Bandwidth Music or speech permitted Duty Cycle Reference Standard *
- 9 - 148.5 kHz 70 dBV/m @ 6 m - No - EN 300 330


So if I understand right, the the emissions above 9 kHz have to be limited below 148.5kHz and 70dbuV/m at a distance of 6 meters from the coil.

No limitations below 9kHz.

Now I only need to find out how to measure that.

Any ideas????

Tinkerer

[pebe]

Here you go.
http://www.compliance-club.com/PLT/R...cification.pdf
Details in para. 5.2

It looks like it is intended to measure transmissions in the horizontal direction. But like most other specs, you are refered to other specs - this time to CISPR16-1. It gets a bit like trying to trace your family tree!

[WM6]

About EMI regulations:

"EMI Regulations
Government regulations in the US and many other countries prohibit electronic products from emitting EMI that could interfere with radio and television receivers. European regulations also include EMI immunity levels. Manufacturers of commercial electronic products generally contend with three types of EMI problems:

• Suppression of internally generated signals to prevent excessive levels of radiated and/or conducted emissions
  The FCC in the United States, CSA in Canada, VCCI in Japan, AUSTEL in Australia, and legislation by EU (European Union) member countries all set certain standards for EMI emission levels that commercial electronic devices must meet before being sold in those countries. Many electronic products sold in the US must be tested and verified or certified for compliance with the FCC’s Part 15 regulations.
  
  
• External ambient interference with equipment operation
  Many companies establish their own specifications for immunity to EMI over a range of phenomena. These may include electrostatic discharge (ESD), radiated immunity, and electricfast transients (EFT). This is not yet a requirement in the US; however, EU regulations currently do include immunity requirements.
  
  
• Internally generated emissions interfering with equipment operation
  EMI from one circuit can interfere with the function of another within the same system or subsystem. Typically called cross-talk, this problem is the most common source of system susceptibility. Cross-talk frequently occurs in densely packaged mobile or portable equipment."

http://www.chomerics.com/tech/EMITHEORY.htm#reg

[Aziz]

Here we go (found some formulas, which could help):

Electrical Field: dB µV/m -> V/m

E = (10^(e/20) ) /1e6
e = 20*lg(E*1e6)

E: electrical field strength in V/m
e: electrical field strength in dB µV/m
lg: log base 10

Magnetic Field: dB µA/m -> A/m (analog to above)

H = (10^(h/20) ) /1e6
h = 20*lg(H*1e6)

H: magnetic field strength in A/m
h: magnetic field strength in dB µA/m

As we know, we only have the magnetic field emission part if the coil is good shielded (no electric field outside of the coil). We should shield the coil as the electric field strength can go quite high due to high flyback voltage.

Aziz

[Carl-NC]

So if I understand right, the the emissions above 9 kHz have to be limited below 148.5kHz and 70dbuV/m at a distance of 6 meters from the coil.

No limitations below 9kHz.

Now I only need to find out how to measure that.

Any ideas????

70dBuV is equal to 3.16mV. This refers to the E-field, which is often specified in EM limits (rather than the H-field) because it's easier to measure. Fortunately, metal detectors operate on magnetic fields and aren't very efficient at producing an E-field.

To measure the E-field, you need a measurement antenna calibrated for use in the frequency of interest. This is another problem, because a VLF antenna for this purpose would be quite large. Perhaps there is another method used for low frequencies, but I've never checked.

- Carl

[Aziz]

Hi Carl,

70dBuV is equal to 3.16mV. This refers to the E-field, which is often specified in EM limits (rather than the H-field) because it's easier to measure. Fortunately, metal detectors operate on magnetic fields and aren't very efficient at producing an E-field.

To measure the E-field, you need a measurement antenna calibrated for use in the frequency of interest. This is another problem, because a VLF antenna for this purpose would be quite large. Perhaps there is another method used for low frequencies, but I've never checked.

- Carl

we can calculate the H-field strength. We only need the distance to the coil (specified measurement distance), coil inductivity and the max. coil current.

Aziz

[Carl-NC]

Hi Carl,

we can calculate the H-field strength. We only need the distance to the coil (specified measurement distance), coil inductivity and the max. coil current.

Aziz

True, but in the near-field the E and H fields aren't necessarily related like they are in the far-field, where E/H = 377 ohms. For a detector the E-field is probably very weak, so even if we can calculate the H-field we can't derive the E-field from that.

I suspect the only way to determine whether the detector E-field exceeds the gov't limitation is to measure it.

[Qiaozhi]

I personally don't recall ever hearing anyone complain that metal detectors are causing interference, except when two detectorists get too close to each other. In fact, it's always the other way round, as shown in these articles ->
http://www.troycustomdetectors.com/s...erference.html
http://detectorstuff.com/2009/08/05/...-dave-johnson/

However, there has been some discussion as to whether pacemakers can be affected by anti-theft systems and metal detectors ->
http://www.fda.gov/MedicalDevices/Sa...ions/UCM062288

[Orbit]

Imagine the receiver to 100MHz ,that receives a radio signal over natural obstacles and the transmitter power 20w ,he will be heard but very little, however, if the transmitter power increase at 150w, This I have personally tried on doing RF transmitters.With 20w I barely caught a signal while I was with 150 w same signal caught mint.