power boost

[Jungle Jim]

Carl,


I hear that one of the things that limits the depth capability of production machines is that the power output is kept below a ceiling set by the FCC. I don't think I would do any harm running around with a machine that exceeds that barier a little. Is it possibly to boost the power and depth of an existing machine without a major internal mod. job? Jungle Jim

[Carl]

Re: power boost

>Carl,


>I hear that one of the things that limits the depth capability of production machines is that the power output is kept below a ceiling set by the FCC. I don't think I would do any harm running around with a machine that exceeds that barier a little. Is it possibly to boost the power and depth of an existing machine without a major internal mod. job? Jungle Jim


Probably, and I am about to try it on one of my machines. Looks like it has a single current-setting resistor in the transmit oscillator so I might actually run a potentiometer to it, vary the transmit power (and battery drain) in the field.


Keep in mind that depth is not proportional to power, not even close. For an ideal omnidirectional transmission (and target response) the received power is inversely proportional to the sixth power of depth, i.e., P ~ 1/(d^6). So if you double the power you get 12% more depth, or if you want to double your depth you need 64x more power. I don't think MDs are quite this bad since the coil field is somewhat directional.


I've been trying to find out exactly what the FCC regs are on this. Their web site and the CFR site are not clear, but imply that regs on "homebrew" circuits are pretty loose, and that sub-9KHz commercial transmitters (which covers most VLF machines) can be up to 10 watts. I've written the FCC for clarification and will post their reply.


- Carl

[Alan]

Re: power boost

I've read that uncontrolled RF devices of which IB detectors belong are limited to 100 milliwatts of output power by the FCC. The other factor limiting increasing the power output is the concurrent TX/RX nature of IB detectors and signal saturation of the RX windings from the TX windings. PI detectors can overcome this because they stop the TX before the RX kicks in. However as with both types of detectors the conductivity of the ground is the limiting factor for RF devices with current technology.


>I've been trying to find out exactly what the FCC regs are on this. Their web site and the CFR site are not clear, but imply that regs on "homebrew" circuits are pretty loose, and that sub-9KHz commercial transmitters (which covers most VLF machines) can be up to 10 watts. I've written the FCC for clarification and will post their reply.


>- Carl

[Carl]

Re: power boost

>I've read that uncontrolled RF devices of which IB detectors belong are limited to 100 milliwatts of output power by the FCC. The other factor limiting increasing the power output is the concurrent TX/RX nature of IB detectors and signal saturation of the RX windings from the TX windings. PI detectors can overcome this because they stop the TX before the RX kicks in. However as with both types of detectors the conductivity of the ground is the limiting factor for RF devices with current technology.


My understanding is that in a true induction balance setup the RX coil never sees any of the field from the TX coil, only from the target. Do most modern IB machines still use geometrically balanced coils, or do they use bucking circuits to force the balance? In the latter case I can see how increasing the TX power will cause problems.


I've also heard the 100mW number but cannot find it in print.


- Carl